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An enumeration of myelinated and unmyelinated fibers in the optio nerve of vertebrates

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North we ste rn Un iversity Library
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NAME AND ADDRESS
DATE
NORTHWESTERN UNIVERSITY
AN ENUMERATION OF MVELINATED AND UNMYELINATED
EIBERS IN THE OPTIO NERVE
OF VERTEBRATES
A DISSERTATION
SUBMITTED TO THE GRADUATE SCHOOL
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
fox the degree
DOCTOR OF PHILOSOPHY
DEPARTMENT OF ANATOMY
By
SIMON RULIN BRUESCH
CHICAGO, ILLINOIS
AUGUST, 1940
ProQuest Number: 10101217
All rights reserved
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uest
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TABLE OF CONTENTS
I.
INTRODUCTION AND STATEMENTOF THE PROBLEM............... 1
II.
REVIEW OF THE LITERATURE.............................. 4
A.
Older literature:
B.
Modern literature:
1.
3.
III.
1876 to the present
of
History of enumerations fibers in nerves
other than the optic.
History of enumerations of fibers in the
optic nerve.
ANATOMY OF THE OPTIC NERVE..........................
A.
3.
3.
B.
Sheaths and vessels
a. Dura mater
b. Arachnoid
c. Pia mater
Interstital cells
a. Neuroglia
b. Microglia
Nerve fibers
a. Origin
b. Structure
Development of the optic nerve
1.
3.
3.
4.
C.
Optic stalfc
Nerve fibers
Myelination of the fibers
Chiasma
Distribution of fibers in nerve and tract
1.
3.
Decussation of fibers
Retinal localization in nerve and tract
MATERIALS AND METHODS............
A.
Materials
1.
3.
3.
B.
39
Adult structure
1.
IV.
ancient timesto 1876
Sources of material
Age of specimens
Clinical histories of human specimens
Methods
1.
Dissection of the nerves
40
TABLE OF CONTENTS
2.
3.
4*
V.
Fixation
a. Osmic acid fixation
b. Fixation for the silver tbchniques
c# Data on fixation of the material
Staining
a* Sectioning of the osmic-stained
material
b.
Silver-staining
Fiber estimates
a* Criteria for selecting sections of
nerves for counting
b. Counting procedure: strip method
Equipment
Procedure
Percentage of fibers actually
counted
4). Calculation of the total
number of fibers
5). Precautions against error
RESULTS............................................. 61
A»
Microscopic descriptions of the nerves
B.
Fiber enumerations
VI,
SUMMARY OF RESULTS.................................. 98
VII.
DISCUSSION......................................... 101
A.
Material
1# Scope of the material
2# Number of specimens in a series
3. Use of single nerve or the pair
B«
Methods
1* Staining
2* Counting
C*
Sources of error
D«
Evaluation of results
1. Relation of the results to the material
2# Relation of the total fiber number to the
age of the specimen
3. Correlations between the results and the
state of development of the visual system
TABLE OF CONTENTS
a*
4.
5.
Description of retinal specializations
1). Rods and cones
a)• Definitions
b). Distribution
3). Specializations for increasing
visual acuity: area centralis,
fovea centralis, macula lutea
a)* Structure
b). Distribution
c). Possible significance of
the fovea centralis
3). Photomechanical movements
a). Definition
b). Distribution
b. Central connections of the vertebrate
optic pathway
c. Visual habits of vertebrates
d. Correlations of retinal specializations,
central connections of the optic path­
way, and visual habits with the findings
in the optic nerve
Present knowledge of myelin and its relation
to this problem
a* Nature of myelin
b. Structure of myelin
c. Origin of the myelin sheath
d. Myelinogenesis
e.
Relation of myelin to nerve function
Electrophysiology of the retina and optic
nerve
a. Retinal action currents
1). Components
2). Relation of retinal action
currents to anatomical
structure
3)* Relation of retinal action
currents to the optic nerve
b.
Action currents of optic nerve
c.
Summary of retinal and optic nerve
action currents and their possible
relation to the structure of the
optic nerve.
VIII. CONCLUSIONS......
3.6$
IX.
ACKNOWLEDGMENTS......
167
X.
APPENDIX............................................ 168
XI.
BIBLIOGRAPHY........................................ 170
XII.
VITA................................................199
I.
INTRODUCTION AND STATEMENT OF THE PROBLEM
The visual system is an important anatomical specialization
which most animals utilise for an adequate maintainence of contact
with their environment#
Man has long recognized its importance
in his own physical existence and thereby has long sought to
understand its construction and function.
Hence, since the
period of the dawn of the written scientific work (the age of
the Greeks and Romans), the literature concerned with the anatomy
and physiology of seeing has been accumulating.
Anatomists and physiologists have done much, particularly
in the past seventy-five years, to establish clearly the funda­
mental structure of the visual system and to explain how these
structures carry out the intricate process of vision.
Clinicians
have utilized these observations and theories and have added
many of their own to them and, thereby, have developed the highly
important and successful clinical specialty of ophthalmology.
These ophthalmologists follow the researches of the anatomists
and physiologists closely and often gain new information of
clinical application from them.
But in addition to watching,
these clinicians have undertaken many researches in recent years
in the "pure science" of the anatomy and physiology of the visual
system and have uncovered many new facts.
This is a clear indi­
cation of the desire of clinicians for newer and more complete
information on the fundamentals of the anatomy and physiology of
the visual system.
But the story of the anatomy and the physiology of the
visual system is far from complete at present.
Much work has
undoubtedly been done without proper correlation;
the clinicians
have instituted and described in the literature certain therapies without any anatomical or physiological explanation for
their success or failure, physiologists have undertaken
experiments and have made observations which they could not
interpret because details of structure had not been worked
out, and anatomists have made observations of anatomical
detail which have remained unused because physiologists and
clinicians have either overlooked or neglected them.
In short,
structure must be known before function can be adequately in­
terpreted, and structure and function must be understood before
therapy can be removed from the classification of an empirical
procedure*
The knowledge of the structure of the grosser aspects of
the human visual system is reasonably complete, but the more
detailed features are as yet incompletely understood.
This
applies to all parts of the visual system, bftt it is particularly
true of the optic nerve*
The origin and termination of the fiber
components of the optic nerve is well established and the indi­
vidual structure of these fibers has been the subject of many
studies.
The functional distribution of the fibers within the
optic nerve, chiasraa, and tracts is now reasonably well under­
stood;
but as yet the numerical composition and the completeness
of the myelination of the fibers is imperfectly known.
It is
not illogical to speculate that there might be some relation
between the number of fibers in an optic nerve and the function
of the visual system:
might not a nerve with 1,000,000 fibers
subserve a better developed visual system and a higher type of
vision than one containing just 1,500 fibers?
Further, the
noting of the presence of unmyelinated fibers in an optic nerve
might be of functional significance for, as will be discussed
later in this paper, unmyelinated fibers might convey a slightly
different impulse than myelinated ones
It is not the purpose
3,
of this paper to settle these points;
but in making the
observations and discussing them, certain trends suggestive
of an explanation will be noted*
As id so often the case, there is much to be learned from
comparative studies made on lower vertebrates*
Often the find­
ings in man cannot be properly interpreted until it is known
what is present in the lower forms;
the complex picture may be
broken down into its simpler components (phylogenetically speak­
ing} and thereby more easily understood.
Therefore, knowing that enumerations of myelinated and
unmyelinated fibers in the optic nerve of man were incomplete
(vide historical review) and, believing that such facts are
necessary before much progress can be made in understanding the
details of the function of the optic nerve, it became the object
of this research to complete these fiber studies which were so
ably begun by other workers in this laboratory several years ago*
Further, believing that the findings in the human optic nerve
would be of enhanced value if similar data were available for
the lower vertebrates, the same type of study was extended to
include representatives of all the important vertebrate groups.
Although quantitative determinations are nothing new in the
field of anatomy, there has been a tendency to neglect this more
tedius form of research for the glamorous physiological anatomical
type ofinvestigation.
Krogh (1939) has gauged the situation well
in these words:
HI believe that when taken up in earnest by competent
anatomists, the field of quantitative anatomy will prove
to be a rich and fruitful one. Many determinations...are
urgently needed as a basis for quantitative physiological
work.w
II.
A#
REVIEW OF THE LITERATURE
Older literature:
ancient times to 1876
The record of man's attempts to gain information about the
anatomical foundation of the sense of vision goes back to the
period of the Greek and Roman influence.
Naturally these early
foundations of our knowledge of the visual system now seem to be
the crudest sort of speculation, but a re-examination of these
ancient concepts is of great value:
it not only gives an under­
standing of how the information we now possess was obtained but,
most important of all, it points out that man's thinking is
largely dictated by current concepts and that an observation is
usually explained by the utilization of these concepts.
This
famous dictum of August Gomte expresses well the debt of the
New to the Old:
"The older the world grows, the more the living are indebted
to the dead."
Hippocrates (B.C. 460-356) described the brain and recognized
it as the seat of intelligence:
"The brain of man, as in all other animals, is double, and a
thin membrane (meninx) divides it through the middle, ...and
veins run toward it from all parts of the body, many of which
are small, but two are thick, — the one from the liver, and
the other from the spleen."
"And men ought to know that from nothing else but thence (from
the brain) come joys, delights, laughter and sports, and sorrows,
griefs, despondency, and lamentations."
"And the eyes, the ears, the tougue and the feet, administer
such things as thebrain cogitates....It is the
brain which
is the messenger to the understanding."
"Since, then, the brain, as being the primary seat of sense
and of the spirits, perceives whatever occurs in the body, if
any change more powerful than usual takes place in the air,
owing to the seasons, the brain becomes changed by the state
of the air."
lippocrates believed that inspired air passed directly to the
brain by means of veins.
He knew little or nothing about nerves
5.
and he confounded them with veins, ligaments and tendons.
The
inspired air, he believed, if free and unobstructed, purified
the brain itself, but if restrained and obstructed, produced
much disturbance of the system and derangment of the mind.
These quotations express such concepts:
"When a person draws in air by the mouth and nostrils, the
breath (pneuma) goes first to the brain..,"
".••the air which enters the veins is of use (to the body) by
entering the brain and its ventricles, and thus it imparts
sensibility and motion to all the members, so that when the
veins are excluded from the air...and do not receive it, the
man loses his speech and intellect, and the hands become
powerless."
has
Plato (B.C. 437-347) discussed whether man^intelligence
and sense through the blood or by air or fire, or whether the
senses of hearing, seeing and smelling depend upon the brain.
He decided that the brain was the seat of intelligence and
perception and was the governing principle of all.
He did not
recognize nerves in the modern sense.
Aristotle (B.C. 384-321) described the brain in theseterras:
"...an insert viscus, cold and bloodless, an organ not to be
enumerated amongst other organs of the body, seeing that it
is of no use except to cool the heart."
He believed that life, perception and sensation resided in the
heart, though he added that some thought they are situated in the
brain, which he denied on account of its assumed bloodlessness.
Aristotle described the connections running between the eyes and
the brain but whether he meant nerves or vessels is uncertain.
At any rate, the existence of optic nerves in all animals was
doubted long after Aristotle's time (Guthrie, 1921).
Galen (A.D. 131-301) was the first to give a description of
the cranial nerves.
The optic nerves were Galen's first pa4r;
the softest of all;
most like the brain itself, and therefore
ie did not consider them true nerves.
They alone of all the
nerves are hollow, he observed, and in them alone can be seen
a lucid spirit flowing to and from the eyes (Guthrie, 1921).
But Galen admitted that it was difficult to convince oneself
of the existence of a canal which would hardly admit a hog's
bristle.
This alleged hollowness of the optic and other nerves
was hotly contested even as late as the latter part of the
seventeenth century.
Galen placed thq site of the origin of the optic nerves in
the fore part of the lateral ventricles.
It is certain, however
that Galen did not trace the optic tracts to their actual origin
in the optic thalami, although he stated that the nerves arise
from that part of the anterior lateral ventricle which is like a
chink or crevice (Guthrie, 1931).
There was extant before the time of Galen an opinion that
there was a true crossing of the optic nerves in the mid-line,
that the right tract was related to the left eye and that the
left tract was similarly concerned with the right eye.
Galen
flatly denied the assertion made by Rufus of Ephesus (A.D. 50)
that such a crossing of the optic nerves occurred.
He believed
that they only joined and reinforced vedbh • other, so as to
prevent double vision, and also to strengthen the power of one
eye when the other was destroyed.
He said that the closing of
one eye caused the pupil of the other to dilate, which proved
that the power of one eye was reinforced by closure of theother.
This use of the chiasma, he said, was told to him by a god in a
dream— a much higher authority than Rufus of Ephesus (Guthrie,
1931).
The death of Galen brought the study of anatomy very nearly
to an end for fifteen centuries.
wrangled with each other;
The various sects of medicine
but worse yet, the whole seience of
medicine "became contaminated by the lowest kind of mysticism,
sorcery, magic and imposture, in spite of spasmodic efforts
of some of the Roman Emperors to check the evil.
Very few
outstanding names aremet with in the period from Galen*s death
to the fall of Rome in A.D. 476.
The magnificent library of
Alexandria with its wealth of medical books was destroyed by
the Saracens in the seventh century.
In imitation of Alexandiia
thb Arabians founded their universities and their teaching in
medicine was based entirely on that of the Greeks, and to them
the modern world owes the survival of the Greek medical liter­
ature.
But they achieved little progress, least of all in the
field of neurology.
The reason for this lack of progress was
that the principles of Islam made human dissection a deadly sin.
It is not known when or where human dissection in aid of anatomi­
cal studies was revived;
it is certain that in 1308 the Senate
of Venice ordered that a body should be annually dissected
(McMurrich, 1930).
Leonardo de Vinci (1452-1519) made valuable contributions to
the study of anatomy, even though these were made primarily from
the viewpoint of the artist and sculptor.
In his anatomical draw­
ings of the nervous system, Leonardo represents the optic nerves
as arising from the anterior ventricle (McMurrich, 1930, fig. 71
and 73).
In one of the figures (fig. 73), he clearly shows the
chiasma and optic tracts, all in their true relations.
He
believed that the “pneuma11 arising in the ventricles flowed
through the optic nerves which were but hollow cylinders to the
eyes.
The site of the formation of the image intrigued Leonardo:
he speaks of it as the visual virtue.
After considerable irreso­
luteness, first locating it in the pupil and then in the lens,
Leonardo finally decided to place it at the end of the optic
nerve (thus placing the visual virtue in what is now known to
be the blind spot of the eye!).
The decision in favor of the
last location was based on an attempt to explain the erection
of the image.
The phenomena of refration had been accurately
described in the eleventh century and Leonardo recognized that
the cornea and lens must produce refraction and therefore in­
version of the image must occur.
Since it never occurred to
him that erection of the image might be a matter of interpre­
tation, located in the brain, he was forced to the conclusion
that there was a second crossing of the rays within the eyeball
The first crossing occurred between the cornea and lens, and
the seoond, he argued, from theanterior surface of the lens to
the head of the optic nerve.
Leonardo noted further, however,
that the double crossing of the rays in the eye might be open
to objection and the erection of the image ascribed to the
decussation of the optic nerve (McMurrich, 1930).
Vesalius (1514-1564) closely followed Leonardo de Vinci;
as McMurrich states:
“Vesalius was undoubtedly the founder of modern anatomy,
but Leonardo was his forerunner, a St. John crying in the
wilderness.11
But even though Vesalius scoffed at Galen*s anatomical works,
his account of the nervous system differed only in minor points
from that of Galen.
The brain to him was the site of the chief
soul, and also manufactured it.
It distributed the animal
spirit through the hollow nerves and thus motion and sensation
were effected.
Vesalius (1543) was the first to note atrophy
of the optic nerve after removal of the eye.
Having dissected
at Padua a woman whose right eye was absent, for a long time, he
observed that the right optic nerve was smaller in its entire
extent than the left.
9.
Eustachius, who died in 1574, traced the optic nerves to
their origin.
In one of his drawings of the brain (1552) he
showed an optic chiasraa;
in another there is no chiasma— the
optic nerves being separate throughout their entire course
(Guthrie, 1921).
To Varolio (1543-1575) we are indebted for a description of
the optic commissure.
Galen, as has been noted, denied that the
optic nerves crossed in the chiasma as Rufus of Ephesus had
asserted.
Varolio (1571) thought he had discovered the origin
of the optic nerves in the cerebrum when he wrote:
"The cerebrum is especially constructed for vision, while the
cerebellum is constructed for audition."
Valverde (1589) stated that he had assured himself that the
crossing of the optic nerves was absent:
in bandits who were
punished in Venice attheir first offense by removal of one of
their eyes and who having committed new crimes were hanged,
Valverde could find no chiasma.
The physical theory of vision owes its development mainly
to the work of several great astronomers and physicists.
Kepler
(1604) was the first to show how the retina is essential to sight,
the part the lens plays in refraction, and that the convergence
cf luminous rays before reaching the retina is the cause of myopia.
Descartes (1637) compared the eye to a camera obscura and its
accommodation was shown to be due to changes in the form of the
lens.
Edme Mariotte (died in 1684) proved that a luminous eye
is due to reflection of light and he also discovered the blind
spot in the retina (1668).
Aranzi (1587) demonstrated the
reversal of the image projected on the retina in cattle and he
3howed the lateral entry of the optic nerve,
Scheiner (1619)
£ave an ingenious demonstration of howimages fall on the human
10.
retina and noticed the changes in curvature of the lens during
accommodation (Garrison, 1921).
Thomas Willis (1621-1675) added much knowledge to neurology
through his great De Anatome Oerebri (1664).
His anatomical
descriptions of the nervous system were the best up to that time
but his ideas about the physiology of the nervous system were as
erroneous as Galen*s.
About the optic nerves, Willis had this to
say:
"Moreover, we advertise concerning the Optick Nerves, that
they as in other living creatures, inclining mutually to
one another, are not united however, unless perhaps towards
the superfices; but they are crossed, and a nerve arising
from the right side ofthe oblong marrow, is carried into the
left eye, and so on the contrary."
He speaks of the optic nerves in his Practice of Physick (1685)
...arising from behind the chambered bodies (thalami) and
descending from thence finally being united, and being again
separated and carried into the balls of the eyes."
Willis thought, however, that the nerves finally formed the hard
coat of the eyeball after reaching the eye.
He also made this
interesting statement:
"We did but notice...that the trunk of the nerve...was as it
were a little bundle of very many fibres or small strings
growing together into one, and of produced parallels, as it
should seem for that end, that the animal spirits, flowing
in the whole nerve, might be moved in so many lines or direct
rays; to wit, whereby they may carry the visible species,
sufficiently refracted from the eye,thence to a common
sensorium by a direct beam, without being intorted."
Sir Isaac Newton (1642-1727) was the first to present a
clear analysis of the relationship ofthe fibers of the optic
nerves to the chiasma and optic tracts.
The first evidences of
Newton's interest in the anatomy of the visual system occur in a
letter refuting A New Theory of Vision by Dr. W. Briggs published
in 1683.
Briggs sought to explain single vision with two'eyes.
He described the optic nerves as arising from two "gibbous
11.
protuberances" in such a manner that those fibers that are in
the apex of the thalami have the greatest tension, while those
in the opposite part have the least tension by reason of a
less flexure.
Every fiber that passes into the upper part of
the right eye from the upper part of one thalamus has a borree
sponding one passing from theupper part of the other thalamus
into the upper part of the left eye, and the same thing occurs
with the lower group of fibers.
The fibers which thus correspond
in site correspond also in tension
“...so that when any impression from an object without
moves both fibres, it causes not a double sensation any
more than unisons in two viols struck together cause a
double sound."
Newton considered this theory very ingenious but he proceeded to
refute it, using these arguments:
1.
The bending of the nerves in the thalami is no proof of a
a difference of tension.
2.
The singleness of the picture arises from the coincidence
of the two pictures, and therefore the explanation must be
sought for in the cause that produces the coincidence.
3.
In answering Briggs' statement that the optic nerves do
not decussate or blend together (he cites proof of this
in fishes and. the chameleon), Newton replied:
"If you say that in the chameleon and fishes thenerves
only touch one another without mixture, and sometimes
do not so much as touch; ftis true, but makes altogether
against you. Fishes look one way with one eye, the other
way with the other...And in those animals which do not
look the same way with both eyes, what wonder if the
nerves do not join? To make them join would have been
to no purpose; and nature does nothing in vain. But
then, whilst in these animals, where 'tis not necessary,
they are not joined, in all others which look the same
way with both eyes, so far as I can yet learn, they are
joined. Consider, therefore, for what reason they are
joined in the one and not in the other..."
Apparently Newton pursued this trend stmll further and elucidated
his theory of the semi-decussation of the optic nerves,
it is
13.
summarized in the 15th Query of his Optics and was included in
a much garbled form in Joseph Harris* Treatise of Optics.
It
should be pointed out that this theory was anticipated to a
considerable extent by M. Rohault in his Traite de Physique
published in 1671 (Brewster, 1855), more than ten years before
Newton»s interest was called to the subject.
Newton*s concept
of the transmission of impulses from the eye to the brain over
the optic nerves is best conveyed by these words:
“Light seldom strikes upon the parts of gross bodies, (as
may be seen in its passing through them;) its reflection
and refraction is made by the diversity of aethers; and
therefore its effect upon the retina can only be to make
this vibrate: which motion then must be either carried in
the optic nerves to the sensorium, or produce other motions
that are carried thither* Not the latter, for water is too
gross for such subtile impressions; and as for animal
spirits, tho* I tied a piece of the optic nerve at one end,
I could not spy the least bubble; a little moisture only,
and the^marrow itself squeezed out...However, what need of
such spirits?...granting me, ...that there are pipes filled
with a pure transparent liquor passing from the eye to the
sensorium, ...the vibrating motion of the aether will of
necessity run along thither. For nothing interrupts that
motion but reflecting surfaces; and therefore also that
motion cannot stray through the reflecting surfaces of the
pipe, but must run along (like sound in a trunk) entire to
the sensorium. And that vision thus made, is very conform­
able to the sense of heading, which is made by like
vibrations. “
Vicq d*Azyr pointed out in 1731 that, while the most exact
anatomists since the time of Galen had denied the crossing of
the optic nerves in the chiasma, he was of the opinion that
“...their medullary substance communicated, and mixed that
of one side with that of the other.11
The histology of the nervous system was neglected in the
eighteenth century after getting off to an auspicious start
during the previous century in the hands of Leeuwenhoeck,
Ruyech and Malpighi.
The reason for the neglect may have been
Stahl*s statement that microscopic study was useless, or
Stenson's belief that nervous structures were far too intricate
13.
to unravel (Guthrie, 1921), or, more likely, the inherent un­
reliability of microscopes of that period and the lack of
adequate sectioning and staining techniques.
This neglect
makes it easy to understand why the status of the optic nerve
at the end of the eighteenth century was not remarkably differ­
ent from that of Galen*s period.
The knowledge of the relation­
ship between the optic nerves, the chiasma, and the thalamus
was still largely speculative in character with no general
agreement on even these elementary questions.
Galen*s idea of
the origin of the optic nerves from the “optic chambers" was
still in vogue, indicating that the relationship of the optic
nerves and tracts to the optic thalamus was still unsuspected.
The first seventy-five years of the ninteenth century saw
the accomplishment of much!
the origin of the optic nerve from
the retina of the eye was established, the problem of the
decussation of the optic nerve fibers at the chiasma was settled,
and the relationship of the optic tract to the lateral geniculate
body and superior colliculus was determined.
Many investigators
contributed to the accomplishment of these important landmarks;
the more important will be briefly mentioned in the following
paragraphs•
Cuvier wrote in 1800:
“It is certain that in all quadrupeds, the principle fascicle
of the optic nerves comes from the nates (superior colliculi)
and the corpus geniculatum externum.“
Gall (1810) confirmed this observation of Cuvier, and further
observed an atrophy of the “nates" associated with optic atrophy,
adding more evidence to an association of the optic nerves with
the superior colliculi.
Tiedemann (1816) also believed that the
optic nerves arose from both the lateral geniculate bodies and
the superior colliculi.
His evidence was based on dissections
14
of human fetuses three to five months of age, and he
11• followed the optic nerves as far as the interior of
the superior colliculi as well as to the surface of the
optic thalamus.11
Magendie (1844) summarized the origin of the optic nerves
in these words:
rlIt does not arise from the thalamus nervi optici, as many
anatomists have thought, but it derives its origin, 1st,
from the anterior pair of those tubercles called the quadrugemini; 2nd, from the corpus geniculatum externum, an
eminence found before, and a little to the outer side of,
these tubercles; 3rd, from the laminae of cineritious
substance, placed before the meeting of the optic nerves
and mammillary eminences, and which is known by the name
tuber cinereum.11
That the question of the complete decussation of the optic nerves
at the chiasma was not yet settled is indicated by this discus­
sion of Magendie1s:
11The most careful researches have been made for the purpose
of determining whether they decussate or are in contact, or
if they really intermix with each other; anatomy has not yet
settled this question...Some have thought that the crossing
of the optic nerves in fishes removed every doubt on the
subject; but this can only be justly considered as amount­
ing to a probability.H
Magendie cited some experiments performed by cutting the optic
nerves of rabbits in various places and observing the changes in
vision, and then concluded:
11With respect to its crossing with that of the opposite side,
no doubt can reasonably exist; the facts that I have reported
I consider demonstrative.H
11M* Pouillet, in his Treatise on Physics, does not agree in
this opinion. He believes that it may be true, perhaps, with
regard to animals, but not in man, and that Woolaston (who
claimed a partial crossing) has spoken only of the latter.
To this I reply, that, with respect to the anatomical arrange­
ments here referred to, man does not differ from the mammiferi.
I will add, that, having had occasion to make my objections in
England, to that profound philosopher, whom the intellectual
world has so many reasons to deplore, he did not appear to
doubt that if the section of the deuussation, over the sella
turcica, produced blindness, it may be concluded that the
crossing is total, andnot partial. I do not think that he
insisted upon hisconjecture after the publication of my
experiments."
15.
These words are an excellent example of how observations made
on lower animals, even in the hands of so great a scientist as
Magendie, can be wrong when applied to man.
And so the controversy continued up into the latter part
of the ninteenth century.
The experimental physiologists were
working on the problem but their experiments were poorly
controlled and often the wrong observation was made after
following out a biilliantly conceived procedure.
In the first half of the ninteenth century, the anatomists
added little to the knowledge of thefiner structure of the optic
nerve.
But along with the improvement of microscopes and the
development of adequate fixing and staining techniques for
nervous tissue, there was brought about a long train of acute
observations on the detail of the nervous system which revolu­
tionized the fundamental concepts of the construction of the
brain and the spinal cord.
Thus thefoundation was laid by the beginning of the last
quarter of the ninteenth century for the study of the microscopic
anatomy of the nervous system.
Progress was to be rapid, for at
last criteria for theinterpretation of the normal aa well as the
pathological function of the nervous system had been established.
16.
B.
Modern literature:
1876 to the present
1. History of the enumeration of fibers in nerves other
than the orotic.
The quantitative composition of nerve trunks has been the
subject of manyinvestigations over a period of at least ninetyfive years; almost from the time anatomists realized that nerves
were made up of minute fibers and were first in possession of
sectioning and staining techniques.
Necessarily these first
determinations were the crudest of estimates;
nevertheless,
they indicate the interest which has long a/ttached to this type
of research.
The moreimportant of these fiber enumerations made on nerves
other than the optic are briefly listed in chronological order
in the following paragraphs*
1845
Rosenthal made estimates of the number of fibers in all
the cerebral nerves except I, XX, and VII in the human.
His estimates were based on the number of fibers counted
in a few squares of an ocular-micrometer.
1859
Stilling estimated the number of fibers in the dorsal
roots of a human set of spinal nerves.
1872
Tergast counted the fibers in the abducens
nerve.
187B
Holl counted the fibers in the dorsal and ventral roots
and the trunk of the three lumbar spinal nerves of the
frog.
1878
Freud made counts on teased preparations of thedorsal
roots of the spinal nerwes of Petromyzon.
1880
Stienon reported two counts of the fibers in the two
roots and the trunk of the spinal nerves: one
speciman was the cervical nerve of a dog, and the
other was a lumbar nerve of the frog.
1882
Birge made counts of the fibers at a single level of
the two roots and trunk of the frog's spinal nerves.
1887
Vashkevitch determined the number of fibers in the
nervus ischiadicvis and nervus medianus of bats, mice,
rats, marmots, rabbits, cats and dogs.
17.
1889
Fritsch determined the numerical relations of the
elements of the electric organ of the torpedo to
the nerve cells and nerve fibers.
Schiller counted the number of myelinated fibers in
the oculomotor nerve of the cat at various ages.
Hodge reported the same research as Schiller!s but
with slightly different results.
1893
Blocq and Ozanoff reported the number of pyramidal
fibers supplying the arm, leg and trunk in man.
1896
Lewin and Gatrle made some enumerations upon the nerves
of the rabbit: they counted the two roots and the
trunk of three of the sacral spinal nerves.
1898
Buhle counted the fibers to be found on the central
and distal sides of the spinal ganglion of the frog.
1899
Hardesty reported counts of the myelinated fibers in
the spinal nerves II to X in the frog.
1900
Dunn counted the nerve fibers ennervating the skin and
muscles of the thigh of the frog (further work on this
subject was reported in 1903 end 1909).
Dale enumerated centripetal and centrifugal myelinated
fibers arising in the spinal ganglia of mammals.
1903
Hatai reported counts of spinal ganglion cells and dorsal
root fibers in the white rat.
1903
Donaldson determined the number of fibers distributed
to the skin and muscles of the frog’s leg.
1904
Ingbert computed the number of sensory and motor fibers
in the dorsal and ventral roots of the spinal nerves.
1906
Boughton studied the numerical relationships of the
oculomotor nerve in the white rat and the cat.
Ranson reported a count of the myelinated fibers in
the spinal nerves of the white rat.
1912
Dunn counted the myelinated fibers in the second
cervical nerve of the albino rat.
1913
Greenman determined the number of myelinated fibers in
the peroneal nerves of the white rat.
1933
Shimbo computed the myelinated fibers in the dorsal
and ventral roots of human spinal nerves.
1930
Duncan counted the myelinated fibers in: l) the human
right genitofemoral nerve and 3) the sciatic nerve of
the rat.
1931
Ono reported the number of nerve fibers in the spinal
roots of the cat.
Davenport and Ranson determined the ratio of cells to
fibers and of myelinated to unmyelinated fibers in
the spinal nerve roots.
1934
Agduhr reported comparative counts made on the roots
of the spinal nerves in the frog, mouse, dog and
human.
Davenport and Bothe made numerical studies on the
human nerves 02, 06, T4, T9, L3, S2 and S5.
1939
Bergstrand reported results of counts made on the
human spinal accessory nerve.
Lassek and Rasmussen published the results of their
fiber counts made on the human pyramidal tract.
1940
Rasmussen published the results of enumerations of
fibers in the human auditory nerve.
Foley and DuBois made a quantitative fiber study of
the cervical sympathetic trunk in the rabbit, cat
and rat.
Holmes and Davenport reported their series of
enumerations of fibers in the dorsal roots of the
cat.
Even though the preceding list probably does not include
all the quantitative nerve fiber studies made on nerves other
than the optic, it does record those that are reasonably wellknown to scientific workers in this field.
The list is suf­
ficiently long, however, to demonstrate the interest which
anatomists have evinced in this sort of research, yet it is
not long enough to show that this line of investigation has
been completely worked out.
19
3.
History of enumerations of fibers in the optic nerve*
Several workers have made estimates of the number of fibers
contained in the human optic nerve.
Krause (1876) seems to have
been the first to make such an investigation.
The result obtain­
ed by Krause was, however, purely a computation based on data
given for man from earlier sources.
Krause accepted the area of
a section of the optic nerve, exclusive of its sheaths, as 10
square millimeters;
the diameter of the nerve fibers to average
0.0003 millimeters;
and the connective tissue (inter- and intra-
fascicular) to total 69 per cent of the cross-sectional area.
Division of the area occupied by nerve fibers by the area of a
single fiber gave 1,000,000 as the apparent nerve fiber content
of the human optic nerve.
Later (1880) Krause came to believe
that these data as to the average size of an optic nerve fiber;
and as to the per cent area occupied by connective tissue were
unreliable and decided that the conclusion previously reached
was correspondingly invalidated.
Kuhnt (1879) attempted another type of calculation;
this
work, however, was a step backward as far as precision was con­
cerned.
This investigator made sections through the lamina
cribosa of the human optic nerve and stained them with osmic
acid;
he then counted a row of fibers from center to periphery.
This gave a r&dius of the circular section in terms of the nerve
fibers encountered.
Using this radial value (r), Kuhnt computed
the total number of fibers directly by using the familiar formula
for determining the area of a circle.
Estimates made in this way
u
for three nerves (3 adults and 1 child) led Kuhnt to conclde
that the total number of fibers in the human optic nerve is
about 40,000.
Although the method itself is very crude, Kuhntfs
20
results are further invalidated by the fact that the optic nerve
fibers lose their myelin sheaths at the level of the lamina
cribosa thus making any estimates of myelinated fibers at this
level worthless.
Zalzer (1880) made a more exact attack on the problem by
counting sample areas of osmic-stained sections and then calcu­
lating the total number of fibers by the method of proportionate
areas.
This investigator used an ocular with a square drawn
upon it and then counted the fibers within this square, making
such counts in seven different regions of the optic nerve.
average number of fibers per square was found to be 300.
The
The
entire cross-section of the nerve was then projected on to tin­
foil and the perimeter and the supporting connective tissue both
traced.
Next the gross area of this tracing was determined by
the use of a planimeter.
By the method of cutting-out and weigh­
ing, Zalzer determined the relative areas occupied by connective
tissue and nerve fibers.
Reducing his sample-count areas and
the total nerve-fiber areas to the same magnification, Zalzer
was able to calculate by simple computation the total number of
nerve fibers.
413,000;
On three adult nerves, he obtained these results:
434,378;
465,558.
Krause retracted his previous estimate of>,40£OOO fibers
(vide page 19) in 1880 and offered another estimate of 440,347.
This figure was apparently arrived at by the use of methods
similar to those described by Zalzer.
Krause does not make clear,
however, the exact manner in which he arrived at his last figure.
In addition to making this estimation, Krause intimated that
there might be many more very small fibers in the human optic
nerve which he could not count by the methods available at that
21.
time.
He suspected that there might he as many uncounted
small fibers present as there were counted larger ones, there­
by holding to the possibility that his figure was only about
one-half the true number (circa 880,000).
Nothing further waa added to this problem until in 1915
when Zwanenburg included some estimates of human optic nerve
fiberw as a part of a more comprehensive quantitative study of
the retina.
Zwanenburg, also using the method of proportionate
areas, made an estimate on one human optic nerve stained by the
Pal-Weigert technique.
His procedure was essentially the same
as that employed by Zalzer except that he actually counted a
far larger number of fibers:
4,374 fibers in 100 different sub­
fields, compared to Zalzer*s counting of 1,400 fibers in 7 differ­
ent fields.
Zwanenburg arrived at the figure of 550,000 as the
total number of fibers in the one human optic nerve he studied.
Another long unproductive period intervened, this time
broken by the work of Schiable in 1934, done in this laboratory.
Schaible was the first to investigate the possibility of the
presence of unmyelinated fibers in the human optic nerve.
He
carried out this investigation by making osmic acid preparations
of one portion of the nerve and silver-stained preparations of
another portion.
These two different staining techniques
demonstrate myelinated fibers (osmic acid) and axis cylinders
of all types of nerve fibers (silver technique).
By making
estimates of the number of fibers present in each type of pre­
paration, Sohaible noted a close correspondence between the two
estimates and concluded that there are no unmyelinated fibers in
the human optic nerve.
He estimated the fibers in two different
optic nerves and obtained these figures:
733,333 and 1,253,750.
22.
Schaible *s investigation established the fundamental technique
which has been utilized by other workers in this department and
which led up to the work to be described in this paper.
Bickeife made further estimates of the fiber content of the
human optic nerve in 1935.
He utilized the same methods of pre­
paration of the material and estimating fibers that Schaible had
adopted.
This time a series of six optic nerves was prepared
and careful estimates were made of their fiber content.
Bickel
arrived at two slightly different averages due to the presence
of two possible means of calculation of the observated data.
These two averages of his series of six human optic nerves are:
1,234,100 and 1,208,263 fibers.
The present status of the fiber content of the human optic
nerve is, therefore, this:
estimates have been made of the
number of fibers by several workers and these figures vary from
40,000 to 1,353,750 fibers.
Except for the work of Zwanenburg,
Schaible and Bickel, no determination rests on a careful study,
but rather represent rough, easily-obtained computations.
Schaible fs estimates have indicated that 8,11 the fibers are
myelinated.
The problem of the total number of fibers in the optic
nerve of vertebrates below man has not engaged the attention of
very many workers.
In fishes, the only previous work that has
been found in the literature deals with the optic nerve of the
conger eel and the blind fishes.
Adrian and Matthews (1927) in
connection with the work on the action of light on the eye, made
some quantitative studies on osmic-stained sections of the conger
eel^ optic nerve.
They made camera lucida drawings of the nerve
and made counts from these on four different specimens.
They
23.
arrived at a count averaging 10,000 fibers.
Using 3 micra as
a dividing point, they estimated that there were about 6,000
small fibers (l to 3 micra in diameter) and 2,000 large ones
(3 to 8 micra) in one of the sections which contained 8,000
fibers.
Charlton (1933) made a detailed study of the optic
tectum and its related fiber tracts in certain blind fishes
(Troglichthys rosea, Amblyopsis spelaeus and Typhlichthys
eigenmanni).
He found an exceedingly small optic nerve (12 to
17 micra in diameter) containing only a few fine myelinated
fibers (Weigart stain), probably averaging not more than 12 in
number.
The amphibian optic nerve has received considerably more
attention than has the fish nerve.
Palmer (1912) studied the
numerical relations of the histological elements in the retina
and optic nerve of Necturus maculosus.
He made Bielschowsky
preparations of the sections of the optic nerve.
these numerical results:
He obtained
1,982 fibers in the nerve near the
eye and 962 fibers at a level near the chiasma.
Palmer's
figures for the retinal elements are of interest in this
connection because, when considered from a relative view-point,
these proportions were secured:
1 1 visual cells, 121 nuclei
in the outer nuclear layer, 175 nuclei in the inner nuclear
layer, 30 ganglion cells, 26 Muller's fibers, 2 optic nerve
fibers in the distal portion of the nerve and 1 optic nerve
fiber in the portion near the chiasma.
study should be particularly noted:
Two points in this
l) the doubling of the
number of optic nerve fibers between the proximal and distal
portions of the nerve, and 2) the presence of 30 ganglion cells
to one optic nerve fiber.
These results seemed to ba so
contrary to what one would expect that Lander (1937), working
in this laboratory, undertook to check the results obtained on
the optic nerve*
Lander prepared sections by Davenports silver
nitrate method (1930) as well as some by the Bielschowsky technique used by Palmer.
In order to prove conclusively that the
axis cylinders were being selectively stained by the Davenport
technique, Lander carried out a degeneration experiment:
the
retina was destroyed with a blunt needle and, after allowing
30 to 35 days to elapse, the animal was killed and optic nerve
preparations made by the Davenport procedure.
It was found that
no argyrophilic fibers were discernable in the stained preparations
of the degenerated nerves;
thus clearly indicating that only
nerve fibers were "being stained.
Lander next carried out a
degeneration experiment along the same lines as the previous one
but this time the optic nerve preparations were stained by the
Bielschowsky technique used by Palmer.
This experiment showed
clearly that the Bielschowsky stain is not specific for nervous
elements, for the sections of the degenerated optic nerve showed
numerous darkly stained bodies comparable in every way to those
found in normal nerves.
Thus Palmer*s results on the Necturus
optic nerve were shown to be incorrect because he counted many
non-nervous elements.
Lander*s counts varied from 345 to 390
fibers, with an average of 363 fibers in 6 specimens.
He found
no significant difference between fiber counts close to the
chiasma and those made at a level of the nerve close to the eye­
ball.
Lander also studied the Necturus retina and could not
recognize any characteristic ganglion cells in the ganglion cell
layer.
Yet he found the total number of nuclei present to be
great (30,744 according to Palmer);
probably most of them are
25.
nuclei of neuroglial elements.
Lauber (1902) made a study of the eye of Cryptobranchus
japonicus.
As a part of his study he stained the optic nerve
by the Pal-Weigert method and set the number of nerve fibers
at 450.
Lander (1937) enumerated the fibers in the optic nerve
of Amblystoma in both osmic- and silver-stained sections.
secured these results:
He
an average of 2,004 fibers in silver-
stained sections and 1,176 fibers in osmic-stained sections.
Howe (in preparation) made a similar quantitative study in this
laboratory of the optic nerve of the bullfrog, Rana catesbiana,
and obtained these figures:
13,208 fibers in the osmic—stained
sections and 30,268 fibers in the silver preparations.
Norris (1938), working in this laboratory, has made the
only study, so far as has been determined, on the reptile optic
nerve.
He enumerated the fibers in the optic nerve of the
turtle, securing these results:
61,562 fibers in the osmic
preparations and 105,040 in the silver.
No quantitative nerve fiber data on the bird optic nerve
have been located in the literature.
But onthe other hand,
several workers, most of them working in this laboratory, have
made fiber studies on the sub-human mammalian optic nerve.
Norris (1938) studied the opossum optic nerve and secured these
results:
55,119 fibers in osmic-stained sections and 82,104
in silver preparations.
Schaible (1934), in conjunction with
his work on the human optic nerve, studied several other mam­
mals:
dog, 139,000 fibers in silver-stained sections and
152,000 in the osmic;
in the osmic;
rabbit, 261,000 in the silver and 333,000
macaque monkey, 1,739,000 in a silver preparation.
Oastanares (1935) studied the dog optic nerve in a larger series,
and secured an average of 147,353 fibers from the silverstained sections.
Putter (1903) made several fiber estimates in the optic
nerve of certain marine mammals and published these results:
Macrorhinus leoninus
Phoca barbata
Phoca vitulina
Odobaenus rosmarus
Otaria jubata
Balaenoptera physalus
Phocaena communis
Delphinapterus leucas
Hyperoodon rostratus
767.000
174.000
147.000
111.000
140.000
157.000
36,100
137.000
77,000
It should be pointed out that these fiber estimates were
determined in a manner similar to Krause1s estimate on the
human optic nerve:
a mathematical proportion was set up
between the cross-sectional area of one nerve-fiber and the
cross-sectional area of the entire nerve.
Although these
estimates may indicate the general range ofthe fiber number
in these marine mammals, it cannot be admitted that they are
tr$te quantitative determinations. As Putter carefully points
i
out, he did not requre absolute fiber counts because the object
of his study was to compare units of fibers to units of the
retinal components.
The question of the presence of unmyelinated fibers in the
optic nerve has been of interest to several workers but few have
made accurate numerical comparisons of osmic- and silver-stained
preparations to actually determine whether such fibers are
present.
Herrick (1893) demonstrated by the use of an osmic
stain that all the fibers in the optic nerve of Hecturus are
unmyelinated.
Palmer (1913) also made the same observant ion
from osmic preparations.
Lander (1937) again carried out the
same sort of procedure on the nerve and confirmed the findings
of Herrick and Palmer.
In a similar study on Amblystoma, Lander
27
found 41 per cent of the fibers unmyelinated,
Howe (in
preparation) determined that 57 per cent of the bullfrog's
optic nerve fibers are unmyelinated.
Norris (1938) noted
that 41 per cent of the turtle's fibers are unmyelinated.
Ho statements have been located in the literature about the
status of the myelination of the bird optic nerve fibers.
Several investigations are available on the degree of
myelination of the mammalian optic nerve fibers.
33 per cent unmyelinated in the opossum.
Horris found
Rochon-Duvigneaud
(1928) stated that all the fibers in the mole's optic nerve
are unmyelinated but cited no experimental evidence to prove
this statement.
Schaible (1934) found 18 per cent of the
fibers in the rabbit optic nerve unmyelinated but he qualified
this figure by stating that he questioned the accuracy of his
estimates made on the osmic acid preparations.
Schaible found
no unmyelinated fibers in the dog and human optic nerves.
The enumerations of fibers of the optio nerve of vertebrates
discussed in the preceding pages are summarised in tabular form
in table I.
TABLE I
SUMMARY OF ENUMERATIONS OF FIBERS OF THE
OPTIO NERVE OF VERTEBRATES
Worker
Year
Adrian and
Matthews
1927
Conger eel
Charlton
1933
Blind fishes
Herrick
1893
Hecturus maculosus
100
Palmer
1913
Hecturus maculosus
l,982(at eye) 100
962(chiasraa)
Lander
1937
Hecturus maculosus
Form Studied
Humber of
Fibers
io of
Unmyelination
10,000
13
363
100
28.
TABLE I (cont *d)
Worker
Year
Form Studied
Lander
1937
Arablystoraa
Lauber
1902
Oryptobranchus
japonicus
Howe
1937
Rana catesbiana
Horris
1938
Norris
1938
Humber of
Fibers
$ of
Unmyelination
2,004
41
450
30,368
57
Turtle
105,040
41
Opossum
82,104
33
1928
RochonDuvigneaud
Mole
Schaible
1934
Dog
139,000
Castanares 1935
Dog
147,852
Schaible
1934
Rabbit
261,000
Putter
1902
Macrorhinus leoninus
Phoca barbata
Phoca vitulina
Odobaenus rosmarus
Otaria jubata
Balaenoptera physalus
Phocaena communis
Delphinapterus leucas
Hyperoodon rostratus
767.000
174.000
147.000
111.000
140.000
157.000
36,100
137.000
77,000
Schaible
1934
Macaque monkey
1,739,000
Krause
1876
Man
1 ,000,000
Kuhnt
1879
Man
40,000
Zalzer
1880
Man
437,645
Krause
1880
Man
440,347
880,000 (?)
Zwanenburg 1915
Man
550,000
Schaible
1934
Man
1,253,750
733,333
Biclcel
1935
Man
1,234,100
1,208,362
100
0
18
0
0
29.
III.
A.
ANATOMY OF THE OPTIO NERVE
Adult Structure
The optic nerve is a part of the brain;
it must be con­
sidered as one of the nerve fiber tracts within the central
nervous system.
in:
It differs from the other cerebrospinal nerves
1) that it lacks the highly specialized reticular connective
tissue framework characteristic of these nerves, and 2) it
pos­
sesses the supporting tissues characteristic of the brain and
spinal cord.
1. Sheaths and vessels
a. Dura mater:
The outer covering of the optic nerve, the dura mater, is
continuous posteriorly with the dura of the brain and anteriorly
with the outer layers of the sclera of the eyeball.
The optic
nerve receives its dural sheath in the optic canal and, at its
anterior exit, it splits into two layers:
the outer goes to
form the peri-orbita, andthe inner continues forward as the exthe
ternal covering of the nerve. The dura isAthickest and toughest
covering of the optic nerve.
Its structural components consist
chiefly of connective tissue, made up of many parallel collag­
enous fibers with an abundance of elastic fibers which follow
the collagenous fibers in a general way;
an endothelium which
forms the thin layer lining the inner side;
numerous nerves
from the ciliary ganglion and the ciliary system of nerves;
and bloodvessels from the ophthalmic artery and vein, small twigs
of which run in a longitudinal network which lies principally on
the surface.
b. Arachnoid:
The arachnoidal sheath represents the middle covering of the
30.
nerve.
This sheath is continuous with the arachnoid of the
brain and it too is inserted anteriorly into the sclera of the
eyeball.
In the orbital portion of the nerve, the arachnoid
is more closely related to the dura than to the underlying pia.
The membrane consists of these layers:
an endothelial covering
on the side next to the dura, beneath this there is a delicate
layer of connective tissue, and inside this is the inner layer
of endothelium just like the outer one.
The subarachnoid trab­
eculae arise from the connective tissue layer of the arachnoid
and from the thick trabeculae of the dural side which penetrate
the arachnoid.
The trabeculae form an actual trabecular net
corresponding to the subarachnoid tissue of the brain and spinal
cord placed between the arachnoid sheath and the pia.
The net
joins the pia, some of the trabeculae ending in terminal plate­
like structures while others sink deeply into the pia.
The arach­
noid is practically devoid of capillaries.
c. Pia mater:
The pial sheath intimately surrounds the optic nerve;
it
is continuous with the same layer of the brain and, as in the
case of the other layers, is inserted anteriorly into the sclera
of the eyeball.
It is made up of three layers of elastic and
collagenous fibers;
the outer is circular, the next mainly
oblique, and the innermost is made up chiefly of longitudinal
fibers.
On the outside of the circular layer there is a covering
of endothelial cells which is continuous with those lining the
arachnoid trabeculae and the inner side of the arachnoid membrane.
Septa sink into the nerve from the inner side of the pial sheath
to form the septal system.
The abundance and form of this septal
system varies widely within the vertebrate group (vide results,
histological descriptions).
The pia is fairly rich in hi0o<i
31
vessels which are derived from the ophthalmic artery and vein*
3. Interstitial cells
The interstitial cells of theoptic nerve may be classified
into two main groups:
neuroglia and microglia.
As would be
expected, since the optic nerve is really a tract comparable to
conducting pathways in the white matter of the brain, these
interstitial cells are the same as those in thewhite matter of
the brain.
a* Neuroglia;
The neuroglia can be subdivided into astrocytes and oligo­
dendrocytes.
The fibrous astrocytes have the samehistologic
characteristics in the optic nerve as those in the white matter
of the brain*
Their arrangement changes only to meet the
structural alterations in the nerve caused by the connective
tissue septal system.
There is an increased number of fibrous
astrocytes in the lamina choriodalis in comparison with the
number in the lamina scleralis and retinal level of the nerve.
The oligodendroglia were notdemonstrated in the optic nerve
until 1936 (Lopez Enriquez), but now they can be readily seen
by the use of the proper staining technique.
They have never
been demonstrated in the retinal level of the optic nerve where
the fibers are all normally unmyelinated;
this suggests a
possible function for oligodendroglia which will be discussed
later in connection with myelin.
The oligodendroglia are group­
ed in rows or cell columns between the nerve fibers, and their
projections follow along parallel to the direction of the nerve
fibers.
Numerically considered, Marchesani (1936) estimated
that they compose two-thirds of the total number of the inter­
stitial cells of the optic nerve.
32.
b* Microglia;
The microglia of the optic nerve, as in the brain, is of
mesodermal origin and it is the only one of the interstitial
cells which gives rise to phagocytes*
microglia cells are few in number*
Numerically considered,
They appear irregularly
distributed within the nerve bundles*
No special differences
have been found between microglia in the optic tract, chiasma
dr nerve*
These cells have never been demonstrated in the
optic nerve-head except under pathological conditions (Gone
and MacMillan, 1933)*
3. Nerve fibers
a* Origin;
Numerous experiments (Cajal, 1911;
Mann, 1928;
Brouwer
and Zeeman, 1926) have conclusively shown that the bulk of the
fibers making up the optic nerve are derived from the ganglion
cells of the ganglionic layer of the retina.
Since these cells
represent the third neuron in the peripheral visual system, the
optic nerve contains the axons which link the retina to the
specialized visual structures of thecentral nervous system.
There is a possibility that a small number of the fibers are
efferent in function (Arey, 1916b) and thus would not originate
in the retina*
This is true of fishes;
whether it occurs in
other forms is not clear at present.
b* Structures
Structurally the nerve fibers are the same sort as those
found in thewhite matter of the central nervous system.
The
fibers are predominately very fine in most vertebrates with a
few coarse ones interspersed between (vide results, histological
descriptions).
The fibers possess no neurilemma sheaths.
The
33
myelin sheaths are small andthin;
neither incisura of Schmidt-
Lanterraaim nor nodes of Ranvier have ever been demonstrated
(Cone and MacMillan, 1933),
Work by Hortega (1928) on myelinated
fibers in the central nervous system shows that the myelin
sheaths are segmented and the clefts causing this segmentation
are formed by protoplasmic projections from oligodendrocytes#
Whether this finding also applies to the myelin sheaths of the
optic nerve fibers is not known.
The connective tissue septal
system is associated with a grouping of the nerve fibers into
parallel bundles.
In the human herve, the number of these
bundles has been placed at from eight hundred to twelve hundred
(Greeff, 1900),
Such bundles of nerve fibers are absent in forms
which possess no septal system:
some elasmobranchs and teleost-
eans (Duke-Elder, 1933).
B.
Development of the Optic Uerve
The opticnerve developes in the substance of the optic stalk.
However, the cells of the stalk itself form only the neuroglial
supporting tissue of the optic nerve because the actual nerve
fibers develop from the ganglion cells of the retina.
The nerve
sheaths and the connective tissue about thecentral vessels, and
the septal system connecting these, arise from surrounding meso­
derm, some of which condenses on the outside of the stalk, while
some accompanies the vessels when the stalk becomes invaginated.
1. Optic stalk
The optic stalk is recognizable from thetime an optic vesicle
is present.
At first it is circular in section, relatively very
short and thick, and having a lumen continuous with the cavity
of the fore-brain proximally and thecavity of the optic vesicle
distally.
These relations are well shown in the 4 millimeter
34*
human embryo.
Invagination begins at the optic cup and continues
along the under sufface of the stalk throughout its entire
length.
brain.
This depression fades away on the under surface of the
As the stalk lengthens, the depressed area (choroid
fissure) deppens in its distal part.
In a 7.5 millimeter human
embryo the stalk is cresentic on section near the cup and circular
near the brain.
The deepened invagination is occupied by a blood
vessel which eventually becomes theintraneural portion of the
central artery of the retina.
By 13 millimeters, the lumen of
the optic stalk has become very small and the stalk itself very
thin.
2. Nerve fibers
The cells of the stalk, commencing in that portion which is
continuous with the inner layer of the ojbtic cup, become somewhat
irregularly arranged and vacuolated when the nerve fibers appear
in the retina.
Then the axis cylinders from the retina grow in
among these vacuolated cells, which disappear as the nerve fibers
increase in number and replace them.
pear in a 17 millilmeter human embryo.
These changes begin to ap­
A slight condensation in
the surrounding mesoderm is appearing at this time as the pre­
cursor of the nerve sheath.
At this stage the nerve fibers extend
just up to the brain but cannot be recognized with certainty in
the chiasma.
The nerve fibers almost completely fill the stalk
by 19 millimeters, although a small portion of the lumen is still
visible.
By 25 millimeters, the stalk is completely filled with
nerve fibers and no trace of lumen can be seen.
Most of the cells
of the original stalk can be seen as glial cells arranged in
longitudinal rows between the bundles of nerve fibers.
The nerve
increases from now onwards at the same rate as the retina and
35
closely resembles the adult nerve except that the fibers are
unmyelinated (Mann, 1928).
3,
Myelination of the fibers
The exact time of the myelination of the fibers is a matter
of some difference of opinion, some authors giving it as before
birth in the human, whereas others time its onset at the ninth
month only.
It is possible that individual variations may occur
in this since the amount of general development at birth is not
absolutely constant.
investigation.
Oertainly it is a point which needs further
Sattler (1915) found that the myelination com­
menced in the central nervous system and spread peripherally
along thenerve fibers.
There was none at 5 months, at about 7
months the myelin sheaths could be traced from the optic tract
to the beginning of the optic nerve, and by term these had reach­
ed the lamina cribosa.
Myelin appears first as minute droplets
of lecithin-containing substance in the protoplasm of the glial
cells.
These droplets then run together and surround the axis
cylinders.
The sheaths are at first very thin, and as develop­
ment proceeds they thicken proximally at first, so that at birth
the myelin sheaths in the optic tract are thicker than those in
the nerve behind the eyeball, whereas in the adult they are the
same thickness.
4. Ohiasma
The chiasma is recognized earliest as that portion of the
floor of the fore-brain which lies between the insertions of the
optic stalks.
It appears as a smooth convexity lying in front
of the region where Rathke*s pouch comes in contact with the
floor of the fore-brain.
As development proceeds, this hollow
convex region projects more and forms a definite ridge uniting
the two optio stalks*
Their lumina open into its cavity, which
can now he called the optic recess*
This recess deepens and
its wall undergoes differentiation in common with the rest of
the wall of the fore-brain*
At 13 millimeters distinct ependy­
mal, mantle and marginal zones can be recognized.
Soon after
this the marginal layer begins to get filled up with nerve
fibers owing to growth into it of fibers from the lower surface
of the optic stalk.
By 25 millimeters the nerve fibers are
well developed and cover the surface between the two optic
stalks, so that a true chiasma may now be spoken of.
The fibers,
after partially decussating in thechiasma, now pass backwards
around the base of the pituitary body, and a definite convexity
can be recognized in the situation of the optic tract*
This
process continues until by 48 millimeters the appearance is that
seen in the adult.
C.
Distribution of Fibers in Nerve and Tract
1* Decussation of fibers
The decussation of the fibers in lower vertebrates is a
complete one.
intact nerves;
In fishes there is a simple crossing of the two
an interesting exception is the herring, in which
one nerve buttonholes through the other.
In amphibians and
reptiles the decussation is a complete one but each nerve breaks
up into large fasciculi which interdigitate with one another
(Gross, 1903).
This same sort of arrangement persists in birds
but in a more intricate form.
In all mammals, except marsupials
and monotremes, a certain number of optic nerve fibers do not
cross in the chiasma (Duke-Elder, 1933).
This arrangement is
believed to be necessary in mammals tolay the foundation for the
possibility of binocular vision, and as such it is a purely
37.
mammalian characteristic*
The crossed fibers, instead of
remaining in widely separated fasciculi as in other vertebrates,
become intertwined and alw&ys remain the more numerous.
This
rearrangement of the fibers at the chiasma must have been a
very gradual development for, as one proceeds from the lower to
the higher mammals, the number of crossing fibers becomes fewer
and fewer.
As previously mentioned, crossing is complete in
monotremes andmarsupials and is nearly so in rodents (Le Gros
Clark, 1931;
Brouwer and Zeeman, 1936).
In the cat, Barris
(1934) observed that the number of crossed fibers is greater
than the uncrossed as was observed by several other investigators
(Ganser, 1883;
Gudden, 1879;
Brouwer and Zeeman, 1936).
In
the monkey the number of crossed and uncrossed fibers is about
equal (Brouwer and Zeeman, 1936).
A few definite bifurcating fibers, one fiber going to one
hemisphere and one to the other, have been found at the chiasma
in higher vertebrates.
None have ever been found in lower verte­
brates where the crossing is complete.
Cajal (1911), in describ­
ing these fibers which bifurcate, concluded that they are efferent
fibers subserving reflex pupillary action and reflex eye move­
ments.
Such a conclusion disagreed with an earlier hypothesis
of Henschen that these are macular fibers.
According to Cone
and MacMillan (1933), work by Putnam (1936) and others showing
no connections of the external geniculate bodies of one side
with the contralateral visual cortex must lead one to seriously
consider the possibility that these branching fibers are macule^r
fibers.
3.
Retinal localization in nerve and tract
Many investigators have worked on the problem of retinal
38
localization of fibers in the optic nerve and tract but the
most valuable contribution was made by Brouwer and Zeeman (1936).
They studied the secondary degenerations occurring after exper­
imentally producing retinal lesions in monkeys.
In the optic nerve there is a certain degree of localization
these workers found, of the peripheral quadrants of the retina*
the fibers from the upper half of the retina are situated above
those from the lower and, further, the temporal fibers lie later­
al and the nasal medial.
The localization of the macular fibers
is more difficult for, although their number is large, they
apparently shift positions during the course of the nerve.
Hear
the eye they are situated laterally in the optic nerve, but to­
ward the chiasma they show a tendency to assume a central locat­
ion.
It is possible that these macular fibers separate those
from the temporal upper and lower quadrants of the peripheral
retina from one another.
Since some of the macular fibers are
found between fibers from the various quadrants of the retina,
it is certain that there is not a very exact localization of the
macula in the optic nerve.
In the neighborhood of the chiasma,
experimental results indicate that the fibers from the upper
half of the macula lie above those from the lower half.
The relations of the fibers in the chiasma, according to
Brouwer and Zeeman, are these:
fibers from the upper part of
the retina generally cross dorsally in the chiasma, and those
from the lower half ventrally*
Immediately before the formation
of the optic tract, the fibers from the dorsal quadrants reach
the ventral half of the chiasma.
The fibers from the upper
nasal quadrants cross somewhat later then do those from the
lower quadrants.
Brouwer and Zeeman state that localization of the fibers
in the optic tracts follows these principles:
the fibers from
the upper quadrant of the retina lie dorsally, those from the
lower ventrally and there is no overlap between these;
the
macular fibers are situated centrally and then gradually bea
come larger in a l^eral direction. It appears that the medioventral portion of the macular fibers overlap the fibers from
the dorsal and ventral parts of the peripheral retina.
The
projection is approximately the same in both crossed and uncfossed tracts.
40.
IV.
A.
MATERIALS AND METHODS
Materials
The selection of the material was governed by two factors:
a desire to secure representatives of each of the orders of the
various vertebrate classes, and the availability of the specimens.
In an extensive survey research of this sort, it is naturally
impossible to examine each species in detail due to limitations
in time and material.
And such a detailed research is probably
not necessary since there is a tendency toward uniformity within
each of the orders of vertebrates, and even among classes in many
instances, so that a few representatives of eaeh show the trend
with considerable exactitude.
In some instances, important
orders have not been studied because of the difficulty in secur­
ing specimens.
Hence, the material used in this study has been
confined largely to those specimens which can be rather easily
secured in this portion of the country.
1. Sources of the material
The source of the material used in this study is very varied.
The Hopkins Marine Laboratory of Stanford University located at
Pacific Grove, California, collected the hagfish specimens from
Monterey Bay.
The brook lampreys were secured during their
breeding period in May from a private collector in Michigan.
The Shedd Aquarium supplied many moribund fishes:
dogfish shark
pups, stingrays, guitar fish, bowfins and hacklebacks.
The bull­
heads were secured from Wisconsin through a local biological
supply house.
The goldfish specimens were purchased from the
local pet shops, and the perches were secured from fishermen
who caught them in Lake Michigan.
The frog material was secured from the physiology laboratory
here at school.
41
The baby alligators were secured from the Lincoln Park
Zoo and the horned toads were collected by the author in
Southern California.
The bird material was secured through purchasing specimens
from local poultry and pet shops#
The bats were collected by an amateur naturalist in
Chicago.
The dog, cat, rabbit, guinea-pig, rat and monkey
specimens were all secured in the medical school from various
departments which happened to be using these common experimental
animals at the time.
at the stockyards.
The pig and sheep nerves were obtained
The human material was all secured from
the autopsy room at Passavant Memorial Hospital through the
cooperation of Dr. Queen.
The material used in this study
is summarized and
classified in Table IX.
TABLE II
SUMMARY AND CLASSIFICATION OF MATERIAL
Class
Cyclo stomata
Elasmobranchii
Order
Family
Hyperotreta
Eptatretidae
Hyperoartia
Petromyzonidae
Selachii
Squalidae
Selachii
Rhinoba/bidae
Selachii
Dasyatidae
Species
Poliostotrema
stouti
Entosphenus
lamottenii
Squalis
acanthias
Rhinobatus
productus
Dasyatis
hastatus
Common
Name
hagfish
brook
lamprey
dogfish
shark
guitar
fish
stingray
Pisces
Ganoidei
Glaniostorai
Halecomorphi
Acipenseridae Scaphirhynchus hacklepla.torynchus
back
Amiidae
Amia calva
bowfin
43
TABLE II (cont!d)
Class
Order
Family
Species
Common
Name
Pisces
Teleostei
Amphibia
Eventognathii
Cyprinidae
Nematognathii
Ameiuridae
Salienta
Linguata
Bufonidae
Linguata
Reptilia
Aves
Ranidae
goldfish
bullhead
toad
Bufo
americanus
Rana pipiens frog
Crocidilini
Crocodylidae
alligator
Alligator
mississippiensis
Squamata
Sauria
Iguanidae
Phrynosoma
coronaturm
Anseriformes
Anatidae
Galliformes
Phasianidae
Charadriiformes Columbidae
Passeriformes
Mammalia
Eutheria
Carassins
auratus
Ameiurus
melas
Chiroptera
Carnivora
Fringillidae
Vespertilionidae
Canidae
Felidae
Rodentia
Leporidae
domestic
duck
Gallus
doraestioa
Coluraba
domestica
Fringilla
canaria
Eptesicus
fuscus
Canus
familiaris
Felus
domestica
Lepus
cunicuius
Muridae
Cavia
Artiodaotyla
Suidae
Primates
Bovidae
Anthropoidea
horned
toad
Cavia
aperea,
Sus
domestica
Ovis aries
Macacus
rhesus
Homo
sapiens
chicken
pigeon
canary
brown
bat
dog
cat
rabbit
white, rat
hooded rat
guineapig
pig
sheep
monkey
man
43
3.
Age of the specimens
Whenever it was possible, fully-developed animals were
used as sources for the optic nerves studied.
However, in a
few instances immature forms were unavoidably used:
a* Dogfish shark pups: these were available due to their
birth in the Shedd Aquarium.
Since noother shark material was
available, it was decided to utilize this material.
The ages
of these sharks varied from 10 to 51 days.
b. Baby alligators:
these were used because they were eajsily
available whereas the adults were not.
The ages of the three
specimens used in this series is not known, but their head-totail measurements were 34, 33 and 45 centimeters respectively.
c. Ducklings and chicks:
these were selected for use because
the osmic preparations of the large optic nerves of the adult
forms were not good enough to enumerate the fibers.
It was
hoped that the smaller nerves of these immature forms would
show improved blackening of the myelin sheaths and these hopes
were realized.
The ducklings were z\ weeks old and the chicks
1-g- weeks at the time they were sacrificed.
d. Oat fetus:
this fetus of 64 days (3 days short of term)
was used because it seemed desirable to check the total number
of fibers in an immature form and compare the result with that
of the adults.
3.
Olinical histories of human specimens
Summaries of clinical histories were recorded for each of
the human specimens secured at autopsy.
These histories are not
as complete as would be desirable, however, because fundus exami­
nations and visual acuity tests were rarely done on these patients.
The pertinent points in these cliMcal histories are summa­
rized in Table III.
44
TABLE III
SUMMARY OF THE KNOWN DATA PERTAINING TO THE HUMAN
OPTIC NERVES USED IN THIS INVESTIGATION
Number
Age
Sex
I
19
Male
Chronis nephritis,
with renal rickets
Tumor of maxilla
due to von Reck­
linghausen’s dis­
ease.
Wore glasses
for 3 years
Nystagmus
since birth
Horizontal
nystagmus
Visual acuity:
R-6/30,
L-6/25
Fundus: hard
to see, but
seemed normal
II
66
Male
Bleeding gastric
ulcer
Myopia for
50 years
Arcus senilis
Visual fields
and central
vision normal
to gross tests
III
64
Male
Post-operative
pulmonary embolus
Carcinoma of
stomach
Glasses for
reading
No positive
findings
Cause of Death
and Associated
Pathology
Visual
History
Findings of
Examination of
Visual System
IV
63 Female
Carcinoma of
pylorus with
metastasis to
L. lung and
meninges
Glasses for
reading
Central
vision and
visual fields
normal to
gross tests
V
70 Female
Pancarditis
on rheumatic
basis
Good
vision
No positive
findings
VI
68 Female
Carcinoma of
biliary system
Good
vision
Conjunctiva
icteric
VII
61 Female
Congestive heart
failure
Perinephritic
abscess
Nothing on
No positive
record, but
findings
patient said
to have had
good vision
VIII
71 Female
Fatty degenerat­
ion of myocard­
ium
Bilateral broncho­
pneumonia
Glasses for
reading
Sclera mud­
dy , otherwise neg.
IX
47 Female
Chronic lymphatic
leukemia
Nothing on
record
Nothing on
record
45.
TABLE I I I
Number
(e orvt 'd )
Age
Sex
X
47
Male
Amyloidosis of
Failing vision Jaundiced sclera
liver (syphilis?) (of recent
Pupils react
onset)
sliiiggishly to
light and
ac commodat ion
No visual tests
made
XI
51
Male
Lymphosarcoma of
None
dorsal mesentery
recorded
with extension
into small intest­
ine
Mesenteric thrombosis
Peritonitis
Lobar pneumonia
XII
67
M©,le
Ruptured mucocele,
with peritonitis
Lobar pneumonia
Cause of Death
and Associated
Pathology
Visual
History
Findings of
Examination of
Visual System
None
recorded
Nothing on
Nothing on
record; it
record
is known
that patient
could see
46.
B«
1«
Methods
Dissection of the nerves
The nerves were dissected from the animals within as short
a period following death as was possible.
In most instances,
the animals were obtained alive and, after being billed (usually
by the injection of a lethal dose of nembutal), the nerves
were immediately removed by a careful dissection.
Thus the
elapsed time between the death of the animal and the immersion
of the nerves in the appropriate fixing fluids was, in most
instances, a period of thirty minutes to an hour.
But in some
specimens, a longer time intervened because the dissections
could not be made immediately postmortem:
certain fishes,
which were obtained after they died in the aquarium;
and the
human nerves, which could not be obtained until the bodies
came to autopsy.
In these instances, the time between death
and fixation varied from an hour to fifteen hours.
In only
one of these specimens was there marked evidence of postmortem
degenerative changes:
one shark pup, which had been dead an
estimated eighteen hours at room temperature and showed such
marked changes that the nerve was discarded.
The optic nerves were completely dissected out and removed
with the chiasma on the proximal end and the eyeballs on the
distal ends.
If the nerves were of sufficient length to allow
division into two portions, this was done;
one portion was used
for the osraic acid myelin sheath stain, and the other for the
axis cylinder silver stain.
In some instances, however, the
nerves were too short to permit such a division;
right and left nerves were used.
then both
The left nerve was usually
used for the osmic stain and the right one for the silver stain.
This use of right and left nerves was necessary in these
specimens:
all of the fishes except the guitar fish, all of
the amphibians, all of the reptiles, all of the birds, and
the bat in the mammalian class.
After being dissected and removed from the animal, the
nerves were immersed in a dish containing physiological saline
solution and then any excess of adherent tissue was teased
away.
Next the nerves were severed from the chiasma and the
eyeballs, using a razor in order to secure evenly cut edges.
The two ends of each nerve were tied to the supporting ends
of a bent glass rod by means of a silk thread.
In the process
of tying each nerve to a glass rod, the nerve was stretched
enough to draw it straight but ne^verenough to make it taut.
The nerve with its attached glass rod was then placed in the
appropriate fixing fluid.
3. Fixation
In view of the plan to study the degree of myelination of
the optic nerves, two divergent methods of fixation were em­
ployed:
one which would make possible differential staining
of the myelin sheaths and the otherwhich would be the basis for
a silver stain to demonstrate axis cylinders of both myelinated
and unmyelinated fibers.
a.
Qsmic acid fixation:
Osmic acid was used both as a fixative and a stain for
those optic nerves where differentiation of the myelin sheaths
was desired.
Since the only information available concerning
the technique of using osmic acid for fish optic nerves (the
forms first studied) was that outlined by Adrian and Matthews
(1937) for the conger eel, numerous variations in the procedure
had to be tried before successful staining was secured.
But it
was found that a successful procedure for fishes did not neces­
sarily give good results on the optic nerves of other classes.
Hence, several techniques were devised in order to secure usable
preparations in all the classes studied.
Tfce use of an aqueous
solution of osmic acid of varying concentration and for differ­
ing durations was the principle utilized at the onset.
The
strength of the solution was varied from 0.5 to 3 percent.
A
duration of 34 to 30 hours in 3 per cent solution gave optimum
fixation for the medium sized fish and reptile optic nerves,
whereas 30 to 48 hours in 1 percent solution gave the best
results for the smaller nerves of these classes.
In attacking
the problem of securing complete blackening of the myelin sheaths
of fibers in large nerves, the well-known fact that osmic acid
vapor has greater penetrating power than the solution was util­
ized (Lee, 1937;
McClung, 1937;
Guyer, 1930).
The strength of
the osmic acid solution used for the production of the vapor
was varied from 3 to 4 per cent.
The nerves were suspended by
means of a glass rod over the solution of osmic acid and then
the lid of the container was replaced tightly to prevent escape
of the vapor.
The duration of exposure of the nerve to the
vapor was usually 34 to 30 hours.
On numerous occasions
portions of large nerves (e.g., human) have been treated by
both the immersion and vapor methods and in every instance the
vapor procedure gave a much superior blackening of the myelin
sheaths.
Since the demonstration of its superiority, the vapor
method has been used on all the nerves studied with successful
results in most instances.
Birds, as a class, have been diffi­
cult to fix and stain with osmic acid, possibly because of the
49
extremely small size of the fibers.
b. Fixation for the silver techniques:
The fixatives used on those nerves destined to be stained
with a silver procedure have also varied as the problem prog­
ressed.
The early fish material was fixed in Hofker*s solution
(1921) (glacial acetic acid, trichloracetic acid, and absolute
alcohol) with the exception of one bullhead which was fixed in
Bouinfs (1897) solution (picric acid, glacial acetic acid, and
formalin).
All the later material was fixed in the n. butyl-
propyl alcohol fixative recently described by Davenport and
Kline (1938):
formic acid, trichloracetic acid, n. butyl
alcohol, and n. propyl alcohol.
c. Data on fixation of the material:
Table XV is a summary of the data on the fixation of the
optic nerves used in this study.
TABLE IV
FIXATION OF MATERIAL
Form
Hagfish
#1
Side of
Animal
Fixative
Duration
(hours)
R
L
Butyl-propyl
2$ osmic vapor
31
31
R
L
Butyl-propyl
2 osmic vapor
31
31
R
L
Butyl-propyl
3$ osmic vapor
13
25
#3
R
L
Butyl-propyl
3$ osmic vapor
11
24
#3
R
L
Butyl-propyl
3$ osmic solution
11
36
#4
R
L
Butyl-propyl
2% osmic solution
13
25
#3
Brook lamprey
' #1
50.
TABLE IV (oont'd)
Form
Shark pup
#1
Side of
Animal
Fixative
Duration
(hours)
R
L
Butyl-propyl
2fo osmic solution
34
34
#3
R
L
Butyl-propyl
2$ osmic solution
24
24
#3
R
L
Hofker*s solution
3$ osmic solution
34
24
#4
R
L
Butyl-propyl
3*fo osmic solution
24
39
R
L
Butyl-propyl
2fo osmic solution
34
24
#3
R
L
Butyl-propyl
osmic solution
24
24
#3
R
L
HofkerTs solution
2^o osmic solution
24
39
Stingray
Guitar fish
#1
R
R
Prox.i
Dist.J
Hofker’s solution
itfo osmic solution
48
71
L
L
Prox.i
Dist.2
Butyl-propyl
2$ osmic solution
24
34
R
L
Butyl-propyl
3$ osmic vapor
5
28
#3
R
L
Butyl-propyl
3$ osmic vapor
5
28
#3
R
L
Butyl-propyl
2fo osmic vapor
15
24
R
L
Butyl-propyl
2fo osmic vapor
13
25
R
L
Butyl-propyl
2<
fo osmic vapor
12
24
#3
Hacklehack
#1
Bowfin
#1
#3
51.
TABLE IV (cont'd)
Form
Goldfish
#1
Side of
Animal
Fixative
Duration
(hours)
R
L
Hofker's solution
1$ osmic solution
37
48
R
L
Butyl-propyl
1<f> osmic solution
31
48
R
L
Hofker*s solution
0#5$> osmic solution
48
48
#3
R
L
Bouinfs solution
1<fo osmic solution
46
34
#3
R
L
Butyl-propyl
ifo osmic solution
39
39
#4
R
L
Butyl-propyl
l*fo osmic solution
34
48
R
L
Butyl-propyl
3$ osmic solution
10
36
R
L
Butyl-propyl
2$> osmic solution
10
30
#1
R
L
Butyl-propyl
2fo osmic vapor
33
34
#3
R
L
Butyl-propyl
2$ osmic vapor
31
23
R
L
Butyl-propyl
2$ osmic vapor
13
35
#3
R
L
Butyl-propyl
3$ osmic vapor
8
30
#3
R
L
Butyl-propyl
2$ osmic vapor
8
39
R
L
Butyl-propyl
4$ osmic vapor
10
36
#2
R
L
Butyl-propyl
4jo osmic vapor
13
35
#3
R
L
Butyl-propyl
4fo osmic vapor
13
34-
#2
Bullhead
#i
Toad
#1
#3
Frog
Alligator
#1
Duckling
#1
53.
TABLE IV (cont’d)
Form
Side of
Animal
Fixative
Duration
(hours)
Chick
#i
R
L
Butyl-propyl
4$ osmic vapor
16
34
#2
R
L
Butyl-propyl
4$ osmic vapor
12
34
#3
R
L
Butyl-propyl
4$ osmic vapor
10
24
R
L
Butyl-propyl
3$ osmic vapor
12
30
#3
R
L
Butyl-propyl
2$ osmic vapor
10
26
#3
R
L
Butyl-propyl
4$ osmic vapor
13
24
#4
R
L
Butyl-propyl
4$ osmic vapor
13
24
R
L
Butyl-propyl
4$ osmic vapor
8
30
#3
R
L
Butyl-propyl
4$ osmic vapor
8
30
#1
R
L
Butyl-propyl
2$> osmic vapor
10
24
#2
R
L
Butyl-propyl
3$ osmic vapor
16
36
#3
R
L
Butyl-propyl
2$ osmic vapor
30
24
#4
R
L
Butyl-propyl
3$ osmic vapor
18
33
#1
R
R
Butyl-propyl
3$ osmic vapor
12
30
#2
L
L
Butyl-propyl
3$ osmic vapor
17
30
#3
R
R
Butyl-propyl
2$> osmic vapor
30
29
#4
R
Butyl-propyl
19
Pigeon
#1
Canary
#1
Bat
Dog
TABLE IV (coat'd)
Form
Side of
Animal
Fixative
Duration
(hours)
Oat
#1
R
R
Butyl-propyl
3$ osmic vapor
15
30
#3
R
R
Butyl-propyl
3$ osmic vapor
31
36
#3 (fetus)
R
Butyl-propyl
R
R
Butyl-propyl
3$ osmic vapor
11
39
L
L
Butyl-propyl
3$ osmic vapor
10
28
Rabbit
#1
#3
Rat (white)
#1
8
R
R
Prox. i
Dist. J
Butyl-propyl
3fo osmic vapor
6
24
R
R
Prox* i
Dist. t
Butyl-propyl
3$ osmic vapor
12
24
R
R
Prox. 15Diet. \
Butyl-propyl
4$ osmic vapor
9
36
R
R
Prox. i
Dist. J
Butyl-propyl
4$ osmic vapor
9
26
R
R
Butyl-propyl
3$ osmic vapor
33
26
$*3
R
R
Butyl-propyl
3$ osmic vapor
10
26
#3
R
R
Butyl-propyl
2$ osmic vapor
10
24
Butyl-propyl
4$ osmic vapor
9
30
Butyl-propyl
4$ osmic vapor
9
30
Butyl-propyl
4$ osmic vapor
9
30
#3
Rat (hooded)
#3
Guinea pig
#1
Pig
#1
#3
*
Unknown
ii
(same)
»
ii
#3
ii
#4
ii
ii
ii
ii
Butyl-propyl
9
54.
TABLE IV (cont!d)
Form
Sheep
#1
#3
Side of
Animal
Unknown
H
(same)
8
39
ii
Butyl-propyl
4$ osmic vapor
8
29
ii
Butyl-propyl
4<fo osmic vapor
8
28
ii
ii
Duration
(hours)
Butyl-propyl
4$> osmic vapor
ii
ii
#3
Fixative
ii
Butyl-propyl
R
R
Butyl-propyl
3<j) osmicvapor
Not known
37
#4
R
R
Butyl-propyl
2 osmic vapor
Not known
27
#5
R
R
Butyl-propyl
3$ osmic vapor
Not known
36
#4
Monkey
#3
Man
8
Butyl-propyl
2<f0 osmic vapor
9
30
Butyl-propyl
2fo osmic vapor
10
26
R
R
Prox. 1
Dist. S
2
Dist. X
Prox. 1
2
Dist. ?1
Prox. 2
Butyl-propyl
2$ osmic vapor
19
32
#4
R
R
Dist. x
Prox. 5
2
Butyl-propyl
2$ osmic vapor
10
38
#5
R
R
Butyl-propyl
2$ osmic vapor
24
34
# 6
R
R
Butyl-propyl
3$ osmic vapor
16
43
#7
R
R
Butyl-propyl
3$ osmic vapor
10
37
#8
R
R
Dist. $
Prox.
Dist. ?1
Prox. 2
1
Dist. 1
Prox. 2
1
Dist. 1
Prox. 2
#9
#10
#1
R
R
#3
R
R
#3
#11
#13
Butyl-propyl
3$ osmic vapor
Not known
30
R
Butyl-propyl
Not known
R
Butyl-propyl
Not known
R
R
Butyl-propyl
12
Butyl-propyl
12
55#
Following fixation, the nerves were washed;
those fixed
with osmic acid were washed in water, and those fixed in the
butyl-propyl mixture were washed in 70 per cent ethyl alcohol#
After washing, the material was dehydrated in a graded series
of ethyl alcohol running up to 95 per cent, then passed into
n. butyl alcohol, and finally embedded in paraffin.
3. Staining
a# Sectioning of the osmic-stained material;
The material was sectioned in the standard manner with no
attempt being made to obtain serial sections.
The sections of
the osmic-blackened nerves were cut 3 to 4 micra in thickness
and mounted on clean slides.
Hotplates with temperatures regu­
lated between 40 and 50 degrees Centigrade were used in the
process of floating and spreading the sections on the slides.
Just as soon as the sections were adequately spread, as checked
by examining them under the microscope with low power, they
were taken off the hotplate and left at room temperature to dry.
This maneuver was found necessary to avoid the rapid bleaching
of the myelin rings due to the effect of the heat.
After dry­
ing was complete (usually twenty-four hours was allowed), the
slides were placed in xylol to dissolve away the paraffin and
then cleared with balsam and a cover slip placed over the
sections.
b. Silver staining:
Sections of optic nerves fixed in anticipation of silver
staining were cut 5 to 6 micra thick and were placed on slides
by means ofthe usual technique.
These sections were then stain­
ed by a silver technique, the type of technique depending upon
the stage of the research.
The initial plan was to stain them
by means of the Davenport silver nitrate technique (1930),
but this gave such uncertain results on the fish nerves that
other techniques had. to be resorted to.
Bodian*s protargol
technique (1936) was next utilized, but this gave undependa­
ble results in most instances.
This unreliability of the
silver tfechniques when used on the optic nerve led to con­
siderable experimentation with new procedures or modifications
of old ones.
Davenportfs modification of Bodian's protargol
technique (1938) was used for a time and this method gave a
much greater percentage of successful preparations than Bodian*s
original technique.
Finally, a combination of the silver nitrate
and protargol techniques was developed (Davenport, McArthur,
Bruesch, 1939).
This rapid silver technique gave the most con­
sistently successful preparations of all the procedures tried
and it was used on all the material except some of the earlier
sections of fish optic nerves where the work of staining and
enumerating had already been completed.
4. Fiber estimates
a. Criteria for selecting sections of nerves for counting:
Many sections were carefully examined under the microscope
before a representative one was selected for counting.
criteria were used in making the selection:
Certain
the section had to
be cut exactly transversely so that all the fibers would show
up as circular dots in the silver stains or as round tubes in
the osmic-myelin stains, it had to be flat on the slide, and it
had to be free from distortion and tearing.
In most instances
these criteria for a countable section were met, but in a few,
due to the small size of the nerve and thereby the fewness of
of the sections, some concessions were made;
never enough,
57.
however, to materially add to the error of the fiber estimate.
b.
Counting procedure:
l).
strip method
Equipment:
The counting of the fibers stained by both osmic and silver
stains was done by the strip method described by Davenport and
Barnes (1935).
A research type binocular microscope with appo-
chromatic lenses was used in making the fiber enumerations.
The
oil immersion objective and the 10X oculars were used, providing
a magnification of 900 diameters.
This magnification proved
adequate, except in the instance of certain of the bird optic
nerves, to permit the detection of even the smallest fibers in
those forms having very small fibers.
This magnification was
used for the fiber enumerations of the bird nerves but it was
very difficult in some (e.g., canary) to identify each of the
smallest fibers.
The special equipment necessary for the strip method of
enumeration consists of a strip provided by cementing two smooth
edges of cover slip gl&ss to the diaphragm of the ocular with
beeswax.
The edges of the cover slips then appear in the field
as two parallel lines.
The width between the lines was made
small enough to enable the enumerator to count the fibers in the
stiip without losing his place.
So naturally, the smaller the
fibers, the narrower the space between the parallel lines must
be.
After reaching the proper adjustment for width, the strip
was carefully fixed in a position by wedging the ocular so that
the movement of the mechanical stage exactly paralleled it.
3).
Procedure:
The first procedure in making an actual estimation was to
determine the total number of strips in the cross-sectional area
53
of the nerve.
This was done Toy setting the lower of the two
parallel lines just at the lowest edge of the nerve where the
fibers begin to appear.
Then by means of a vertical movement
of the mechanical stage the lower line was moved to the place
previously occupied by the upper line.
Landmarks had to be
noted before each vertical movement in order to make the re­
placement of the lines accurate.
These landmarks were most
often unusually shaped fibers, large fibers, connective tissue
septa, neuroglial cells, et cetera.
Each vertical movement
was counted and the final count taken as the total number of
strips in the cross-section of that particular section of an
optic nerve.
With this procedure completed, the actual count­
ing of the fibers was carried out.
Occasionally this procedure
was carried out again after the enumeration of the fibers had
been completed in order to check on the total number of strips.
3).
Percentage of fibers actually counted:
The percentage of the total fibers actually counted varied
from 4 to 100 percent.
The tendency was to count a higher per­
centage in the smaller nerves because this could be done without
materially adding to the counting time.
An example of a complete
fiber count is the brook lamprey nerve.
In the majority of
species, however, estimates rather than total counts were made.
These estimates were made by counting a certain number of strips
and then making an estimate based on these partial counts.
The
first strip at the lowest edge of the nerve section was arbi­
trarily counted in every case and then every 5th, 7th, 10th,
15th, 30th or 35th strip, depending upon the sample frequency
decided upon.
The decision as to the sample frequency was based
on the size of the nerve, the regularity of the distribution of
the fibers, and the smoothness of the contour of the nerve#
A large, round nerve with very few or small connective tissue
septa and a regular distribution of the fibers can be estimated
quite accurately with the actual counting of only a few strips.
On the other hand, a highly irregularly shaped nerve with many
large connective tissue septa would require the actual count­
ing of a high percentage of its fiber content to insure an
accurate estimate#
4).
Calculation of the total number of fibers:
When the figure for the actual number of fibers counted in
a certain number of strips was secured, the following simple
proportion established the calculated total:
Number of strips counted
Total number of strips
5).
_
—
Number of fibers counted
Total number of fibers
Precautions against error:
The possibility of counting supporting neuroglial fibers
in the silver-stained preparations was minimized because the
improved silver technique stained such elements very poorly, if
at all.
As a further precaution to avoid this possibility of
error, every doubtful fiber was carefully examined by focussing
up
and down its length in the section.
be
followed completely through the thickness
was not counted.
If the fiber couid not
ofthe section, it
No silver precipitates were present in the
preparations stained with the improved rapid technique, so there
was no possibility of confusing these with fibers.
The sharp
contrast between the dark purple to black color of the axis
cylinders and the pale neuroglial and connective tissue fibers
made the distinguishing of the nerve fibers relatively easy in
the silver-stained preparations.
Greater difficulty was
experienced in distinguishing the osmic-blackened myelin
sheaths "because their "borders were often pressed so closely
together that they could not "be clearly outlined even by
careful focussing.
This condition was occasionally accentu­
ated in the central portions of the nerves where the blacken­
ing of the myelin sheaths was not as complete as at the
periphery.
In most cases, however, it was felt that the
individual fibers could be made out reasonably accurately
by careful focussing and comparison with neighboring ones.
t
61.
V.
RESULTS
A.
Microscopic descriptions of the nerves
1. Fishes
HAGrFISH:
The nerve is small, averaging 74.3 micra in its smallest
diameter and 76.1 micra in its largest.
As these dimensions
indicate, the nerve is nearly round in shape.
The average
calculated area of the cross-section is 0.00415 square
millimeters.*
There are no connective tissue septa, although delicate
glial fibers can be seen separating the nerve fibers.
The nerve fibers vary greatly in size, some being sur­
prisingly large and others so small as to make individual
identification difficult.
The fibers are evenly distributed
throughout the nerve.
Neuroglial cellular elements are difficult to identify.
BROOK LAKPREY:
This nerve averages 88.B micra in its largest diameter
and 83.6 in its smallest.
The average calculated area of the
cross-section is 0.00591 square millimeters.
The b®ook lamprey nerve contains an axial core of large,
pale cells.
Their nuclei stain well with hematoxylin and
cresyl violet, showing masses of chromatin but no nucleoli.
* These dimensions were determined bymeasuring the silver-stained
sections used for making the total fiber estimates with an ocular
micrometer. They are averages of as many nerves as were included
in a series. The areas are slightly inaccurate in that they are
calculated on the assumption that these nerves have smoothly cur­
ved contours. The areas are total, i.e., they include connective
tissue septa, blood vessels, et cetera,that are contained within
the nerve.
62.
These cellular elements rarely extend out from the core to
mix with the fibers.
The core averages 26.6 micra in diameter,
or has an average cross-sectional area of 0,00056 square
millimeters.
r
The fibers, of varying sizes, are peripherally ar^nged
about the central core of cells.
and are not interrupted by septa.
They are evenly distributed
Fibrous tissue is very
scanty within the nerve.
DOGFISH SHARK:
The nerve averages 744 micra in its largest diameter and
595 micra in its smallest.
The average area of the cross-
section is 0.348 square millimeters.
Thenerve contains a prominent connective tissue septal
system, the septa being arranged in strips passing across the
nerve from one side to the other.
These septa split the fibers
into almost rectangular shaped fascicles which extend from one
side of the nerve to the other.
There is no radiation of the
septa from the center toward the periphery as is customarily
seen.
The fibers are large, rather uniform in size, and are
closely packed in the fascicles.
Neuroglial elements are few and are widely scattered.
STINGRAY:
This nerve averages 371 micra in its largest and 310 micra
in its smallest diameter.
The average cross-sectional area is
0.090 square millimeters.
There is a prominent connective tissue septal system with
an arrangement which shows little regularity.
There is a ten-
dancy for the principal septa to radiate from the center toward
64.
the periphery, each then breaking up into secondary and
tertiary septa.
These often appear discontinuous, i.e.,
there are heavy strands of connective tissue within the nerve
which do not appear to be connected with other septa in any
single section.
There is the whole pattern of irregularity
and incomplete fasciculation present in this nerve.
Fibers are rather uniform in size with the medium-sized
group predominating, and they are closely packed in the ir­
regular, incomplete fascicles.
Neuroglial elements are rarely seen.
GUITAR FISH:
This largest of the fish nerves measures 884 micra in
its longest dimension and 787 in its shortest.
Its average
cross-sectional area is 0.547 square millimeters.
The septal system is fairly prominent.
It forms incomplete
partitions radiating from the center toward the periphery.
It
resembles the stingray system but is not a,s robust proportion­
ately.
The nerve fibers are large, some being of great size, and
all are larger than those seen in other fishes.
The fibers are
very loosely packed in the fascicles.
Neuroglial elements are scarce.
HAOKLEBAOK:
The nerve measures 406 micra by 187 micra.
The shape is
unusual in that it resembles that of a horseshoe.
measurements, it was figuratively straightened out.
In making the
The cross-
sectional area is 0.061 square millimeters.
The septal connective tissue is scanty in amount.
Its
arrangement is so irregular that no pattern can be identified.
65
The fibers are mostly medium-sized and are loosely packed
in the nerve.
The neuroglial elements are scanty in amount.
BOWFIN:
This nerve measures 834 by 606 micra.
The average cross-
sectional area is 0.393 square millimeters.
The septal system is well-formed and unique in its arrange­
ment:
the system consists of two distinct sets of septa, l) a
peripheral set passing from the outer pial sheath toward the
center but never reaching it, and 2) a central set which gives
rise to septa radiating out toward the periphery but never quite
reaching it.
These two sets of septa so interdigitate that the
fibers are pressed into a continuous, highly crinkled sheet.
The effect in cross-section is that of the wall of an old-fashion­
ed purse when the string is drawn tight.
The nerve fibers are moderately large, rather uniform in
size, and are evenly distributed in the highly folded sheet just
described.
The neuroglial elements are not conspicuous.
GOLDFISH:
The nerve averages 431 micra by 360 micra.
The average
cross-sectional area is 0.133 square millimeters.
The septal system is prominent and it divides the fibers
into numerous small round fascicles.
These are equally distrib­
uted throughout the entire cross-section of the nerve.
Each
fascicle is completely divided from its neighbor by the peri­
neurium.
The fibers are mostly small with a fewlarger ones between.
The neuroglial elements are abundant.
66.
BULLHEAD:
The nerve averages 443 Toy 365 micra*
Its average cross-
sectional area is 0.137 square millimeters.
The septal system is incomplete and inconspicuous.
There
are a few scattered strands of connective tissue but these show
no apparent design.
The fibers are small in size and are regularly distributed.
The neuroglial elements are abundant.
3.
Amphibia and reptiles
TOAD:
The nerve averages 311 by 171 micra.
The average calculated
area is 0.043 square millimeters.
There are no connective tissue septa in these preparations,
although the usual glial fibers are present.
The nerve fibers are medium to small in size and are evenly
distributed throughout the nerve.
Neuroglial nuclei are present but are infrequent.
FROG:
The nerve measures 493 by 349 micra and its average crosssectional area is 0.135 square millimeters.
There are no connective tissue septa present.
Occasional
blood capillaries can be seen dipping inward from the pia carry­
ing a few connective tissue strands with them but these are in­
frequent and the connective tissue does not seem to enter into
the formation of a septal system.
The nerve fibers are mixed large and small, and are not
arranged in fascicles.
Neuroglial elements are abundant.
67.
ALLIGATOR:
The three nerves in this series average 670 by 525 micra.
Their average area is 0,377 square millimeters.
Connective tissue septa are rarely seen, hence fasciciilation is nearly absent.
The nerve fibers are mostly small and areuniformly dis­
tributed throughout thenerve.
Neuroglial elements are present but they are scant in amount.
HORNED TOAD:
The nerves measure 565 by 473 micra and their average crosssectional area is 0.310 square millimeters.
The connective tissue septa are clearly demonstrated and
the nerve fibers are divided by them into many small fascicles.
The septa tend to be incomplete, however, so the fascicles often
merge on one or more sides with neighboring ones.
The nerve fibers are uniformly small and are evenly dis­
tributed.
Neuroglial elements are present but inconspicuous.
3.
Birds
DUCKLINGS'
The average measurements are 1213 micra by 1038 micra.
The
area of the cross-section ofthe nerve averages 1.08 square milli­
meters.
The connective tissue septa are abundant, but they are thin­
ner when compared with certain other forms.
The septa,! system is
moderately complete, resulting in an easily recognisable
fasoiculation of the nerve fibers.
There are mostly small-sized nerve fibers present and they
appear to be uniformly distributed throughout the nerve.
68.
The neuroglial elements are not conspicuous.
CHIOK:
The chick nerve is slightly smaller than that of the
duckling:
1069 toy 908 micra#
Its average cross-sectional
area is 0.763 square millimeters.
The connective tissue septa are present in great numbers
tout they are so interrupted, when examined in cross-section,
that fasciculation of the fibers is difficult to make out.
The nerve fibers are uniformly ofa small size and are
evenly distributed throughout the nerve.
The neuroglia.1 elements are abundant.
PIGEON:
This large nerve measures 1729 toy 1540 micra and its
calculated average area is 2.09 square millimeters.
The connective tissue septa are abundant tout they are
interrupted, giving a pattern of numerous islands of connective
tissue scattered throughout the nerve.
This naturally makes
for the same sort of incomplete fasciculation seen in other
bird optic nerves.
The nerve fibers are medium- to small-sized, with the
latter group predominating.
The axis cylinders are closely
packed and are evenly distributed throughout the nerve.
The neuroglial nuclei are conspicuous in every field.
CANARY:
This nerve is smaller than that of the other birds studied:
it measures 804 toy 775 micra and ha,s an average area of 0.490
square millimeters.
The connective tissue septa are numerous tout each is thin
and incompletely attached to its neighbor when viewed in cross-
69,
section.
This arrangement makes it difficult to recognize
any definite fasciculation of the nerve fibers.
The nerve fibers:
a few medium-sized fibers are scatter­
ed throughout the nerve but they are slightly more abundant in
the center;
small.
the vast majority of the fibers are exceedingly
Many axis cylinders are so fine that they can just be
seen at a magnification of 900 diameters.
The osmic acid
preparations showed a similar close packing of small nerve fibers;
many were so small that it did not seem feasible to attempt to
enumerate them.
The neuroglia,! elements are not abundant.
4. Mammals
BAT:
This small nerve measures
cross-sectional area is 0.0108
136 by 108 micra andits average
square millimeters.
The connective tissue septa are robust for so small a
nerve.
They divide the fibers into fascicles, most of which
are completely demarcated from
neighboring bundlesbythese
heavy bands of binding tissue.
The nerve fibers are uniformly of a small size and are
closely packed together in their fascicles.
The neuroglial elements are rarely seen.
DOG:
This nerve measures 1454 by 1443 micra and has an average
cross-sectional area, of 1.65 square millimeters.
The connective tissue septa are robust and abundant;
they divide the nerve fibers into numerous completely separated
fascicles.
The nerve fibers are highly variable in size:
very large
fibers predominate, but many exceedingly fine fibers can be
seen interspersed between them.
This nerve, more than any
other studied in this series (except that of the cat), shows
such a marked variations in the size of its nerve fibers.
The neuroglial elements are not abundant.
CAT:
This nerve measures 1308 by 1148 micra and its area is
1.18 square millimeters.
The connective tissue septa are robust and divide the
nerve fibers into numerous fascicles.
This septal system
very closely resembles that of the dog in its pattern.
The nerve fibers are predominately large but numerous
small fibers are scattered between them.
The neuroglial elements are inconspicuous.
RABBIT:
The diameters of this nerve are:
1065 by 785 micra.
a
average cross-sectional area is 0.656 squre millimeters.
The
The rather robust connective tissue septa do not form a
completely connected septal system pattern;
they appear as
islets of light-staining tissue interspersed at irregular
intervals between the nerve fibers.
The nerve fibers are mostly of a small size with a few of
medium size scattered between them.
The neuroglial elements are conspicuous.
RAT:
This rodent nerve measures 583 by 475 micra and has a
cross-sectional area of 0.318 (figures obtained on white rat
nerves but hooded rat nerves are very similar).
The connective tissue septa are nearly absent, there being
71.
merely a few delicate strands of tissue scattered at irregular
intervals throughout the cross-section of the nerve.
No fasci­
cular pattern can Toe recognized.
The nerve fibers vary greatly in size:
the majority are
small, but many medium—sized fibers can be located in any field
examined* The neuroglial elements appear to be sparse in amount.
GUINEA PIG:
This nerve measures 630 by 519 micra and its calculated
cross-sectional area is 0.256 square millimeters.
The connective tissue septa are inconspicuous.
The few
that are present are small, discontinuous islets of binding
tissue*
No definite fascicula/tion cam be described.
The nerve fibers are uniformly of a small size and are
evenly distributed.
The neuroglial elements are conspicuous.
PIG: •
This large nerve measures 3183 by 3470 micra and its area
is 6.17 square millimeters.
The connective tissue septa are very robust and form a>
pattern which separates groups of fibers into definite fascicles.
The nerve fibers are medium-sized with some scattered smalland large-sized ones.
The fibers are quite loosely packed in
the nerve.
The neuroglial elements are not abundant.
SHEEP:
The sheep nerve measures 3717 by 3019 micra and has an
average cross-sectional area of 4.31 square millimeters.
The septa are heavy and form a continuous connective tissue
72.
network which splits the nerve fibers into conspicuous fascicles.
The nerve fibers are mostly medium-sized with scattered
large and small ones distributed over the cross-section.
The neuroglial elements are abundant.
MONKEY:
The monkey nerve measures 2138 by 1954 micra and has an
average cross-sectional area of 3.24 square millimeters.
The septa are robust, but less so than in the ungulates.
They divide the fibers into fascicles, but when viewed in crosssection the fascicles are rarely completely surrounded.
The nerve fibers are remarkably uniform in size:
in ganeral,
a rather small size.
The neuroglial elements are abundant.
HUMAN:
The human optic nerve measures 4617 by 3382 micra and has
an average cross-sectional area of 8.65 square millimeters— the
greatest of any nerve used in this study.
The connective tissue septa are well-developed, although
they are somewhat less robust than those seen in the monkey.
The fascicles tendto be only partially surrounded by connective
tissue when viewed in cross-3ection.
Numerous corpora amylacia
axe present in the nerves from older individuals.
The majority of the nerve fibers are small but great
numbers of medium-sized fibers can be seen throughout the nerve.
The fibers are loosely packed in their fascicles.
The neuroglial elements are conspicuous in every field.
73.
B.
Fiber enumerations
The fesults of the fiber enumerations made in this study
are presented in tabular form in the pages that follow.
TABLE V
THE HUMBER OF FIBERS IN THE
OPTIO NERVE OF OYOLOSTOMES
A.
Specimen Stain
Number
.
Number of
Strips
Counted
Hagfish
Number of
Fibers
Counted
Total
Number of
Strips
Total
Number of
Fibers
$0 of
Fibers
Counted
Silver
Osmic
14.5
0
1,416
0
14.5
0
1,416
0
100
0
Silver
Osmic
19.5
0
1,743
0
19.5
0
1,743
0
100
0
AVERAGES «
*
Silver
Osmic
17.0
0
1,579
0
17.0
0
1,579
0
100
0
1
3.
B.
Brook lamprey
l*.
Silver
Osmic
24
0
4,955
0
24
0
4,955
0
100
0
2.
Silver
Osmic
24
0
4,769
0
24
0
4,769
0
100
0
3.
Silver
Osmic
33
0
5,477
0
33
0
5,477
0
100
0
4.
Silver
Osmic
27
0
5,666
0
27
0
5,666
0
100
0
AVERAGES •
Silver
Osmic
27
0
5,217
0
27
0
5,217
0
100
0
9
74.
TABLE VI
THE NUMBER OF FIBERS IN THE OPTIC
HSRVE OF E LA SMOBRAHC HS
A.
Specimen
Humber
Dogfish shark pups
Stain
Humber of
Strips
Counted
Humber of
Fibers
Counted
1.
Silver
Osmic
30
39
31,633
30,319
100
153
108,115
108,131
30.0
19.8
3.
Silver
Osmic
30
37
10,873
13,153
197
133
107,089
89,416
10.1
20.3
3.
Silver
Osmic
35
48
37,391
36,404
171
340
133,834
132,030
20.4
20.0
4.
Silver
Osmic
17
30
9,885
9,353
166
308
96,512
97,381
10.3
9.6
Silver
Osmic
33
31
17,443
18,633
158
188
111,385
106,709
15.3
17.2
Total
Humber of
Strios
Total
io of
Humber of Fibers
Counted
Fibers
(Estimated)
AVERAGES;
B.
Guitar fish
1.
Silver
Osmic
35
17
7,397
7,534
345
167
73,549
74,010
10.3
10.3
3.
Silver
Osmic
31
34
11,347
11,001
143
163
76,733
74,519
14.8
14.6
Silver
Osmic
33
31
9,373
9,367
194
165
74,636
74,264
13.5
13.4
AVERAGES:
75.
TABLE VI (con-t'd)
C.
Specimen
Humber
Sting ray
Stain
Number of
Strips
Counted
Number of
Fibers
Counted
Total
Number of
Strips
1.
Silver
Osmic
14
20
3,975
8,817
67
99
42,953
43,639
30,9
20.3
2.
Silver
Osmic
17
27
7,520
7:,:469
83
133
36,715
36,431
30.5
20.5
3.
Silver
Osmic
17
23
7,864
7,907
85
115
39,330
39,535
20.0
30.0
Silver
Osmic
16
23
8,119
8,066
78
116
39,663
39,365
20.5
30.3
$ of
Total
Number of Fibers
Counted
Fibers
(Estimated)
AVERAGES:
76.
TABLE VII
THE NUMBER OF FIBERS IN THE
OPTIO NERVE OF GANOIDS
A.
Haokleback
Specimen
Number
Stain
Number of
Strips
Counted
Number of
Fibers
Counted
.
Silver
Osmic
32
19
2,772
2,799
110
94
13,360
13,848
20# 0
20.3
3.
Silver
Osmic
24
13
3,157
3,038
130
90
15,785
15,190
30.0
20.0
3,
Silver
Osmic
17
17
3,355
3,379
83
83
10,877
11,615
20.7
30.5
Silver
Osmic
31
18
2,738
2,739
104
89
13,507
13,551
20.3
20.2
1
Total
Number of
Strips
$ of
Total
Number of Fiber*
Fibers
Countec
(Estimated)
AVERAGES:
B.
1
.
3*
Bowf in
Silver
Osmic
19
20
11,010
11,366
190
193
110,010
108,717
10.0
10.3
Silver
Osmic
31
21
12,376
12,059
301
305
117,451
117,719
10.4
10.3
Silver
Osmic
20
21
11,642
11,662
196
199
113,735
113,218
10.2
10.3
AVERAGES:
77
.
TABLE VIII
THE NUMBER OF FIBERS IN THE
OPTIC NERVE OF TELEOSTS
A.
Specimen
Number
Goldfish
Stain
Number of
Strips
Counted
Number of
Fibers
Counted
1.
Silver
Osmic
12
8
10,537
4,733
60
86
52,683
50,879
30.0
9.3
2.
Silver
Osmic
11
12
8,871
7,900
66
83
53,336
53,983
16.7
14.6
Silver
Osmic
12
10
9,704
6,316
63
84
53,954
52,431
18.4
12.0
Total
Number of
Strips
Total
$ of
Number of Fibers
Counted
Fibers
(Estimated)
AVERAGES:
B.
Bullhead
1.
Silver
Osmic
10
10
3,737
3,668
67
70
25,038
35,676
14.9
14.9
2.
Silver
Osmic
11
9
3,439
3,500
77
63
24,009
24,500
14.3
15.1
3.
Silver
Osmic
9
10
4,460
4,356
58
65
38,339
28,314
15.7
15.4
4.
Silver
Osmic
9 '
10
5,695
4,242
46
68
39,118
38,846
18.9
14.9
Silver
Osmic
10
10
4,330
3,941
63
67
36,636
36,834
17.0
15.1
AVERAGES:
78
TABLE IX
SUMMARY OF THE TOTAL NUMBER OF FIBERS IN OSMIC- AND
SILVER-STAINED SECTIONS OF THE OPTIC NERVE OF FISHES
Specimen name
and number
Osmic-stained
fibers
Hagfish
1.
2.
AVERAGES:
Brook lamprey
1.
3.
3.
4.
AVERAGES:
Shark pup
1.
*2*
3.
4.
AVERAGES:
luitar fish
1.
2.
AVERAGES:
3ting ray
1.
2.
3.
AVERAGES:
Sgokleback
1*
3.
3.
AVERAGES:
Silver-stained
fibers
Difference
fo
Difference
0
0
1,416
1,743
1,416
1,743
100.0
100.0
0
1,579
1,579
100.0
0
0
0
0
4,955
4,769
5,477
5,666
4,955
4,769
5,477
5,666
100.0
100.0
100.0
100.0
0
5,317
5,317
100.0
108,121
89,416
133,020
97,281
108,115
107,089
133,834
96,512
-6
17,675
1,804
-769
112,474
113,150
676
0.5
74,010
74,519
73,549
76,723
-1,561
3,304
-3.1
2.8
74,264
74,636
373
0.4
43,639
36,431
39,535
42,953
36,715
39,320
-686
394
-215
-1.4
0.9
-0.5
39,865
39,663
-202
-0.3
13,848
15,190
11,615
13,860
15,785
10,877
12
595
-738
0.1
3.8
-6.8
13,551
13,507
-44
-0.3
0.005
16.4
1.3
-0.7
'Shark pup #2 is omitted from the averages in that.series; refer
to discussion, under heading of material, for the reasons for
this omissioh.
79
TABLE IX (oont*d)
Specimen name
and number
Osmic-stained
fibers
Bowfin
1.
3.
Silver-stained
fibers
Difference
$
Difference
108,717
117,719
110,010
117,451
1,393
-263
1.3
-0.3
AVERAGES: 113,318
113,735
517
0.5
50,879
53,983
52,683
53,236
1,993
-757
3.8
-1.4
53,431
53,954
523
1.2
35,676
34,500
38,314
38,846
35,038
34,009
28,339
29,118
-638
-491
25
273
-2.5
-3.0
0.1
0.9
36,834
26,262
-308
-0.9
Goldfish
1.
3*
AVERAGES:
Bullhead
1.
3.
3.
4.
AVERAGES:
.
80
TABLE X
STATISTICAL ANALYSIS OF THE TOTAL NUMBER OF NERVE
FIBERS IN THE OPTIC NERVE OF FISHES
dame and
Average Standard Probable Co-•efficient
dumber of Stain Fiber Deviation Error
of variation
Specimens
Estimate (S.D.)
(c.v.)
of S.D.
Probable
Error
of C.V.
Hagfish
(2)
Silver
Osmic
1,579
0
231
0
77.9
0
Lamprey
(4)
Silver
Osmic
5,217
0
388
0
93
0
Shark
(3)
Silver 113,150
Osmic 112,474
24,450
23,570
6,735
6,216
luitar
fish (2)
Silver
Osmic
74,636
74,264
2,087
254-
704
86
2.79
0. 34
0.94
0.13
3ting
ray (3)
Silver
Osmic
38,663
39,865
2,548
3,958
703
816
6.43
7.4-0
1.77
2.27
lackleaack (3)
Silver
Osmic
13,507
13,551
3,038
3,246
837
615
3owfin
(2)
Silver 113,735
Osmic 113,218
3,735
4,501
1,256
1,518
3.23
3.97
1.10
1,34
>oldfish
(3)
Silver
Osmic
52,954
52,431
273
1,552
93
523
0.51
2.97
0.17
0.99
3ullhead
(4)
Silver
Osmic
26,626
26,834
2,143
1,641
514
391
8.05
6.13
1.91
1.46
14.6
0
7.4-4
0
31.6
30.0
22.5
16.6
4.9
0
1.77
0
5.9
5.5
6.3
4.6
81.
TABLE XI
THE NUMBER OF FIBERS IN THE
OPTIC NERVE OF AMPHIBIA
A.
Specimen
Number
Stain
Number of
Strips
Counted
Toad
Number of
Fibers
Counted
Total
Number of
Strips
$, of
Total
Number of Fibers
Counted
Fibers
(Estimated)
1.
Silver
Osmic
14
8
4,885
1,027
42
75
14,655
9,638
33.3
10.7
3.
Silver
Osmic
9
8
1,675
1,147
87
75
16,193
10,754
10.3
10.7
Silver
Osmic
13
8
3,380
1,087
65
75
15,433
10,191
21.8
10.7
AVERAGES:
B.
Frog
1.
Silver
Osmic
13
15
3,023
. 1 *612
113
143
28,466
15,368
10.6
10.5
3.
Silver
Osmic
13
13
3,103
1,551
133
127
39,359
15,153
10.6
10.2
Silver
Osmic
13
14
3,063
1,581
118
135
28,913
15,260
10.6
10.4
AVERAGES:
82
TABLE XII
THE NUMBER OF FIBERS IN THE
OPTIC! NERVE OF REPTILES
A.
3pecimen
Number
Stain
Number of
Strips
Cotinted
Alligator (baby)
Number of
Fibers
Counted
Total
Number of
Strins
$ of
Total
Number of Fibers
Counted
Fibers
(Estimated)
1.
Silver
Osmic
8
9
5,042
3,973
112
133
70,588
58,713
7.3
6.8
2.
Silver
Osmic
10
12
7,480
5,363
143
180
106,964
80,445
7.0
6.6
3.
Silver
Osmic
14
17
9,684
7,113
300
255
138,343
106,695
7.0
6.6
Silver
Osmic
11
13
7,403
5,483
152
189
105,398
81,951
7.1
6.6
AVERAGES:
B.
Horned toad
1.
Silver
Osmic
14
13
11,318
7,133
132
130
105,769
71,330
10.6
10.0
3.
Silver
Osmic
16
11
14,933
8,554
153
110
143,370
85,540
10.4
10.0
3*
Silver
Osmic
16
14
14,349
8,866
153
140
137,318
88,660
10.4
10.0
Silver
Osmic
15
13
13,530
8,181
146
127
138,784
81,840
10.5
10.0
AVERAGES:
83.
TABLE XIII
SUMMARY OF THE TOTAL NUMBER OF FIBERS IN OSMIC
AND SILVER-STAINED SECTIONS OF THE OPTIC
NERVE OF AMPHIBIANS AND REPTILES
Specimen name
and number
Osmic-stained
fibers
Silver-stained
fibers
Difference
€i°
Difference
Toad
3.
AVERAGES:
9,638
10,754
14,655
16,193
5,027
5,438
34.3
33.5
10,191
15,433
5,332
33.9
15,368
15,153
33,466
39,359
13,098
14,307
46.8
48.4
15,360
38,913
13,652
CD
.
1.
58,713
80,445
106,695
70,588
106,964
138,343
11,876
36,519
31,648
16.8
34.8
22.9
81,951
105,398
33,347
21.5
71,330
85,540
88,660
105,769
143,370
137,313
34,539
57,830
48,553
32.6
40.3
35.4
81,840
138,784
46,944
36.1
Frog
1.
3.
AVERAGES:
Alligator
1.
3.
3.
AVERAGES:
horned toad
1.
3.
3.
AVERAGES:
84.
TABLE XIV
STATISTICAL ANALYSIS OF THE TOTAL NUMBER OF
FIBERS IN THE OPTIC NERVE OF
AMPHIBIANS AND REPTILES
Name and
Number of Stain
Specimens
Average
Fiber
Estimates
Standard Probable Co-efficient Probable
of variation Error
Deviation Error
(S.D.)
of C.V.
of S.O.
(C.V.)
Toad
(3)
Silver
Osmio
15,433
10,191
1,087
796
Frog;
(3)
Silver
Osmic
38,913
15,360
631
140
Alligator Silver
(3)
Osmic
105,398
81,840
39,500
28,570
10,878
7,868
37.5
34.8
10.33
9.53
Horned
toa,d (3)
138,784
81,840
36,580
10,530
7,317
3,941
20.6
13.3
5.67
3.53
Silver
Osmic
367
369
7.05
7.81
3.37
3.63
313
50.5
3.18
0.98
0.74
0.33
85
TABLE XV
THE HUMBER OF FIBERS IU THE
OPTIC HERVE OF BIRDS
A,
Specimen
Humber
Duckling
Stain
Humber of
Strips
Counted
Humber of
Fibers
Counted
Silver
Osmic
21
18
38,319
38,079
310
366
416,090
414,945
6.7
6.7
2.
Silver
Osmic
31
30
38,346
28,587
313
295
431,140
421,638
6.7
6.8
3*
Silver
Osmic
17
23
25,911
37,768
355
319
388,665
402,636
6. 6
6.8
Silver
Osmic
30
20
27,459
38,145
392
293
408,632
413,073
6.7
6.8
1
.
Total
Humber of
Strips
fo of
Total
Humber of Fibers
Counted
Fibers
(Estimated)
AVERAGES:
B.
1
.
3.
Chick
Silver
Osmic
19
17
27,828
28,393
285
353
417,430
431,051
6.7
6.7
Silver
Osmic
15
18
37,365
29,279
325
261
410,474
424,546
6.7
6.9
Silver
Osmic
17
18
27,597
28,785
255
257
413,948
422,798
6.7
6.8
AVERAGES:
86.
TABLE XV (cont'd)
C*
Spec imen
Number
Stain
Pigeon
Number of
Strips
Counted
Number of
Fibers
Counted
Total
Number of
Strips
Tot9,1
$ of
Number of Fibers
Fibers
Counted
(Estimated)
1.
SilVei*
Osmic
20
26
50,530
66,754
399
384
1,008,073
985,905
5.0
6.8
2.
Silver
Osmio
19
32
49,281
47,348
373
439
964,870
931,336
5.1
5.1
Silver
Osmic
20
24
49,906
57,001
385
406
986,471
953,630
5.1
6.0
AVERAGES:
D,
Canary
1.
11
24,134
Silver
Osmic Not counted*
307
454,303
5.3
2.
10
21,091
Silver
Osmic Not counted*
190
400,739
5.3
199
427,516
5.3
AVERAGES:
Silver
11
33,612
•Thenerve fibers were well-stained in the osmic-preparations but
they were so small and so closely packed together that it seemed
immpossible to enumerate them accurately. From the appearance
of these osmic-stained preparations it would be logical to
predict that all the fibers in the canary are myelinated.
87.
TABLE XVI
SUMMARY OF THE TOTAL NUMBER OF FIBERS IN
OSMIC- AND SILVER-STAINED SECTIONS
OF THE OPTIC NERVE OF BIRDS
3peciraen name
and number
Duckling
1.
3.
3.
Osmic-stained
fibers
Silver-stained
fibers
Difference
$
Difference
414,945
431,638
403,636
416,090
431,140
388,665
1,145
-498
-13,971
-
AVERAGES: 413,073
408,633
-4,441
-
431,051
434,546
417,430
410,475
—3,631
■14,071
-0.9
-3.4
AVERAGES: 433,798
413,948
-8,850
-3.1
985,905
931,336
1,008,073
964,870
33,168
43,534
3.3
4.5
AVERAGES: 953,630
986,471
33,851
3.3
Chick
1.
3.
Pigeon
1.
3.
Canary
1.
Not
Counted
AVERAGES:
454,303
400,739
437,516
0.3
0.1
-3.6
1.1
88.
TABLE XVII
STATISTICAL ANALYSIS OF THE TOTAL
NUMBER OF FIBERS IN THE
OPTIC NERVE OF BIRDS
B=-'..
=
Name and
Number of Stain
Specimens
Standard Probable
Average
Fiber
Deviation Error
Estimates (B.D.)
of S.D.
Co--efficient Probable
of variation Error
of C.V.
(C.V.)
Duckling
(3)
Silver
Osmic
408,633
413,073
33,055
13,050
6,349
3,319
5.64
3.93
1. 55
0.80
Chick
(3)
Silver
Osmic
413,948
433,798
4,910
3,470
1,656
833
1.18
0.61
0.39
0.31
Pigeon
(2)
Silver
Osmic
986,471
953,630
30,548
45,660
10,548
15,399
3.09
4.89
1.05
1.65
Canary
(3)
Silver
437,516
37,880
13,776
8.86
3.99
89.
TABLE XVIII
THE NUMBER OF FIBERS IN THE
OPTIC NERVE OF MAMMALS
A.
Specimen
Number
Bat
Stain
Number of
Strips
Counted
Number of
Fibers
Counted
1.
Silver
Osmic
6
7
1,331
776
39
33
6,433
3,657
20.7
31.2
3.
Silver
Osmic
6
8
1,479
793
28
37
6,903
3,665
31.4
31.6
3.
Silver
7
1,550
Osmic Not counted
31.5
6,975
32.2
4.
Silver
Osmic
8
8
1,485
966
40
36
7,425
4,387
30.0
22.2
Silver
Osmic
7
8
1,461
845
32
35
6,934
3,903
31.1
31.6
Total
Number of
Strips
d}a Of
Total
Number of Fibers
Counted
Fibers
‘
11 '
AVERAGES:
B.
Dog
1.
10,741
Silver
32
Osmic Not counted
473
158,763
6.8
2.
Silver
Osmic
31
33
10,330
10,337
465
475
154,800
153,400
6.7
6.7
3.
Silver
Osmic
36
38
9,253
9,425
387
406
137,736
136,663
6.7
6.9
Silver
Bulbar end
16
Chiasmal end 34
8,507
8,183
310
476
164,833
163,396
5.1
5.0
Silver (all 4 nerves)
153,712
6.1
Silver (nerves #3 and #3)
Osmic (nerves #3 and #3)
146,263
145,031
6.7
4.
AVERAGES:
6.8
90,
TABLE XVIII (cont *d)
C.
Specimen
Humber
Stain
Cat
Humber of
Strips
Counted
Humber of
Fibers
Counted
Total
Humber of
Strips
$ of
Total
Humber of Fibers
Counted
Fibers
(Estimated)
1.
Silver
Osmic
36
29
12,389
8,071
353
436
121,481
118,560
10.3
6.8
2.
Silver
Osmic
23
33
7,853
7,799
326
485
116,361
114,622
6.8
6.8
3.
Silver
(fetus)
9
4,898
123
66,395
7.3
118,871
116,594
8.5
6.8
AVERAGES:
Silver (excluding #3)
Osmic
D.
Rabbit
1.
Silver
Osmic
26
32
27,345
27,477
359
313
272,397
268,759
10.3
10.0
2.
Silver
Osmic
24
29
26,239
25,453
335
388
256,825
252,774
10.3
10.0
Silver
Osmic
25
31
36,787
36,465
247
300
264,611
360,767
10.1
10.0
AVERAGES:
E.
White rat
1.
Silver
Osmic
36
38
15,293
13,374
128
137
75,388
60,055
30.3
20.4
3.
Silver
Osmic
34
26
15,386
11,780
166
139
74,337
58,447
30.4
30.2
Silver
Osmic
30
37
15,289
12,027
147
133
74,813
59,261
20«4
20.3
AVERAGES:
91
TABLE XVIII (oont'd)
F.
Specimen
Number
Hooded rat
Stain
Number of
Strips
Counted
Number of
Fibers
Counted
1.
Silver
Osmic
11
16
7,950
8,003
108
158
78,055
79,019
10.2
10.1
2.
Silver
Osmic
14
15
3,219
7,915
140
149
82,190
79,322
10.0
10.1
Silver
Osmic
13
16
8,085
7,959
134
154
80,133
79,120
10.1
10.1
Total
Number of
Strips
Total
$> of
Number of Fibers
Counted
Fibers
(Estimated)
AVERAGES:
G.
Guinea pig
1.
Silver
Osmic
22
20
13,600
13,010
213
200
131,054
130,100
10.4
10.0
2.
Silver
Osmic
16
IS
12,480
13,542
156
179
121,680
134,193
10.3
10.0
Silver
Osmic
19
19
13,040
12,776
184
190
126,367
127,147
10.3
10.0
AVERAGES:
H.
Pig
1.
Silver
Osmic
33
34
33,271
32,949
758
671
663,669
650,358
5.0
5.1
2.
Silver
Osmic
41
39
32,879
35,919
820
761
657,580
700,881
5.1
5.1
3.
Silver
Osmic
40
43
35,382
35,109
790
847
698,795
691,565
5.1
5.1
4.
Silver
Osraic
42
40
35,793
34,941
825
791
703,077
690,958
5.1
5.0
Silver
Osmic
40
39
33,831
34,729
801
768
680,780
683,413
5.1
5.1
AVERAGES:
93.
TABLE XVIII (cont'd)
I.
Specimen
dumber
Sheep
Stain
Number of
Sftrips
Counted
Number of
Fibers
Counted
1.
Silver
Osmic
3?
38
25,760
25,695
675
696
644,000
638,704
4.0
4.0
3.
Silver
Osmic
37
33
36,221
36,512
673
808
653,546
649,142
4.0
4.1
Silver
Osrnic
37
31
25,991
36,104
674
753
648,773
643,933
4.0
4.1
Total
Number of
Strips
Total
of
Number of Fiber?
Count e<
Fibers
(Estimated)
AVERAGES:
J.
Monkey
1.
Silver
Osmic
40
46
81,033
87,746
596
685
1,307,391
1,371,905
6.7
6. 8
3.
Silver
Osmic
44
48
87,500
90,744
660
706
1,305,682
1,333,937
6.7
6.8
3.
Silver
Osmic
26
33
56,213
60,263
514
641
1,111,388
1,167,513
5.1
5.2
Silver
Osmic
37
41
74,915
80,517
591
677
1,208,120
1,257,818
6.3
6.3
AVERAGES:
TABLE XVIII (cont'd)
K.
3pecimen
^
Slumber
Stain
Man
dumber of
Strips
Counted
Humber of
Fibers
Counted
Total
Humber of
Strips
Total
# of
Humber of Fibers
Counted
Fibers
(Estimated)
1.
Silver
Osmic
39
43
38,490
36,345
572
624
564,776
540,116
6.8
6.7
3.
Silver
Osmic
73
78
76,003
81,557
1,095
1,157
1,140,030
1,309,763
6.7
6.7
3.
Silver
66
53,527
1,317
1,068,107
5.0
4.
Silver
66
51,434
1,310
1,016,991
5.1
5.
Silver
42
36,069
1,050
901,725
4.0
6•
Silver
51
40,865
1,275
1,021,635
4.0
7.
Silver
49
36,754
1,320
915,099
4.0
8.
Silver
48
34,383
1,321
871,448
3.9
9.
Silver
50
48,508
1,239
1,201,850
4.0
10.
Silver
11.
Silver
49
41,454
1,204
1,030,830
4.0
IS.
Silver
45
37,538
1,133
936,532
4.0
AVERAGES:
#1 and #3 Silver 56
#1 and #2 Osmic 60
57,346
58,955
833
891
852,403
874,939
6.7
6.7
45,643
1,305
1,009,433
4.5
Hot counted*
All except,Silver 54
#1 and #10
*Axis cylinders showed evidences of degenerative changes. See
Olinical Histories, Table III, page 45, for a possible cause
for this finding.
94.
TABLE XIX
SUMMARY OF THE TOTAL HUMBER OF FIBERS IN
OSMIC!- AND SILVER-STAINED SECTIONS
OF THE OPTIC NERVE OF MAMMALS
Specimen name
and number
Bat
1.
2.
3.
4.
AVERAGES:
3og
1.
2.
3.
4.
AVERAGES:
Sat
1.
3.
AVERAGES:
babbit
1.
2.
AVERAGES:
fhite rat
1.
2.
AVERAGES:
Hooded rat
1.
2.
AVERAGES:
*uinea pig
1.
2.
AVERAGES:
Osmic-stained
fibers
Silver-stained
fibers
3,657
3,665
2,776
3,337
43.3
46.8
4,387
6,433
6,902
6,975
7,425
3,038
40.9
3,903
6,934
3,031
43.7
153,400
136,663
158,763
154,800
137,736
163,559
1,400
1,063
0.9
0.9
145,031
146,263 (#2,#3)
153,713 (all 4)
1,232
0.9
118,560
114,622
131,481
116,261
2,931
1,639
2.3
1.4
116,594
118,871
2,277
1.9
268,759
252,774
273,397
356,825
3,638
4,051
1.4
1.6
260,767
364,611
3,844
1.5
60,055
58,447
75,388
74,337
15,333
15,890
20.2
31.4
59,361
74,813
15,562
20.6
79,019
79,322
78,055
83,190
-964
2,968
-1.3
3.6
79,120
80,132
1,002
1.2
130,100
124,193
131,054
121,680
954
-3,513
0.7
-2.1
127,147
136,367
-780
-0.7
Di fference
*
Difference
95.
TABLE XIX (eont'd)
3pecimen name
and number
Osmic-stained
fibers
Silver-stained
fibers
Difference
$
Difference
650,353
700,881
691,565
690,958
663,669
657,580
698,795
703,077
13,411
-43,301
7,230
12,119
2.0
—6.6
1.0
1.7
633,413
680,780
—2,633
-0.4
638,704
649,142
644,000
653,546
5,296
4,404
0.8
0.6
643,923
648,773
4,850
0.7
1,271,905
1,333,937
1,167,513
1,207,391
1,305,682
1,111,288
-64,514
—28,355
-56,335
-5.3
-2.3
-5.5
AVERAGES: 1,357,818
1,208,130
-49,698
-4.3
540,116
1,209,763
564,776
1,140,030
34,560
-69,732
4.3
—6.1
874,939
852,403
-33,536
-2.5
Pig
1.
3.
3.
4.
AVERAGES:
Sheep
1.
3.
AVERAGES:
Monkey
1.
3.
3.
Man
1.
3.
AVERAGES:
3.
4.
5.
6.
7.
3.
9.
10.
11.
13.
1,068,107
1,016,991
901,725
1,031,625
915,099
871,448
1,201,850
Hot counted
1,020,820
936,532
AVERAGES (#2 to #12):
1,009,433
96.
TABLE XX
STATISTICAL ANALYSIS OF THE TOTAL
NUMBER OF FIBERS IN THE
OPTIC NERVE OF MAMMALS
Name and
Average Standard Probable Co-efficient Probable
Number of Stain
Fiber
Deviation Error
of Variation Error
of S.D.
(C.V.)
of C.V.
Specimens_______ Estimates (S.D.)
Bat (4)
(3 )
Silver
Osmic
6 934
3 903
533
559
127
154
7* 68
14.32
1.83
3.94
Dog (4)
(3)
Silver
Osmic
153 712
145 031
15,980
11,840
3,808
3,993
10.38
8.16
2.48
2.75
Cat (3)
Silver
Osmic
118 871
116 594
3,691
3,789
1,245
941
3.11
2.39
1.05
0.80
Rabbit
(3)
Silver
Osmic
264 611
260 767
11,010
11,310
3,628
3,813
4.16
4.34
1.40
1.46
White rat Silver
Osmic
(3)
74 813
59 261
672
576
227
344
0.90
0.97
0.30
0.33
Hooded
rat (2)
Silver
Osmic
80 133
79 130
2,930
144
988
61
3.66
0.18
1.33
0.06
Guinea
Pig (3)
Silver
Osmic
126 367
127 147
6,330
4,686
2,335
1,580
5.24
3.68
1.77
1.24
Pig
(4)
Silver
Osmic
680 780
683 413
40,380
33,161
9,633
7,903
5.93
4.85
1.41
1.16
Sheep
(3)
Silver
Osmic
648 773
643 933
6,750
7,383
1,436
3,484
1.06
1.17
0.35
0.39
donkey
(3)
Silver
Osmic
1,208 130
1,257 818
106,900
104,320
39,420
38,680
8.85
8.48
3.44
3.33
&an (10)
(3 )
Silver
Osmic
1 ,009 433
874 939
357,300
473,500
38,820
159,670
25.50
52.89
3f85
18.25
97.
TABLE XXI
SIZE AHD CBOSS-SEOTIONAL AREA OF 8ILV2RSTAIEED VERTEBRATE OPTIC NERVE;
ESTIMATED NUMBER OF FIBERS
PER UNIT AREA
Form
Av. Largest Av. Smallest Calculated
Bimension
Dimension
Area,
(micra)
(micra)
(sq. mm.)
Hagfish
Brook'lamprey
Dogfish shark
Sting ray
Guitar fish
Hackleback
Bowfin
Goldfish
Bullhead
76.1
88.8
744
371
884
406
824
’431
443
74.3
83.6
594
310
787
187
606
360
365
0.00415
0.00591
0.348
0.090
0.547
0.061
0.392
0.122
0.137
Total Calculated
Fibers Fibers
Per sq. mm.
1,579
5,217
113,150
39,663
74,636
13,507
113,735
53,954
26,262
380,000
930,000
330,000
440,000
140,000
220,000
390,000
430,000
310,000
Toad
Frog
311
493
171
349
0.042
0.135
15,433
28,912
370,000
210,000
Alligator
Horned toad
670
565
525
473
0.377
0.210
105,298
138,784
380,000
610,000
Duckling
Chick
Pigeon
Canary
1313
1069
1729
804
1038
908
1540
775
1.08
0.763
2.09
0.490
408,632
413,948
986,471
437,516
380,000
540,000
470,000
870,000
Bat
Dog
Cat
Rabbit
Rat
Guinea pig
Pig
Sheep
Monkey
Man
136
14-54
1308
1065
583
630
3182
2717
2138
4607
108
1443
1148
785
475
519
2470
3019
1954
2382
0.0108
1.65
1.18
0.656
0.218
0.356
6.17
4.31
3.24
8.65
6,934
153,712
118,871
264,611
74,812
126,367
680,780
648,773
1,208,120
1,009,433
640,000
90,000
100,000
400,000
340,000
490,000
110,000
150,000
370,000
120,000
474
402
617
1304
1781
374
360
499
1085
1330
0.188
0.088
0.243
1.11
2.63
48,967
23,167
117,041
559,142
429,240
380,000
29'0,000
495,000
565,000
380,000
AVERAGES:
Fishes
Amphibians
Reptiles
Birds
Mammals
VI.
SUMMARY OF RESULTS
Fishes showed an optic nerve composed of either all unmyelin­
ated fibers (Cyclostomata) or all myelinated fibers (Pisces).
The following averages of total fiber estimates (obtained from
silver preparations) were secured:
hagfish, 1,579;
5,217;\ dogfish shark pup, 112,817;
ray, 39,663;
brook lamprey,
guitar fish, 74,636;
sting
shovel-nosed sturgeon (hackleback), 13,555;
(dogfish), 113,735;
goldfish, 52,955;
bowfin
bullhead, 26,626.
The amphibian optic nerve revealed various degrees of unmyelination of its nerve fibers:
47.6$.
toad, 33.9$;
Average total fiber estimates were:
frog (Rana pipiens)
toad, 15,423;
Rana
pipiens, 28,912.
The reptilian optic nerve also showed some unmyelination
of its nerve fibers:
baby alligator, 22$;
horned toad, 36$.
The averages of the total fiber estimates were:
105,298;
alligator,
horned toad, 128,780.
The bird optic nerve showed complete myelination of all its
fibers.
These large nerves showed characteristically high fiber
estimates with exceedingly small fibers.
fiber estimates were secured:
pigeon, 986,471;
These average total
duckling, 408*633;
chick, 413,948;
canary, 427,516.
The mammalian optic nerve revealed great variation in the
myelination of its fibers:
bat, 43.7$;
white rat, 30.8$;
all
the other forms studied showed complete myelination of their fibers
Great differences in the total number of fibers contained within
the mammalian optic ner^e were noted in these estimates:
6,934;
74,337;
dog, 153,712;
cat, 118,871;
hooded rat, 80,122;
1,308,120;
man, 1,009,433.
rabbit, 264,611;
guinea pig, 126,367;
bat,
white rat,
monkey,
Northwestern
University
Uterary
Statistics on the dimensions of the optic nerves studied and
the number of fibers per unit area were summarized in Table XXI,
page 97.
As a group, the amphibians had the smallest optic
nerve with a fiber concentration of 390,000 per square millimeters.
The fish optic nerve was the second smallest but it had a much
higher fiber concentration:
380,000 fibers per square millimeter.
The reptile nerve was considerably larger than that ,of amphibians
and fishes, and it had a high fiber concentration of 495,000 fibers
per square millimeter.
The bird nerve was markedly larger than
that of reptiles, amphibians and fishes, and had the remarkably
high average fiber concentration of 565,000 fibers per square
millimeters.
The mammalian nerve was the largest of all the forms
studied, being more than twice the area of the bird nerve, but it
dad the lowest fiber concentration of any of the vertebrate
a
classes: 380,000 fibers per sqrjre millimeter.
Even though previous workers had found no appreciable differsnce in fiber estimates made on the chiasmal and bulbar ends of
the optic nerve, this point was rechecked on the long dog optic
lerve with these results:
bulbar end, 164,833;
chiasmal end,
163,396.
The rat results suggest the possibility of the existence of
some relationship between albinism and unmyelination of the optic
lerve fibers:
white rat (albino eye), 30.8$ fibers unmyelinated;
looded rat (pigmented eye), no unmyelinated fibers.
The number of fibers in the optic nerve may increase in the
postnatal period.
Shis:
The evidence in favor of such an increase is
1) baby alligator, 34 centimeters in length, 70,588 fibers;
53 centimeters in length, 106,964 fibers;
45 centimeters, 138,343;
3) baby chick (1^- weeks), 413,948;
adult chicken, 863,843
fibers;
3) cat fetus (64 days— 3 days before term), 66,395
fibers;
adult cat, 118,871 fibers.
101
VII*
A.
1*
Discussion
Material
Scone of the material:
The vertebrate material secured for this quantitative
optio nerve study represents a compromise between the availa­
bility of the various forms in this local region and an attempt
to obtain a representative of most of the great phylogenetic
groups*
Some classes have been morethoroughly studied than
others;
this is particularly true of mammals, for here such
variations in results occurred that an even more thorough
study than was possible would have been desirable*
If a class
showed considerable uniformity in results, e*g*, amphibians,
reptiles and birds, with the study being made on widely dis­
tributed orders within the class, there was a tendency to limit
the number of representatives*
A further limitation often
lead to the passing from one class to another:
the time factor*
Even though further material could be secured without great
difficulty, this was occasionally not done because insufficient
time was available to allow for a more thorough probing into
that particular class*
Since a survey of the Shtire vertebrate
soale was the prime objective of this study, detailed studies
of any one group had to be subordinated.
3*
Number of specimens in a series:
The problem of how many specimens to include in each group
series arose early in this study and it was then decided to est­
ablish a minimum of two with no set maximum, although actually no
more than four have been studied in any series except the human*
Many of the series represent just the amount of material that
was available and, even though one might desire another specimen
.
102
or two to round out the series better, this was usually not
possible without great difficulty,
3,
Use of single nerve or the pair?
As previously stated, both nerves of some specimens were
used in making these osmic and silver comparative fiber studies
because the nerves were too short to divide.
It is conceded
that the ideal procedure would be to divide a nerve into halves,
yet technically this is not always possible.
There is no data
available on the optic nerve to indicate what variation in the
fiber content occurs between the two sides but the nature of
the nerve would lead one to suspect that such a variation would
not be very great under normal circumstances.
Calculations
based on Rasmussen's (1940) figures for nerve VIII in the human,
show a maximum variation of 13,7 per cent between the right and
left sides.
The results obtained in this study on right and
left optic nerves show a muoh smaller discrepancy between the
two nerves, in fact, so small a difference that it is apparently
well within the experimental error factor,
B, Methods
1,
Staining:
The more recent staining techniques have done much to in­
crease the discreteness of the individual fibers and thereby to
increase the accuracy ofthe estimates by diminishing interpre­
tative ambiguity.
The sections stained by the silver nitrate-
protargol technique exhibit a remarkable contrast between the
purplish-black axis cylinders and the pale supporting elements.
This marked differentiation makes the procedure of counting
easier and practically rules out the possibility of confusing
axis cylinders and connective tissue strands.
The later sections
stained “by osmic acid are, in most instances, improved over
those prepared in the earlier part of the work, yet the
ambiguity of the fibers is still an important factor in the
making of enumerations on this type of material*
The size of
the nerve, the average size of the fibers contained in the
nerve, and some unknown species factor all contribute to the
ease of stainability with osmic acid*
In general, the larger
the nerve the more difficult it is to stain adequately with
osmic acid*
But there are conspicuous exceptions to this,
leading one to wonder if there might be another factor, which
varies from species to species, that exerts an influence on
the staining qualities of the myelin sheaths, e*g*, the very
large human and dog optic nerves blacken with ease, whereas
the smaller cat nerve is exceedingly difficult to prepare.
Hence, even though one gained considerable experience with
the stainability of optic nerves when they are exposed to osmic
acid, there always exists an uncertainty which cumulative
empirical observations do not dispell*
The bird optic nerves
were all very difficult to prepare, both because of the large
size of the nerves and the minuteness of the diameters of the
majority of the fibers*
As a group, the fishes offered the
least staining difficulties and birds the most.
Although the great difficulty often experienced in
preparing countable osmic-blackened sections was often dis­
heartening, persistence would usually yield some preparations
which were adequate for the purpose at hand.
The vapor method
has been mgoh more successful than the immersion technique,
although even it has occasionally failed.
The use of 4 per cent
osmic acid solution in the stead of 3 per cent, initiated late
104
in the study, shows considerable promise*
But even this
improvement leaves something to be desired;
perhaps this is
an inherent difficulty which can only be resolved by some
fundamental changes in the method, or possibly not at all*
It is certain that at present osmic acid is the best agent
available for the staining of myelin sheaths;
many workers
have tried other methods described in the literature, but
inferior results have forced them back to the use of osmic
acid.
That the fundamental difficulty may be an anatomical
one is intimated by the peripheral location of the myelin
sheath in relation to the other fiber structures*
The fibers
are so closely packed in a nerve that, even though the myelin
be adequately blackened, the rings appear continuous and may
resist resolution as entities under the microscope.
The
absence of a neurilemma sheath in the optic nerve also con­
tributes to this lack of demarcation between the individual
fibers*
This is particularly true ofnerves composed of numer­
ous small fibers which are, at times, so close to each other
that only an experienced eye can detect their boundaries.
The
end-result, in areas where fibers are densely packed, is a
blackened mass perforated by numerous spaoes— the latter
representing either the central space occupied by the shrunken,
unstained axis cylinder or a spaoe between fibers due to the
abutting of several of these circular structures.
Theoretically,
the space occupied by the axis cylinder should be round or oval
in contour whereas spaces between fibers should be irregular;
actually the spaces are often so small that this criterion fails.
However, the condition just described is not true of all optic
nerves stained with osmic acid nor is it usually present
,
105
throughout any particular nerve.
One must reiterate the
conclusion of Schaible (1934) that the silver—stained sections
are superior for enumerating purposes, yet with care reasonably
accurate estimates can be secured from the osmic—stained
sections.
3. Counting:
There is no method for the enumeration of nerve fibers as
yet available which is not open to some criticism.
Three
principal methods are found described in the literature:
the
ocular net, the photomicrographic, and the strip method.
Zalzer (1880) used an ocular with a single ruled square
for counting fibers in the human optic nerve.
He selected areas
for counting where the fibers were the most distinct so natural­
ly he did most of his counting at the extreme periphery where
the fibers are larger and less densely packed.
Since fibers are
not uniformly distributed throughout an optic nerve, the error
resulting from choosing a certain region for counting can be
very great.
A further difficulty in using a square lies in the
increased oppBrtunity for counting a fiber more than once.
Unless the square be very small it would be impossible to keep
the counted fibers separated from the uncounted ones.
The
squared ocular net was used by Lewin and Gaule (1896), Dunn
(1903), Duncan (1930), Ono (1931) and Rasmussen (1940).
Variations of the nets have also been described by Bazin (1919).
The method of counting fibers through the use of the photo­
micrographs as guiding maps appears to have been first used by
Hardesty (1899).
In this method photomicrographs of a high
magnification are taken and then the fibers are checked off with
colored ink as counted from the corresponding field under the
,
106
microscope.
If the entire cross—section of a nerve is counted,
the method's accuracy is limited by the care with which the
procedure is carried out and the ambiguity of the fibers.
How­
ever, it is impractical to count all the fibers of a large
nerve so the use of the sample areas materially shortens the
process of enumeration.
When using sample areas the accuracy
depends upon the uniformity of the distribution of the fibers,
the accuracy of the measurement of the counted area and of the
total cross-sectional area, and the ambiguity of the fibers.
In addition to Hardesty, this method was used by Dale (1900),
Hatai (1903), Duncan (1930), and by several wokkers in this
laboratory:
Schaible (1934), Lander (1937), Bickel (1935),
and Castanares (1935).
Adrian and Matthews (1937) counted the fibers in the optic
nerve of the conger eel from camera lucida drawings.
This method
is obviously impractical for use on nerves with high fiber
content.
The strip method was first used by Davenport and Bothe
(1934) to enumerate the cells in large dorsal root ganglia.
Davenport and Barnes (1935) did further work on the method and
tested its accuracy.
Small nerve fibers were counted by both
the photomicrographic and the strip method and a maximum differ­
ence of 3.9 per cent between the two counts obtained.
Then
these workers considered the possibility of estimating the
number of fibers in a cross-section of a nerve trunk by counting
only a small percentage of the total fibers.
These estimates
were found to agree reasonably well with the total counts;
obviously with a greater error when very small percentages were
counted.
Norris (1938) used this strip method in estimating
.
107
.
the number of myelinated and unmyelinated fibers in the optic
nerve of the turtle and opossum.
He found the method rapid,
adjustable to any size of nerve, and accurate.
The accuracy
of this method is dependent upon the regularity of the shape of
the nerve, the carefulness of the setting of the strip, and the
adequate identification of every fiber counted*
In the hands of
careful workers, this method is believed to be the most reliable
of the various procedures for the enumeration of nerve fibers.
0.
Souroes of error
The use of right and left optic nerves for comparing estimates
of osmic-blackened sections with silver-stained ones introduces
a definite possibility of error due to actual variations in the
total number of fibers between the nerves of the same animal.
The closeness of the check between the two counts in most inst­
ances, however, indicates that this variation between individual
nerves in a pair of optic nerves probably is negligible and is
well within experimental error under normal conditions.
There is a possibility of error in the staining techniques,
particularly the silver procedure.
It is quite within the realm
of possibility that many exceedingly small fibers were not stain­
ed by the techniques used.
The only absolute check upon this
possibility would be a comparison of the nerve fiber counts with
ganglion cell counts. The only retinal ganglion cell count that
a
is a^ilable was made on the dog by Gore (1937), and this worker,
comparing his ganglion cell counts with fiber estimates made on
optic nerves stained with Davenport's (1930) alcoholic silver
nitrate technique, found a reasonably close correspondence
between the two.
On spinal nerves, Holmes and Davenport (1940)
have secured very close checks on their dorsal root counts
and dorsal ganglia enumerations.
These nerve roots were stain­
ed by the silver nitrate-protargol technique and lend strong
evidence to the liklihood that this method stains all the axis
cylinders.
Earlier silver methods are certainly not above
suspicion on this point, e.g., Ono's (1931) exacting enumerat­
ions made on cat spinal nerves may be unacceptable because it
is possible that the method he used did not stain all the fibers.
The conclusion that the silver nitrate-protargol method stains
all the axis cylinders is inferentially supported in this study
by the appearance of the stained sections and the closeness of
the checks between the several units of the same species.
One
would be suspicious of the possibility of there being unstained
axis cylinders if they were noted to become gradually smaller
with no apparent grouping into size categories.
But if the size
abruptly ceases diminishing at a certain diameter, as was noted
in the nerves prepared for this study, it would seem logical to
conclude that there are no smaller unstained axis cylinders
present*
This observation, in conjunction with the closeness of
the agreement between the units of a series, coupled with the
correspondence between the ganglion cell and dorsal root estimates
in the cat series of Holmes and Davenport, gives strong support
to the belief that all the axis cylinders are stained by the
silver nitrate-protargol technique and,therefore, that this source
of error was not present in this study.
The counting technique has certain inherent errors but these
can be minimized by careful attention to detail.
The figures
presented as the total number of myelinated and unmyelinated
109
fibers in an optic nerve are estimates secured by counting a
certain percentage of the fibers, except in the instance of the
hagfish and brook lamprey, where total counts were made.
The
number of fibers actually counted in the making of these est­
imates varied from 4 to 33 per oent of the total number.
Differing percentages were counted because of extremes in nerve
and fiber size, and regularity of nerve shape, made any set
schedule impossible.
Rasmussen (1940), using ocular squares,
made total counts and estimates on the same human VUIth nerve
and found that estimates made by counting roughly l/9th the
area of the nerve showed a maximum variation of 5 per cent from
the figure secured by making a total count.
Similar comparisons
have been made on a few of the smaller nerves used in this study
and, provided well-stained sections with a regular outline are
used, estimates made by counting 10 per cent of the total fiber
content are well within 5 per cent of the total count.
It is
believed that estimates made on very large nerves by counting as
few as 5 per cent of the total number of fibers are as accurate
as those made by counting higher percentages of the fibers in
small nerves.
Probably the chief source of error in the strip method is
inaccurate setting of the strip.
One must make a mental picture
of some landmark in relation to one line of the strip and then
cautiously adjust the mechanical stage so that the second line
will exactly replace the former.
When counting a large nerve
this method of advancing the strip must be carried out hundreds
of times;
hence, it is difficult to prevent some error despite
the utmost care in performing the movement.
This eTror, although
doubtless present in the results of this study, has been
.
110
minimized by carefully repeating the strip settings until a
series of close agreements was obtained.
It addition to the
correct setting of the strip, it was noted that the selection
of the first strip to count influenced the final estimations
slightly in the smaller nerves.
For this reason a uniform
initial setting of the strip was always carried out in order
to keep this factor constant:
the first strip at the lowest
edge of the nerve was always counted, thus permitting the last
counted strip at the top of the cross-section to fall entirely
by chance.
The possibility of confusing axis cylinders with strands of
supporting tissue has been minimized by the use of the silver
nitrate-protargol staining technique.
There is a greater possi­
bility of error in the osmic estimates because of the occasional
inability to resolve the fibers clearly due to the closeness of
their myelin sheaths.
Initially minimum and maximum estimates
were made on the osmic sections but with increased confidence
and experience, it was felt that this was an unnecessary compli­
cation of the counting technique.
P.
1.
Evaluation of results
Relation of the results to the material:
The results obtained in the fishes are summarized in Table
IX (page 78) and reveal that all the fibers in the optic nerve
of thehagfish and brook lamprey are unmyelinated, whereas all
the fibers in representative elasmobranchs, ganoids and teleosts
are myelinated.
The sole exception to these otherwise uniform
results occurred in the dogfish shark pup series.
Shark pup #3
(starred in Table, IX, page 78) shows, a deficit of about 16 per
.
111
.
cent in its osmic estimations when compared with the silver
total.
The other three specimens in this series show close
agreements between their osmic and silver estimates.
Careful
recounts showed that the enumeration was correct so ome must
conclude that this deficit actually existed in the left optic
nerve of this particular shark pup.
to either of two things:
Such a deficit may be due
1) the presence of about 17,000 un­
myelinated fibers in this left optic nerve, or 3) the presence
of some unilateral malformation of that eye and nerve, leading
to a diminution in the number of fibers on that side.
The close
agreement of the estimates on the other members of this series
and the absence of any comparable occurrence in the other groups
leads to the conclusion that this particular discrepancy is most
likely due to a malformation which produced a partial optic
atrophy on that side of the animal.
The findings of two instances
of optic atrophy and deformities of the eyes in four baby chicks
also points to the likelihood that this also occurred in the
baby shark series.
Upon reaching this conclusion, the animal
was dropped from the series and its totals were not averaged in
the final figures on myelinated and unmyelinated fibers.
In the amphibian series, the results obtained on the toad
may be deceptive:
mens were used:
the preparations were inferior.
Two speci­
in one (#3 in the series), the osmic and silver
preparations were reasonably good;
in the other (#1), the
preparations were so poor that they bordered on failure.
Both
osmic preparations were enumerated first and it was felt that
both of these were accurate, with #3 being the most reliable.
Then the silver-stained sections were counted, and in this group
of estimates the reliability is questioned, although it is felt
112
that the results on #2 can be trusted.
.
The fact that the
results on #1 check closely with #2 may thus be purely coin­
cidental but since the estimates were painstakingly made, it
is felt that they at least give some inkling as to the situat­
ion in the toad.
The frog estimates are believed to be reliable,
and since the toad shows the same trend as the frog, the accuracy
of the toad results becomes more acceptable.
The finding of a
large percentage of unmyelinated fibers in the toad and the frog
checks well with Lander's finding of --only unmyelinated fibers
in Necturus and 41 per cent unmyelination in Amblystoma, and
Howe's finding of 57 per cent unmyelination in the bullfrog.
The finding of unmyelinated fibers in reptiles (21.5 per
i
cent in the alligator and 36.1 per cent in the horned toad)
agrees with Norris' report of 41 per cent unmyelination in the
optic nerve of the turtle.
are believed to be accurate.
The results obtained in reptiles
The great variation between the
three members of the baby alligator series is of great interest
in view of its possible bearing on the question of an increase
in the number of neurites in the optic nerve in postnatal life.
The discovery of complete myelination of the fibers in the
optic nerve of birds is entirely in line with what one would
expect, as will be pointed out later.
These results are consider­
ed to be reliable even though the osmic-stained sections in
birds were not as good as in some of the other groups.
The very
high fiber content of these nerves and the extremely small size
of the individual fibers were important factors in the difficulty
of securing adequate osmic preparations.
Even though the osmic
acid blackened the myelin sheaths perfectly, the fibers were so
113
small and packed so closely together that resolution and
individual identification was very difficult*
This tendency
was so extreme in the osmic preparations of the canary nerve
that no attempt was made to enumerate the myelinated fibers.
The mammalian results show great variation, perhaps indi­
cative of the great dissimilarities that exist in this class
of vertebrates.
The finding of 43*7 per cent unmyelination of
the fibers in the bat, and of 20.6 per cent in the white rat
can be compared with Norris* discovery of 33 per cent in the
opossum.
All the other mammals studied in this work showed
complete myelination of their fibers.
The finding of 20.6 per cent unmyelination in the optic
nerve of the white rat and no unmyelination in the hooded rat
is of great interest.
In seeking for an explanation for this
unusual finding, one is tempted to attribute it to the lack of
pigement in the white rat's eye.
But such an explanation is
unwarranted on the basis ofthis meager evidence;
however, the
discovery is of great enough interest to be worth a further
investigation into the state of the myelination of the fibers
in the optic nerve of other albino forms.
The excess of 4.3 per cent of the osmic averages over the
silver in the monkey is an indication of the tendency to over­
enumerate in osmic preparations where the fibers are very small
and closely packed together.
It is not believed that the osmic
results are so high as to obscure the presence of any unmyelin­
ated fibers.
Only two osmic preparations were enumerated in the human
series beoause Schaible (1934) had previously concluded, on the
.
114
basis of substantial evidence, that there are no unmyelinated
fibers present in the human and these two in this series
confirmed that belief.
The remaining nerves in the series
were stained with osmic acid as well as silver but only est­
imates were made on the silver preparations.
The series was
made a large one because the material was readily available
and enough variation was found to make it desirable to run a
long series in order to establish the range of normal variation.
The very low estimate of 564,776 fibers in nerve #1 is of
interest because it brings up the question of just how great
a variation from the average can occur and still be considered
normal.
The clinical history of the patient from whom this
nerve was obtained (vide Table III, page 44) gives some evidence
indicative of the presence of a partial optic atrophy.
In view
of this evidence suggesting abnormality, nerve #1 was excluded
from the final averages in the human series.
3.
Relation of the total fiber number to the age of the
specimen:
Boughton (1906) found an increase in the number of fibers
in the oculomotor nerve of the white rat and the cat between
birth and adulthood.
It is erroneously stated in Quain (Vol.
Ill, p.239, 1907) that this work was done on the optic nerve.
Whitnall (1933), using only Boughton's observation, states that
there is an increase in the number of optic nerve fibers with
age.
Hence, statements in the literature that there is an
increase in the number of fibers in postnatal life are all
based on Boughton's work on the oculomotor nerve.
The wisdom
of applying results obtained on another cranial nerve to the
optic nerve should be seriously questioned.
.
115
.
Some evidence has been accumulated in this study, however,
indicating that this increase in the fibers might actually occur
in the optic nerve.
The evidence is this:
Baby alligator. 34centimeters inlength, 70,588 fibers
33
#n
a 9 106,964
«
45
“■
11 ,138,343
■
Chicken. l i weeks old, 413,948 fibers
full grown,
863,843
11
Cat,
fetus of 64 days, 66,395fibers
full grown,
118,871 11
Biekel (1935) estimated the fibers in an eleven month
infant and found about the same number as in the adult and so
concluded
that there is no increasewith age. But the increase
may have occurred between birth and elevenmonths, soBiekel*s
conclusion should be restated in this form:
there is no increase
in the fibers in the human optic nerve after the eleventh m8nth*
This problem is far from being settled;
careful studies
should be carried out in different forms and at regular age
intervals to determine fcbw great the increase is and when it
ceases.
In view of this possibility of an increase, the total
fiber numbers for the dogfish shark, alligator, duckling and
chick may be too low.
3.
Correlations between results and state of development of
the visual system
a.
Description of retinal specializations
1).
a).
Rods and cones
Definitions:
The percipient elements of the retina are all comparable
in general organization but each has its distinctive character­
istics.
These definitive traits have led to the division of the
116
visual cells into two great subgroups:
rods and cones.
.
These
rod and cone elements of the retina are usually considered to
be specialized neuro-epithelial cells and not nerve cells (Arey,
1932).
A number of characteristics point to this conclusion,
such as the epithelial appearanoe of their outer parts and
their position as boundary cells in the cavity of the primitive
optic vesicle.
The differentiation of rods from cones has often been sub­
ject to much discussion.
The presence of confusing forms, or
even the suggested possibility of the occurrence of intermediate
forms, has led to many disputes and has resulted in much con­
fusion in the literature.
Walls (1933b) has established this convincing set of
criteria for the differentiation of rods from cones on a mor­
phological basis:
RODS
CONES
Connected in multiple to bipolar
cells.
Connected singly, or nearly
so, to bipolars.
Outer segment of maximal volume
in order to carry on maximum
photomechanical activity and
therefore cylindrical.
Outer segment of minimal
volume, and thus conical
unless segment is filament­
ous, as in Muridae.
Contain visual purple when darkadapted.
Never contain visual purple.
Elongate in light and contract
in darkness, if they move at
all.
Elongate in darkness and
contract in light, if they
move at all*
Predominate in nocturnal ani­
mals, sometimes to the exclu­
sion of oones.
Predominate in dtumal ani­
mals, sometimes to the ex­
clusion of rods.
Often have a compact end-knob
devoid of later branches.
Always have a dendritic
end-bulb.
Never contain colored oildroplets.
Often contain colored oildroplets.
117
The histogenesis of the percipient elements has been
variously interpreted:
I 1.
Cones are specialized rods whose development has just
extended farther (Parsons, 1915;
3*.
Kerr, 1919).
Cones represent early stages in the formation of rods
(Bernard, 1903;
3 1•
Cameron, 1905, 1911).
Both rods and cones develop simultaneously and differ­
entiate divergently from a relatively indifferent
progenitor (Detwiler and Laurens, 1931).
The third view seems to be the soundest and there is solid
experimental data to support this belief.
The relationship between rods and cones and the life habits
of animals has been the subject of considerable research.
Wunder
(1935, 1936) studied the visual oells and pigmented retinal
epithelium of fishes in relation to their life habits*
interesting correlations were found in this study:
Several
rods of
bright-light fishes are large, robust, and few in number;
on
the other hand, dim-light fishes have extremely small and slender
rods which are present in great numbers (the only exceptions
were in dim-light fishes with poorly developed eyes).
showed no such correlations:
cones.
Cones
both types have long and robust
Apparently, then, the dim-light habitues command cones
as efficient as those of the bright-light group.
Only dim-light
fishes with poorly developed eyes (e.g., bullhead) have small
cones reduced in number.
Detwiler (1939) makes this statement in relations to the
influence of visual habits on the structure of the eye:
N...so closely correlated is the mode of life of the animal
with the structural make-up of the eye, that one can predict
with reasonable assurance something of the habits o f the
.
118
.
animal as well as its visual ability from a histological
examination of its retina.■
8
b).
Distributions
The relative distribution of rods and cones varies widely
between and within the various classes of vertebrates.
This
distribution will be summarized in the paragraphs to follow.
Myxoids:
nothing has been located in the literature.
Petrpmyzons:
both rods and cones are present.
The rods are
large, few in number, and evenly distributed in the retina
of the brook lamprey (Walls, 1938).
No twin cones are
present.
Elasmobranchs • dogfish sharks and the rays, nocturnal verte­
brates, possess only rods in their retinas (Schultze, 1868;
Greeff, 1899;
Arey, 1933;
Ganoids and teleosts:
Bayliss, 1936;
Detwiler, 1939).
Salmonidae possess both rods and cones,
with many cones being the twin type (Bayliss, 1936).
The
true eels (Anguillidae) have rods and cones in their retinas,
with all the cones of the single type.
On the other hand,
the conger eels have only rods (Bayliss, 1936).
Goldfish
have very small rods and both single and double cones
(Howard, 1908).
The bullhead retina contains both rods
and cones (Arey, 1916a;
Welsh and Osborn, 1937).
The
pollack, mackeral and gunard similarly possess both rods
and cones, the cones being mostly of the twin types
(Bayliss, 1936).
Urodeles:
rods and cones are both present (Detwiler and
Laurens, 1931;
the double type.
Johnson, 1935);
some of the cones are of
The ratio of rods to cones, as stated
by Laurens (1917) for Amblystoma, is 4:3.
The ratio in
119
Heoturus, as given by Howard (1908), Is:
cones, 1;
Anuxa:
rods, i;
.
single
and double cones, 1.
both rods and cones are present but the rods are the
predominant type (Howard, 1908;
1935).
Crawford, 1936;
Granit,
The frog has small rudimentary cones and very
large wods of two types:
a) “violet“ rods with a long
cylindrical external segment, and b) smaller “green“ rods
with a long delicate internal segment (Duke-Elder, 1933).
Testudinata:
and double
turtles and tortoises possess largelysingle
cones with a few rods (Walls, 1934a).
Rhynchocephalia: Sphenodon is the subject of disagreement
in the literature.
Walls (1934a) states that this reptile
possesses,a pure rod retina.
On the other hand, Mann (1933)
describes it as a pure cone retina and mentions the presence
of several types of cones.
In view of the fact that this
reptile is nocturnal in its light habits (Gadow, 1909;
Walls, 1933), it would be most unusual to fine a pure cone
retina.
Crocodilini:
the alligator has a retina made up predominately
of rods, but a few single and double cones have been seen
(Laurens and Detwiler, 1931;
Squamata:
Walls, 1934a).
the chamelion possesses a pure eone retina (Rochon-
Duvigneaud, 1933).
The geoko, a nocturnal lizard, has a
pure rod retina (Detwiler, 1933a;
Crawford, 1936).
The
horned toad, a common dirunal lizard, has a pure cone
retina (Detwiler and Laurens, 1930).
Snakes have a pure
cone retina (Walls, 1934a).
Struthioniformes:
the ostrich has mostly cones, but some rods
can be located in the periphery of the retina (Kajikawa,l923j
130
Casuariiformes:
.
the periphery of the emu retina is largely
made up of rods whereas the central area contains more
cones than rods (Kajikawa, 1933)*
Anseriformes:
both rods and cones are present in the goose
(Ka jikawa, 1933)•
Charadriiformes:
pigeon*
cones dominate the retinal picture of the
The area centralis is completely free of rods
(Granit, 1935)*
Coraciiformes:
the owl was once described as possessing a
pure rod retina, but later observers have questioned this*
Granit (1935) states that the owl*s retina probably contains
many cones*
Hess (1913) has placed the figure for the
number of cones at 3,500,000*
Marsupials:
the opossum, a nocturnal animal, possess abundant
rods and scanty cones*
type (O'Day, 1936;
Some of the cones are of the double
Walls, 1939a)*
The kangaroo has both
rods and cones, with only single cones*
The rod/cone ratio
is said to be 3-41 1 (Hoffman, 1876)*
Insectivores:
it is questioned whether rods or cones are
present in the shrew.
At any rate, the number of percipient
elements is said to be few (Woollard, 1934).
In the mole,
the identification of the visual elements is also disputed.
Rochon-Duvigneaud (1938) states that they resemble cones
more than they do rods whereas Sloanaker (1903) describes
both rods and cones*
Ghiroptera:
the bat is believed to possess a pure rod retina,
although the matter does not seem conclusive (Detwiler, 1934;
Kolmer, 1934)*
131
Carnivores:
the oat has a retina made up largely of rods
with a very few cones (Granit, 1933).
Rodents:
the rabbit has a retina predominately of rods
(Granit, 1933).
The field mouse has a pure rod retina
and the rods are described as being very numerous
(Detwiler, 1934).
The white rat has both rods and cones
(Walls, 1934b).
Primates:
Tarsius, a nocturnal primate, has a pure rod
retina (Woollard, 1934;
Detwiler, 1940).
The marmoset
has both rods and cones, with a macula containing only
cones (Woollard, 1934;
Detwiler, 1939).
has both rods and cones (Detwiler, 1939).
Macacus rhesus
Man has both
rods and cones, their distribution varying with the region
of the retina.
The fovea contains only cones, about 0.8
millimeters from the fovea there are small rods and large
cones with the rods being slightly more numerous, 3 milli­
meters from the fovea the rods are far more numerous than
the cones, at the ora serrata the cones are again more
numerous (Duke-Elder, 1933).
Krause (1879) has calculated
that there are 130,000,000 rods and 7,000,000 cones in the
human retina;
Zalzer (1880) places the number of each at
about half Krause’s figures.
In general, then, the relative distribution of visual cells
in most fishes, amphibia, and mammals is such that there are more
rods than cones.
rods;
In birds, there are usually more cones than
whereas the retina of most diurnal reptiles is composed
exclusively of cones.
One should note that nocturnal vertebrates
may lack cones (e.g., sharks and rays, geckos, field mouse,
.
132
tarsius);
.
yet there are other nocturnal forms that have a
few rudimentary cones (e.g., owl, opossum, rabbit, cat).
3).
Specializations for increasing visual acuity:
a^ea
centralis, fovea centralis, macula lutea
A local modification of the retina for maximal visual acuity
is of widespread occurrence among vertebrates.
This modified
area may be small or extensive, a broad horizontal band across
the fundus or a suprapapillar crescent, round or in the shape
8- variously oriented oval, located centrally or temporally
even as far as the periphery and slightly above or below the
horizontal meridian.
The name usually given to this region of
high resolving power is area centralis.
If the regions contains
a transparent yellow pigment, as it does in some primates
(Dimmer, 1907), it is called a macula lutea.
ia not always smooth;
The area centralis
it may contain a central depression which
might be round or oblong and shallow or deep, the fovea centralis.
a).
Structure:
An area centralis may be distinguished from the surrounding
retina only by the visual cells being more numerous in it per
unit retinal area.
In this highly developed region, the cones,
which mediate sharper vision (Duplicity Theory), take precedence
\
over the rods.
Their diameters are reduced, and they are tight­
ly packed together.
This results in a piling up of the outer
nuclear layer, and the other layers are correspondingly thicken­
ed by an actual increase in the ratio of ganglion cells to bi­
polar cells and that of bipolar cells to visual cells.
T{ie area
centralis represents a compromise between the development of
vision of high acuity but with low sensitivity, and vision of
high sensitivity but with low acuity.
123
The foveal depression is created by the centrifugal dis­
placement of the retinal elements in the two neuronic layers;
hence, the inner nuclear, inner plexiform and ganglion cell
layers are virtually absent.
The fovea is thus a “thin spot11
in the retina located at the base of the visual axis.
b).
Distribution:
The distribution of the area centralis in the vertebrate
scale is of considerable interest.
Kalt (1905) considered it
to be entirely absent from fishes but other workers (Carriere,
1885;
Krause, 1886;
Hess, 1913) have found evidence indicat­
ing that in elasmobranchs and teleosts some approach to central
specialization is present.
amphibians;
An area centralis is found in some
it occurs regularly in reptiles, but it is not
until birds are reached that a well—developed area can be recog­
nized (Duke-Elder, 1933) and it is in this group that two foveae
may occur in diurnal birds of prey (Wood, 1917).
Among mammals,
although a central area is absent in some (rat, mouse, sheep),
the majority show some such specialization, usually in the form
of a shallow depression.
Only in some monkeys and man is there
a true macula lutea with a fovea centralis subserving binocular
and stereoscopic vision.
The profiles of foveae show numerous rather striking
variations.
Of all the foveae, the most highly developed are
those of the diurnal sauropsidans.
Walls (1937) called the
fovea of a lizard or of a bird convexiclivate (sloping convexly)
in contrast to an extreme concaviclivate form in man.
The
temporal fovea of birds (e.g., hawk), which is newer so welldeveloped as the nasal one, may also be concave but not so
broadly as that of man.
Shallow foveae occur in poor-sighted
.
124
birds, e.g. woodpecker (Walls, 1937).
The semidomesticated
pigeon has a shallow and variable fovea;
and owls have practically none.
.
the domestic fowl
The reptile fovea, as already
intimated, is customarily deep and well—developed;
yet
Sphenodon, whichhas secondarily adopted nocturnality (Walls,
1934a) has a broadened, shallow fovea.
The fovea of the honi­
ed toad is particularly well-developed (Detwiler and Laurens,
1930) and is often cited as the best example of a maximal fovea.
When compared to some of these foveae in lower forms, the human
fovea actually seems crude.
o).
Possible significance of the foveas
This “thinning” at the fovea has been traditionally regarded
as an adaptation to permit light to reach the oones unimpeded.
This explanation does not seem to fit the observations made on
the foveal profiles, however.
If such an explanation be true,
then why does the homed toad have a much deeper fovea than that
of the hawk or of man?
Undoubtedly both have a visual acuity
far superior to that of the horned toad.
Walls (1937, 1940) has
sought for an optical e3qplanation for the function of the fovea.
He has theorized that the retinal image is enlarged by refraction
at the clival (sloping) surface of the fovea.
The proof of this
theory depends upon the accurate determination of the actual
retinal refractive indexes and some such measurements have been
made (Valentin, 1879).
Use of these measurements indicates that
the human fovea is too shallow to make for any appreciable image
expansion, whereas approximately a 30 per cent enlargement of the
image would occur in the hawk's fovea.
This has le&d Walls (1940)
to conclude that the human fovea has degenerated, like that of
Sphenodon, owls and pigeons, from a once much more deep and
abrupt depression.
125
5) .
a).
.
Photomechanical movements
Definitions
Rods, cones and retinal pigment undergo positional changes
in bright and dim light.
The chief natural agent that produces
these demonstrable effects on the retinal pigment is light.
Although these movements are quite like those seen in the more
Usual types of dermal chromatophores, they were not noted until
1856 when H. Muller mentioned such movements but it was not until
1877 that Boll and Kuhne independently discovered that suoh move­
ments are correlated with light and darkness.
The myoid of visual cells is the part that actively responds;
its change in length may be as great as 10: 1.
The cone myoid of
fishes, anura and birds elongates in darkness and shortens in
light.
In urodeles, reptiles and mammals the change is slight.
The rod myoid responds in exactly the opposite manner:
gates in the light and shortens in the dark.
it elon­
This movement
occurs extensively in fishes, anura and birds, bbt is negli­
gible or absent in urodeles, reptiles and mammals (Arey, 1932).
b).
Distribution:
Kuhne expressed the belief that a retinal pigment migration
would be found in the eyes of all vertebrates, but this opinion
has not been entirely confirmed.
The eyes of fishes almost in­
variably exhibit a well-pronounced pigment migration;
Walls
(1938), however, found no evidence of retinal pigment migration
in the brook lamprey, certain elasmobranchs, such as Scyllium,
and in Torpedo (Garten, 1907) the pigment cells are completely
devoid of pigment.
In amphibians the retinal pigment migration may be as
pronounced as it is in fishes.
This has toeen studied in the
126
frog, toad, Triton, Neeturus and Amblystoma.
.
In these forms,
and especially in Neeturus, the migration is said to be much
less pronounced than in the frog (Parker, 1933).
The movement of the retinal pigment in reptiles was a
question of muoh uncertainty to the older workers.
More recent
work has favored the view that reptiles show a slight but un­
questionable pigment migration.
Detwiler (1916) demCnstrated
such a movement in three turtles and one lizard.
Laurens and
Detwiler (1921) showed this process in Alligator mississippiensis.
Detwiler (1923a,b; 1924) made further discoveries of its
presence in the eye of the lizards Eremias and Gecko.
The
limited amount of these migrations as compared with what is
seen in the frog is probably what lead to the confusion of the
early workers*
In birds a retinal pigment migration is readily demonstrable.
Angelucci (1873) noted it in the owl;
van Gen^ren Stort (1887a,
b) and Garten (1907) observed it in the pigeon;
Krause (1894)
showed it in the common fowl, Gallus domestica.
In all these
Instances the migration is much more pronounced than in reptiles
and approaches the conditions found among lower vertebrates.
Inmammals a retinal pigment migration has never been satis­
factorily demonstrated,
Angelucci (1878) claimed that such a
migration is present in the rabbit but this was not confirmed by
Deutschmann (1882).
Garten's careful tests (1907) on the ape,
ox, rabbit and rat fai&ed to demonstrate any significan differ­
ences.
Detwiler's observations (1934) on the field mouse and
bat showed a pigment, scanty in amount and motionless under
changes in illumination.
These results lead to the conclusion
that in mammals the retinal pigment migration is at best
137
reduced to its lowest terms if in fact it occurs at all.
It
would be inferred from these conclusions that such a pigment
migration would also be absent from man (Arey, 1915b).
b_»
1).
Central connections of the vertebrate optic pathway
Mvxinoids:
The optic nerves are exceedingly small and Jansen (1930)
was unable to trace them to the region which He regarded as the
optic tectum, although he thought that they might reach that
area in company with postoptic commissural fibers.
The absence
of eye muscle nuclei and vestibular connections in correspondence
with the great reduction of the eyes and the absence of eye
muscles and of the acoustic-lateral apparatus makes absolute
identification of the mesencephalon and optic tectum exceeding­
ly difficult (Areins Kappers, Huber and Crosby, 1936).
3).
Petromvzon:
All the associated nervous structures of the eye are com­
pletely developed, and as a result the mesencephalon has the
chief characteristics which are found in higher vertebrates.
The major portion of the optic tract seems to end in the optic
tectum (Ariens Kappers, Huber and Crosby, 1936).
The optic
tectum is still partly ependymal in character but it shows a
surprising development of lamination when the degree of organ­
ization of the other parts of the brain is considered (Huber and
Crosby, 1934).
Within the tectum there is correlation of optic
impulses with those from the cervical cord and bulbar centers
by way of a spino-mesencephalic tract and a bulbar lemniscus.
A few optic fibers terminate in a small group of cells whioh have
shifted peripheralward under the neurobiotactio influence of the
.
138
optic tract;
.
these are probably to be regarded as a primitive
lateral geniculate nucleus (Halmgren, 1919).
.3)... Elasmobranchfl:
The midbrain is better developed than in cyclostomes because
of the more marked development of the optic nerve and the increase
in secondary projection paths of the medulla and spinal cord.
The optic tract is the most important afferent tract of the tectum
(Areins Kappers, Huber and Crosby, 1936).
After its total decus­
sation near the region of the preoptic recess, the tract passes
dorsocaudalward along the lateral wall of the diencephalon,
giving off in its course collaterals to the lateral geniculate
nucleus.
Before reaching the tectum, the tract divides into two
branches, a smaller medial and a larger lateral, and these
branches, with fibers of the transverse commissure and brachium
tecti, form the stratum medullare externum.
The optic fibers
are distributed to the upper third of the tectum and pass on
light impulses to various intercalated neurons which lie near to
the superficial surface of the tectum, these in turn making con­
nections with the acoustic and lateral line centers.
Thus the
tectum in elasmobranchs is a center for the correlation of optic
with other exteroceptive impulses;
an expression of the fact
that the gravi static and photostatic impulses are here strongly
correlated and that they are inrelation with the primitive vital
sensibility of the body and particularly the head.
4).
Ganoids and teleosts:
The optic tectum is present in all forms but its degree of
development shows important differences in various forms.
cytoarchit ecture of the tectal region shows a considerable
The
advance over that in elasmobranchs.
Soon after its total
decussation, theoptic tract gives off a small bundle, the
fasciculus medialis nervi optici, which enters the dorso—
frontal portion of the diencephalon (Jansen, 1939).
The main
optic tract runs dorsocaudalward along the external wall of
the diencephalon#
As it reaches the level of the lateral
geniculate nucleus it gives off fibers to this nucleus;
these
fibers are believed to originate in the posterior quadrants of
the retina (Zeeman and Lubsen, in Ariens Kappers, 1921).
Before
entering the tectum, the main optic tract divides into two parts:
medial optic tract and lateral or ventral optic tract.
Both
divisions either enter the outer part of the tectum, the stratum
opticum, from which fibers turn into the stratum fibrosum et
griseum superficial©, or certain of the fibers swing directly
into this superficial receptive layer (Jansen, 1939;
1934).
Header,
No basal optic root such as is present in the amphibians,
reptiles, birds and mammals has been demonstrated in gadoids
and teleosts.
Blind fishes have a relatively small optic tectum
and a reduced number of cell layers receiving optic tract fibers.
The optic nerves have rarely, if ever, been actually traced from
the eye to the brain in these forms (Ramsay, 1901;
1933).
Charlton,
As in elasmobranchs, the tectum is a correlative center
for optic and non-optic exteroceptive impulses which have to do,
for the most part, with determining the relations of the body to
surrounding objects.
5).
Amphibians:
The morphologic and histologic characteristics of the brain
of Neeturus indicate that it is primitive even if in certain
respects degenerate in type (Herrick, 1917).
The tectum has
130
.
several rather indistinct layers, suggesting that there has not
been as yet a differentiation of a layer chiefly receptive in
character, such as is found in teleosts and reptiles.
The optic
nerve of Neeturus has retained its embryonic character, the
space inside being continuous with the third ventricle.
optic bundles have been divided into three groups:
The
a superficial
or marginal bundle which terminates in the tectum, an axial
bundle which passes more deeply to the tectum (Herrick was unable
to trace this bundle to its termination), and a basal optic
bundle which passes caudal ward to a nucleus in the base of the
midbrain, comparable to the nucleus of the basal optic root of
reptiles.
The optic tectal region of the frog is relatively large
when compared with Neeturus.
However, it still retains much of
a primitive character when this area in the frog is compared
with that of higher forms.
Examination of the optic tectum of
the frog reveal? features resembling the pattern described for
fishes on the one ha&d and for birds and reptiles on the other*
The optic tectum of urodeles is more primitive than that of anura,
both being less highly developed than fishes.
The optic tracts
of anura have been divided into three bundles, the same as
described for Neeturus;
in this group, however, the axial bundle
can be clearly traced to the optic tectum (Wlassek, 1893).
It is
evident that the tectum receives a considerable number of afferent
fibers and so, in general, it can be stated that the midbrain is
an important correlation center, both for optic impulses and
those various vital Impulses which play a role in primary
muscle sense*
131
6 ).
.
Reptiles:
The optic nerves are completely crossed at the chiasma,
where they interlace in the characteristic manner described
for the alligator by Gross (1903).
The main or marginal bundle
of optic fibers runs caudalward and dorsalward to the optic
tectum, which it enters, forming the stratumoptiaum.
The optic
tectum itself has been found divisible into fourteen l&yers
(Huber and Qrosby, 1933), indicating a high grade of functional
activity.
An axillary optic tract has been described which
enters, to a considerable extent, into the medial and ventral
parts of the pars ventrails of the lateral geniculate nucleus.
A basal optic bundle, similar to that of amphibians, has been
demonstrated in reptiles (Huber and Crosby, 1926;
1930, 1933).
Shanklin,
Structurally, the optic tectum of reptiles is
definitely superior to that of amphibians;
the tectum has
become the main sensory correlation center of the brain and the
richness and complexity of the afferent sensory impulses has
thereby been increased.
7).
Birds:
The optic tectum in the majority of birds oonsists of two
very conspicuous lateral eminences, the optic lobes.
This tectum,
while not different in principle from that of reptiles, is still
more highly developed, showing an even greater differentiation
into layers.
The marginal optic tract carries fibers to the
lateral geniculate nucleus, nucleus externus, nucleus superficialis synencephali and to the tectum itself (Huber and Crosby, 1939).
The medial optic tract (or axillary tract) is primarily related
to the nucleus isthmo—opticus but secondarily connects with the
lateral genioulate nuoleus and the tectum (Huber and Crosby, 1939)
133
.
The basal optic root is larger than in reptiles and can be
traced to the nucleus of the basal root.
In relation to the
tectum are the cells of the mesencephalic root of the trigeminal
nerve and it receives ascending fibers from the spinal nucleus
of V.
Obviously the tectum is a correlation center in whioh
optic impulses are related to other Impulses of exteroceptive
and proprioceptive character.
8).
Mammals:
In mammals, the tectal portions of the midbrain are formed
by the superior and inferior colliculi, of which the superior
colliculi are comparable to the optic tectum described for lower
forms.
In lower mammals the superior colliculi serve for visual
and other types of reflexes.
In higher mammals the optic fibers
reaching them are probably more particularly, although not ex­
clusively, for light reflexes.
These superior colliculi show
less lamination and are relatively smaller in size when compared
to reptiles and birds;
this reduction of the superior colliculus
is to a considerable extent in inverse proportion to the develop­
ment of the dorsal part of the lateral geniculate nucleus, since
this latter center assumes some of the functions formerly carried
out by the superior collicul&s.
Bernheimer (1899)carried 70 per
cent and von Mbnakow (1905) 80 per cent of all optic tract fibers
to the lateral geniculate nucleus.
Phylogenetically, the lateral geniculate nucleus grows up
particularly as a nucleus intercalated in the course of fiber
bundles between the retina and the mesencephalon and as a part of
a discharge path from the midbrain to the thalamus.
These tecto­
thalamic connections are of great importance in forms below
133
.
mammals and in lower mammals, and in these forms the ventral
part of the lateral geniculate nucleus is highly developed.
The amount of tec to—thalamic connections grows less and less
when passing from lower to higher mammals and other funotions
come to dominate the thalamic center.
This is the projection
of optic impulses on the cortex, and presumably the different­
iation of the pars dorsalis of the lateral genioulate nucleus
goes hand in hand with the differentiation of visual centers
within the cortex.
C.
Visual habits of vertebrates
1 ) * Qvclogtomes:
The hagfish eye is not visible externally because it is
completely covered with integument.
Its location can be detect­
ed, however, by the presence of an unpigmented area immediately
over the subcutaneous eye.
The ocular muscles are atrophic;
their presence is denied by some (Ariens Kappers, Huber and
Grosby, 1936).
This greatly regressed eye probably functions
only for primitive photoreception;
it is customarily described
as being HblindH (Walls, 1933a).
The brook lamprey spends several years in the mud as a larva,
then metamorphoses and lives several months more as an adult,
still buried in the mud.
It swims free to the surface only for
a few days of breeding on the inception of sexual maturity and
then dies (Okkelberg, 1921).
Thus the brook lamprey has a pre­
ponderant period of darkness in its visual life cyole:
a
“nocturnal11 existence with probably little dependence on its
visual system.
Walls (1935), however, pointed out that the
undisturbed brook lamprey is diurnal in its breeding habits and
134
.
is apparently not nocturnal in its feeding activities.
Anatomically, the eye shows little adaptation for a nocturnal
existence;
hence Walls concluded that at present we must say
that they may he indifferent to night and day.
The importance
of visual processes in their life adtivities is uncertain.
The
faculty of perceiving images, if present at all, is certinaly
poorly developed;
the optic functions are limited to primitive
photostatic functions concerned with the directing of movements
of the animal (Ariens Kappers, Huber and Crosby, 1936).
3).
Elasmobrahohs and pi sees:
Little is known about the visual discrimination of these
fishes.
Reeves (1919) found that certain fresh water teleosts
could discriminate between two lights when the intensity ratio
is 1:2.
Other workers have carried out similar experiments and
it seems that there is considerable variation in brightness
acuity from species to species;
nevertheless, the faculty appears
to be as well-developed in fishes as in other lower vertebrates.
Form discrimination has been the subject of some experimentation,
the results generally indicating that this aspect of vision is
present to so$e degree.
The question as to whether or not fishes
possess color vision is still a matter of controversy.
a mass of experimental evidence both pro and oon;
There is
with the bulk
of the pro evidence consisting of ill-controlled experiments
which prove nothing.
Warner (1931) reviewed the many and con­
flicting data on the subject and came to the conclusion that some
fishes, at least, do respond to color as such.
He considered it
very probable, however, that the capacity to discriminate colors
is far less developed in some species than others.
135
3),
Amphibia:
The detection of differences in brightness seems to be a
function of age:
very young tadpoles do not orient toward a
source of light, but at a later age they show either a positive
or a negative orientation, depending upon the intensity of the
light*
Adult Anura sire usually positive to all but such intense
photic stimulation as direct sunlight;
they move out of the
direct sunlight and face the source of light.
been observed in Urodeles.
This has also
Very little is known about form and
size discrimination in amphibians.
Dickerson (1906) concluded
that the frog is unable to distinguish between a lighted space
and a white solid.
As with the fishes, the problem of color
vision is not settled.
Most workers believe that amphibians do
possess color vision, including Hess (1910, 1911, 1912), who
has repeatedly denied its presence in fishes.
4).
Reptiles:
Hess (1910) studied the visual acuity of twelve genera of
turtles under various phases of darkness adaptation.
He found
that the turtle eye is much less sensitive to brightness than
the human.
Oasteel (1911) carefully studied form discrimination
in the painted turtle and found that it readily learns to dis­
criminate between two type patterns.
In general, it seems like­
ly that reptiles possess size and form discrimination of a certain
degree within the range of the normal prey of a species.
It is
generally agreed that reptiles possess oolor vision, although
the evidence for this is hardly more substantial than it is for
fishes and amphibians.
136
5).
Birds:
Vision plays a most important role in the life activities
of birds:
maintenance of normal posture (a blindfolded fowl
sinks to the earth or turns over on its side), feeding responses
(a hen will starve to death in the dark even when surrounded
with grain}, and normal flight and nesting activities all depend
upon visual cues.
Johnson (1914, 1916) carried out a systeifr-
atic study of pattern discrimination in the chick and found its
acuity markedly inferior to that of primates;
it is likely
that birds of flight, however, have much better outline vision
than the domestic fowls.
The evidence for color vision for
birds is more conclusive than for the lower vertebrates.
Lashley (1916) found that the range of color vision in domestic
folrts is about the same as for man;
however, the bird is much
less able to discriminate between closely adjacent wave-lengths
within a given spectral band.
Hamilton and Coleman (1933) found
the pigeon*s eye only slightly less sensitive to differences in
wave-length than the eye of normal man.
They found the pigeon*s
“hue discrimination curve11 definitely of the same type as that
of man, having regions of low threshold in the yellow and in the
blua-green.
6).
Mammals:
Sub-primate mammals depend less upon vision in their life
activities than do birds and primates.
Rodents, especially,
make little use of vision (e.g., blinded rats learn a maze about
as readily as normal animals).
of vision at close range:
Apparently dogs make little use
blinded dogs or normal dogs working
in the dark learn to operate a latch box as well as when vision
.
137
is possible.
Sub-primates possess brightness discrimination
but it is of an inferior sort.
Apparently size discrimination
is not bery acute in most mammals*
all experiments up to the
present demonstrate only the ability to detect gross size dif­
ferences.
Form vision seems to be rather poorly developed in
the dog.
The bat apparently does not rely on vision but rather
upon hearing and touch for its dexterity in flight at dusk.
Evidence against the possession of color vision has been report­
ed in these forms:
porcupine, rabbit, calf, adult bull and cat.
Evidence for the dog is conflicting, but the better-controlled
experiments indicate a lack of color vision (Pavlov, 1937).
Walton (1933) has reported strong evidence for qualitative hue
discrimination in the albino rat.
The vision of primates is probably as efficient as that of
man in most respects.
Accurate comparisons between man and lower
primates in visual acuity are lacking.
However, visual cues play
a very important role in primate learning and a highly developed
type of vision is probably present in all forms.
d.
Oorrelation of retinal specializations, central connections
of the ootic pathway* and visual habits with the findings
in the optic nerve
Hagfish:
Retina:
no data was found in the literature concerning the
percipient elements in this form.
Central connections:
the optic pathways have not been traced
to the optic tectum, resulting in great difficulty in even
identifying the degenerate optic centers.
Visual habits:
have a primitive
usually considered to be “blind11, but may
type of photoreception.
138
Optic nerve:
all the fibers are unmyelinated and the lowest
total number, 1,579, of any vertebrate studied was found in the
hagfish.
The findings in the nerve agree well with the degen­
erate type of visual system that occurs in this form.
Brook lanmrey:
Retina:
both rods and cones are present, with the cones
predominating.
No photomechanical movements, and no area centralis
have been described for this form.
Central connections:
the fundamental vertebrate plan is
present but the optic centers are not considered as well-developed.
A very primitive lateral geniculate nucleus may be present, while
the optic tectum is still partly ependymal.
These oentral optic
connections are advanced over the hagfish but they are still of
a primitive sort.
Visual habits:
the faculty of perceiving images is absent
or very poorly developed, hence the eyes are used for reception
of primitive photostatic functions concerned with the directing
of movements of the animal.
Optic nerve:
myelinated.
greater:
as in the hagfish, all the fibers are un­
The total fiber content is, however, eonsiderably
5,317.
This would seem to correlate well with the
state of the development of the visual system in the brook
lamprey.
Dogfish shark
guitar fish
Sting ray
Retina:
these elasmobranchs are said by many workers to have
pure rod retinas.
The presence of photomechanical movements is
questioned, but the possibility of the presence of a primitive
139
.
sort of area centralis must be entertained.
Central connections:
the optic centers are well—developed,
far better than in cyclostomes.
The optiG traot is the most
important afferent tract of the tectum, so that the tectum is
the correlation center for the optic with other exteroceptive
impulses.
Visual habits:
branchs.
little is known about the vision of elasmo­
For the most part, they are deep marine forms and
must lead a highly ••nocturnal11 type of visual life.
Vision
must play an important role, however, because photostatic and
gravistatic impulses are strongly correlated in these forms.
Optic nerve:
this is a nerve which contains all myelinated
fibers and a total number far greater than that in cyclostomes.
The shark nerve contains 113, 817 fibers, the guitar fish
74,636 and the stingray 39,663.
These findings are consistent
with the presence of a reasonably well-developed visual system
which plagre an important role in the lives of these animals.
Haoklebaok
Bowfin
goldfish
Bullhead
Retina:
it is believed that all these fishes possess both
rods and cones in their retinas*
and more numerous than the cones*
In general, the rods are small
There is some evidence to
indicate that a primitive area centralis may be present.
There
are well—developed photomechanical movements*
Central connections:
the cyt©architecture of the tectal
region shows an advance over the elasmobranch group, although
there is little difference between them in their grosser aspects.
As in elasmobranchs, the tectum is a correlative center for
140.
optic and non-optic exteroceptive impulses.
Visual habits:
little is clearly known, although vision in
these forms is important in their life activities.
They can
distinguish differences in light intensity up to a certain po£nt,
and a slight amount of form discrimination seems to be present.
Color vision is disputed, but the trend is to credit these forms
with color vision.
Optic nerve*
the nerve varies greatly in size in this
group, although all the nerves have in common the complete
myelination of their fibers.
Tie total number of fibers in
each nerve falls in the same range as that of the elasmobranchs,
each group showing great variation between the species that
were studied in it:
goldfish, 53,954;
haokleback, 13,507;
bullhead, 36,636.
bowfin, 113,735;
The nerve, then, shows
little difference from that of the elasmobranchs although the
other portions of the visual system are slightly advanced over
that of these sharks and rays.
Toad
Retina:
both rods and cones are present but the rods are
the predominant type.
is doubtful;
The presence of a primitive area centralis
photomechanical movements are as pronounced as in
the fishes.
,
Central connections:
the optic tectum retains much of a
primitive character when it is compared with that of higher forms.
It has features resembling both fishes and reptiles.
Visual habits:
these are not very well known, but the
amphibian eye is apparently inferior to the fish eye in bright­
ness, form and size discrimination.
Most workers feel that
amphibians possess color vision although the proof is still
141.
based on inconclusive experimental evidence*
Optic nerve:
there are some unmyelinated fibers present in
all the species studied in this class.
The toad has 33.9 per
cent and the frog 47.5 per cent unmyelination.
Lander (1937)
found 100 per cent unmyelination in Neeturus and 41 per cent
in Amblystoma.
Howe (in preparation) found 57 per cent un­
myelination in the bullfrog*
low:
toad, 15,433;
The total number of fibers is also
frog, 38,913.
Neeturus and 3,004 in Amblystoma;
bullfrog.
Lander found 362 fibers in
Howe enumerated 30,368 in the
Thus the optic nerve of amphibians is inferior anat­
omically to that of fishes;
vision in this class is also be­
lieved to be of an inferior sort;
and the central connections
of the visual system of Urodeles are definitely more primitive
than those of fishes, but in Anura these regions show a trend
which places them between fishes and reptiles.
Alligator
H o m e d toad
Retina:
the alligator retina is made up predominately of
rods, whereas the horned toad has a pure cone retina.
possess an area centralis and both have a fovea.
Both forms
The horned toad
fovea Is particularly well-developed and is often cited as the
best example of a maximal fovea.
Photomechanical movements
probably occur but they are very slight when compared with those
of fishes and amphibians.
Central connections:
the optic tectum is highly laminated,
indicating a high grade of functional activity.
Structurally,
the optic tectum is superior to that of amphibians.
It has be­
come the main sensory correlation center of the brain and the
richness and complexity of the afferent sensory impulses have
142.
thereby been increased.
Visual habits:, in general, it seems likely that reptiles
have size and form discrimination of a certain degree within
their normal prey range.
color vision.
It is generally agreed that tjiey possess
The alligator is "nocturnal11 in its light habits
whereas the horned toad is "diurnal1*.
Optic nerve:
all the forms studied show the presence of
some unmyelinated fibers, but they are always outnumbered by the
myelinated ones:
per cent;
alligator, 31.5 per cent;
homed toad, 36.1
turtle (Norris, 1938), 41 per cent unmyelination.
The total number of fibers is moderately high and is surprisingly
uniform:
turtle (Norris), 105,040;
toad, 128,784.
alligator, 105,398;
homed
Thus there is a decrease in the percentages of
unmyelinated fibers from that of amphibians and an increase in
the total number of fibers over that of both fishes and amphi­
bians.
The retina and central connections of reptiles are better
developed than those of fishes and amphibians;
vision, on the
other hand, is either equal or inferior to that of fishes and is
definitely superior to that of amphibians.
Except for the
presence of unmyelinated fibers, the reptilian optic nerve is
somewhat better developed than that of fishes.
Here the correl­
ation is not as clear-cut as one would like.
Duckling
Chick
Pigeon
Canary
Retina:
both rods and cones are present in these birds, but
the cones dominate the retinal picture.
and area centralis occur.
A well-developed fovea
The fovea is relatively shallow due to
the domesticated or semi-domesticated habits of these birds.
143.
Photomechanical movements are readily demonstrable and they
are much more pronounced than in reptiles.
Central connections:
the optic tectum, consisting of the
large optic lobes, is still more highly developed and shows an
even greater differentiation into layers than reptiles.
The
tectum is, as in lower forms, a most important correlation
center for optic and non—optic impulses.
Visual habits:
vision plays a most important role in the
life activities of birds.
Pattern discrimination is inferior
in domestic fowls but is probably well—developed in birds of
prey.
Color vision is accepted as a capability of the bird eye
and there is evidence to indicate that it is only slightly less
sensitive to differences in wave-lengths than the eye of normal
man.
It is generally believed that birds of prey have a very
high visual acuity but no controlled experiments on this have
been located in the literature.
Optic nerve:
the nerve contains only myelinated fibers
and its total fiber content is high:
413,948;
pigeon, 986,471;
duckling, 408,633;
canary, 437,516.
chick,
These fibers are
exceedingly small so that birds have more fibers per unit area
of nerve than any other class of vertebrates.
These findings
correlated well with the high degree of development of the bird
eye and central optic connections, and the well-known excellence
of avian vision.
Sub-nrimates:
Retina:
bat, dog, oat .rabbit, rat, guinea nig, nig.
sheep
the nocturnal bat is believed to possess a pure rod
retina, whereas the nocturnal cat and rabbit have many rods and
very few cones.
The rat has a predominately rod retina but cones
are present in considerable numbers.
The pig and sheep both
144.
possess rods and cones but information is lacking as to which
type predominates.
A& area centralis is present in all these
forms except the rat and sheep.
There are no photomechanical
movements.
Central connections*
thesuperior colliculi are the centers
for visual and other types of reflexes.
These superior colliculi
show less lamination and are relatively smaller in size than the
corresponding structures in reptiles and birds, but with this
decrease the dorsal part of the lateral geniculate nucleus hag
increased.
In all the sub—primates the dorsal part of the lat­
eral geniculate body is poorly developed whereas the ventral
part is highly developed, indicating that tecto—thalamic radi­
ations play a more important role in sub-primates than in
primates.
Visual habits:
these sub-primate mammals depend less upon
their vision than do birds and primates.
especially, make little use of vision.
Bats, rodents and dogs,
Brightness and size
discrimination do not seem to be very well-developed.
Most forms
probably possess color vision but it is believed to be absent
in the rabbit, dog and cat.
Optic nerve:
the sub-primate nerve shows great variation in
the presence of unmyelinated fibers:
33 per cent unmyelination;
per cent.
bat, 43.7 per cent;
white rat, 20.8
All the other forms showed complete myelination of
their fibers.
82,104;
oppssum (Norris, 1938),
The total number of fibers vary greatly:
bat, 6,934;
dog, 153,712;
264,611;
white rat, 74,812;
126,367;
pig, 680,780;
cat, 118,871;
hooded rat, 80,122;
sheep, 648,773.
opossum,
rabbit,
guinea pig,
In general, these
results reflect the decreased dependence the suh-prlmates
145*
place on the visual system*
Primates:
Retina*
monkeyf m r m
both rods and cones are present in the retinas of
Maoacus rhesus and man, but rods dominate in actual numbers even
though the macula lutea contains only cones*
The fovea is
shallow when compared to that of reptiles and certain birds*
Ho photomechanical movements have been found*
Central connections:
the superior colliculi are largely
concerned with light reflexes and are less well-developed than
in the sub—primates*
The dorsal portion of the lateral genic—
e
ulate body is large and ^.1-organized and is highly different­
iated;
this development has occurred in conjunction with the
great increase in complexity of the visual centers in the
cerebral cortex*
Conscious vision is apparently the great
forward step in visual specialization that has ocoured in man*
Visual habits:
visual cues play a very important role in
primate learning and a highly developed type of vision is
present in all forms*
Optic nerve:
the primate nerve contains only myelinated
fibers and these are present in great numbers:
1 i208,130;
man, 1,009,423*
monkey,
These findings correlate
well
with the high state of specialization of the retina and optic
centers in primates*
146
4+
Present knowledge of myelin and Its relation to this
Sfeoblem
ft* Nature of myelin:
Myelin is a complex lipoid substance, s&mi—fluid in nature,
which forms a glistening white envelope surrounding the axis
cylinder of certain peripheral and central nerve fibers.
Chemically, myelin is a poorly-known mixture of glycolipins,
s
phospholipins, galactose, inosite, potassium, sodium and calcium
(Biggaxt, 1936)*
In the ordinary preparations of nerve fibers
for microscopie study, utilizing such fat solvents as ether,
alcohol or chloroform, most of the myelin is dissolved.
Best
preservation is obtained by the use of osmium tetraoxide both
as a fixative and a stain*
Myelin may also be preserved, al­
though less successfully, by fixation with formalin or any of
various mixtures containing salts of chromium or certain other
heavy metals*
Fixation by these methods isusually followed by
staining with hematoxylin, giving myelin sheftths a dark blue
color (Maximow, 1934).
If myelinated fibers are treated with a hot mixture of
alcohol and ether, the lipoid structures are extracted but the
space occupied by the myelin is not left empty.
There remains
a delicate fibrillar framework, called neuro-keratin by many,
which has been interpreted in various ways.
Some believe that
this reticulum of the myelin sheath is part of the cell of
Schwann of that segment, but others have challenged the actual
existence of neuro-keratin because it is not demonstrable in
the living fibers.
Hoerr (1936), using the Altmann-Gersh
freezing-drying method, demonstrated to his own satisfaction,
147.
that the neuro-kerati# network is a fixation artefact.
The physical make-up of myelin has been investigated
recently by roentgen-ray diffraction by Schmitt and his
associates (1935).
They found that lipoid fluid crystals are
apparently disposed perpendicular to the length of the fiber
and are oriented radially with respect to the center of the
fiber.
They also noted significant changes in the roentgen-
ray diffraction figures*
b*
Structure of myelins
In peripheral nerves the myelin sheath is interrupted by
the nodes of Ranvier, thus dividing the nerve into nodal seg­
ments of varying lengths*
The length of the nodal segments
may vary from 0*05 to 1 millimeters, depending upon the type
of fiber and the kind of animal*
In general, the thicker the
fiber the longer are the segments (Maximow, 1934).
Between
nodes the myelin sheath is partially interrupted by the incis­
ures of Schmitt-Lantermann*
These a r e oblique, circular clefts
which subdivide the myelin sheath into •myelin segments1. Bito
(1926) presented evidence for the existence of these clefts in
the living myelin sheath but de Henyi (1939a,b,c) considered
them to be fixation artefacts*
These incisures have not been
demonstrated in the optic nerve (Gone and MacMillan, 1932);
nor have typical nodes of Ranvier been found, although the
caliber of the myelin sheath, when cut longitudinally, is un­
even.
Myelinated fibers in the central nervous system are
segmented (Hortega, 1928) and this segmentation is produced by
protoplasmic projections of oligodendrocytes pressing on the
sheath.
These findings led Hortega to conclude that the organ-
148.
ization of the myelin sheath is identical in the central
peripheral nervous systems*
But the myelin sheaths of the
optic nerve fibers must be studied further before any complete
comparisons can be made between them and the remainder of the
nervous system*
c*
Origin of the myelin sheaths:
The origin of myelin is unsettled*
In peripheral nerves it
is believed that myelin is laid down by the axis cylinder with
the cooperation of the neurilemma cells (Cowdry, 1938).
myelin sheath is, accordingly, a part of the neuron*
The
In the
central nervous system and the optic nerve, where neurilemma
sheaths are lacking, one would expect that myelin might have a
modified mode of origin*
Most investigators would Gonnect the
oligodendroglia with myelin formation.
Hortega (1928) believed
that oligodendroglia are analogous to the neurilemma cells of
the peripheral nervous system and he felt that they are either
related to the formation of myelin or play an important part in
its metabolism*
Evidence is accumulating to indicate a close
relationship in the optic nerve between oligodendroglia and
myelin formation*
It is well known that in the human (and in
many lower animals) the myelin does not extend through the
lamina oribosa except under anomalous conditions*
Investigators
have, accordingly, searched the unmyelinated optic papilla and
so far have been unable to find any oligodendroglia (Berliner,
1931;
Cone and MacMillan, 1932) although these cells are
numerous in the myelinated portions of the optic nerve.
Berliner
noted the presence of oligodendroglia in the retina in the
anomalous condition of myelinated nerve fibers in the retina
and concluded that oligodendroglia may be responsible for the
149,
medullation of these fibers.
Berliner also studied the rabbit*s
optic nerve head and retina (the rabbit*s retinal fibers are
normally myelinated) and found this picture:
1) no lamina
cribosa, so that glial cells extend directly into the retina,
3) rows of oligodendroglia in the retina, especially where
myelination is heaviest, and 3) there iB a complete absence of
oligodendroglia where the nerve fibers are non-myelinated.
Further evidence, although indefinite, gathered from chronic
degeneration experiments points to a close relationship between
oligodendroglia and myelination:
when the nerve fibers degen­
erate, the character of this cell group changes (Cone and
MacMillan, 1933)*
Hortega (1938) also noted that oligodendro­
glia are absent in old areas of demyelination in periaxial
encephalitis.
These data offer presumptive evidence that there
exists a synergetic and obligatory symbiotic relationship be­
tween oligodendroglia and myelination of nerve fibers in the
central nervous system.
d.
Myelinogenesis:
Since Flechsig (1895) originated the view that the degree
of myelination might be correlated with funcational capacity,
the time of myelination has been the subject of numerous re­
searches.
Flechsig stated that myelination is found in pro­
jection paths always before association paths, and in the sensory
before the motor ones, and in peripheral before central paths.
Tilney and Casamajor (1924) found that Flechsig*s myelinogenetio
theory applies to the kitten.
But on the other hand, Gonzalez
(1929) cited evidence to show that Flechsig*s theory does not
apply to the white rat, man, or in certain cases to the cat.
150.
Vogt (1908) wont so far as to state that myelination depends on
the number and size of the fibers developed in the pathway, and
is not correlated with the function of the fibers.
The evidence at hand suggests that myelination broadly
follows the phylogenetic pattern of the nervous system and that
there is a close correlation between myelination and functional
activity.
Nerve fibers can conduct impulses before becoming
myelinated but it is an inexact and uncertain impulse (Arnott,
1937).
Furthermore, stimulation may hurry myelination, e.g.,
Held (1897) flashed light in one opened eye of a young rabbit
and then studied the myelination of the optic nerves and found
much more myelination in the nerve corresponding to the eye
stimulated with light.
This experiment convinced Held that
light specifically hastens myelination*
But, according to
Klapkowish (1934), this can occur without a specific stimulation,
e.g., the guinea pig b o m with eyes open and optic nerve fibers
myelinated.
In Held’s opinion this finding in the guinea pig
is due to the longer gestation period:
rabbit, 28-30 days;
guinea pig, 63-65 days.
The problem of myelinogenesis in relation to the optic
nerve is far from settled at present and much more work needs
to be done before the answer can be hoped for.
e.
Relation of myelin to nerve function:
The relation of myelin to nerve function is a matter of
great uncertainty, although there Is some evidence which gives
a partial picture of its role.
The obvious suggestion would be that it is the highly
resistant membrane needed for propagation of the nerve impulse.
151.
But this cannot be the oase, however, because experimental
evidence indicates that unmyelinated fibers conduct inpulses
perfectly well*
Data on myelination in the development of
the fetus (Keene and Hewer, 1931) show that fibers which are
to be myelinated in the adult do conduct inpulses before they
receive their sheath although it may be an inexaot and un­
certain impulse.
Regeneration experiments also demonstrate
that fibers can conduct impulses before their myelin sheaths
have been restored.
A second rather obvious suggestion is that the myelin acts
as an insulator to the axis cylinder, preventing action currents
from one fiber stimulating adjacent ones.
But unmyelinated
fibers also show an isolation of impulses, although probably not
so perfect as in myelinated fibers (Gerard, 1931).
In this
connection, the fact that myelinated fibers invariably lose
their myelin sheath as they approach their peripheral endings
where reactions of surrounding tissue rather than conduction is
important, may be of significance.
This same situation is true
wherever nerve fibers meet other nerve fibers or cells, as in
the cortex or region of neuropil (Maximow, 1934).
Myelin may be a factor in influencing the conduction rate
of nerve fibers.
The conduction rate of myelinated fibers is
strikingly faster than in unmyelinated ones (Erlanger, 1930),
Since the ensheathment of an axis cylinder with a layer of
resistant myelin must insulate it from the tissue fluids, one
would expect myelinated nerve fibers to conduct much slower than
unmyelinated ones.
But the presence of interruptions in the
myelin (nodes of Ranvier, incisures of Schmitt-Lantermann, and
152
"their equivalents in "the central nervous system) must greatly
change the conditions of current flow.
It is conceivable that
the nodes, being exposed points, would concentrate the action
and that this could result in the excitation of a more distant
resting point, thus allowing the nerve impulse to jump from
node to node*
Lill@9s (1925) well-known iron wire model
demonstrates that this theoretical discussion is a possibilityt
iron wire lying free in nitric acid conducts rapidly but when
it is surrounded by a glass tube it conducts more slowly;
if,
however, the glass tube be broken across at several points
(thus simulating nodes of Ranvier), the conduction becomes more
rapid than when no glass tube is present and the activation
jumps from node to node*
Myelin may play a role in nerve metabolism.
It is general­
ly conceded that unmyelinated nerves fatigue more easily that
myelinated ones(Levin, 1927;
Furusawa, 1929)*
It is intrigu­
ing to postulate that myelin may serve as a food reserve for the
active axis cylinder but such a possibility has never been
proved*
However, changes in myelin structure following brief
activity of a nerve have been reported (Stubel, 1913;
1927;
Katsuma, 1927).
Auerbach,
The relationship between degeneration
and regeneration of myelinated nerve fibers also enters into
the picture.
It is known that the appearance of myelin, observ­
ed under polarized light, alters when a current is passed through
a nerve (Stefl, 1928).
Further studies are necessary before any
more definite relation between myelin and nerve metabolism can
be described*
153.
5*
Elec trophy si ology of the retina and optic nerve
a.
Retinal action currents*
Retinal action currents were discovered by the Swedish
physiologist Holmgren in 1865 (Gxanitfc 1933) and since then
the knowledge of them has proceeded hand in hand with the
development of eleotrophysiology in general*
Gotch (1903)
obtained the first curves embodying all the features of the
process with the aid of the sufficiently fast capillary electro­
meter.
v. Brucke and Garten (1907) and piper (1911), using the
string galvanometer, showed that the responses to light are
fundamentally alike for the various vertebrate eyes*
These
workers established the main features of the retinal action
currents and work since then has been one of refinement of
technique and minute analysis of the main components previous­
ly noted*
1)*
Components;
The main components of the retinal action currents may be
summarized in this way:
negative dip, a}
thefirst reaction to illumination is a
then follows a positive rise, the b^-wave, which
at high intensities drops fairly rapidly;
rise, the c-wave, appears;
next a slow secondary
and finally, on cessation of stimul­
ation there is a positive off-effect, the d-wave.
As previously
noted, this general analysis of the retinal action currents
applies to all vertebrate eyes.
Granit (1933) has analyzed the complex retinal potential
arising on stimulation with white light and has broken it down
into three components*
manner:
He summarized these processes in this
Process I (PI) rises slowly after a long latency and falls in
a similar manner*
It is positive in the usual representation
of the retinal action potential*
responsible for the o-wave*
This component is chiefly
It is easily removed by ether;
it is augmented with slight asphyxia.
In thedark-adapted
cat PI is only present with large areas and high intensities
of stimulation*
Kohlrausch (1918) observed that the 0,-wave
is more marked in the dark-adapted eye of nocturnal animals,
and in the light—adapted eye of diurnal forms*
This suggests
that this wave appears whenever an eye functions under con­
ditions most appropriate for the particular retina in quest­
ion*
Granit also suggested that this component appears
when the retina is especially active, and is not at all or
only slightly concerned with the discharge of impulses.
He
suggested that PI might well represent some process of
importance for the maintenance of a continued discharge*
It is unlikely that PI is connected in any way with pigment
or rod or cone movements*
Its reactions to ether and
asphyxia indicate a process of central origin (retinal
synapses, cell bodies)*
Process II (Pit) rises rapidly as the positive b-wave of the
complex response, then falls fairly rapidly at high inten­
sities, less rapidly at low intensities, and continues
hidden by PI under the £-wave of the complex action potential.
It is the only process that can be detected at all intensit­
ies capable of giving a detectable response*
is associated with the production of impulses*
This component
155.
Process III (PHI) is of negative sign, first appearing as
the a-wave of the composite potential, then its further
course is hidden, but by its return to zero at cessation
of stimulation the positive off—effect is elicited as a
release phenomenon.
PHI, therefore, seems to be concern­
ed in some way with the inhibition of impulses.
3).
Relation of retinal action currents to anatomical
structure:
With this knowledge of retinal action currents, correlat­
ions with anatomical arrangement of structures in the retina
are suggested.
Granit (1933, 1934, 1935), along with other
workers in this field, has included statements concerning the
chief features of the retinal anatomy of the species studied
but has so far been reluctant to make definite correlations.
This statement of Granit*s (1933) sums up the present status of
these correlations:
M.*.our knowledge of retinal physiology still is at the stage
when even quite elementary facts have to be established about
the nature of the processes concerned. Little can therefore
be gained by theorizing extensively about the significance of
this work for the subject of vision.11
3).
Relation of retinal actions currents to the optic nerve:
The significance of retinal action currents in relation to
the electrophysiology of the optic nerve has not escaped obser­
vation.
Although Frohlich (1913) made some observations in the
cephalopod eye and optic nerve, it remained for Adrian and
Matthews (1937) to apply the method of study of action currents
to the vertebrate optic nerve.
Using a capillary electro­
meter and valve amplifier, these workers recorded the action
currents of impulses in the optic nerve of the conger eel and
156.
of the frog when the retina is illuminated.
They found that
these action currents do not differ appreciably in time rel­
ations or in grouping from those of other sensory nerves, and
their size is not affected by the strength of the stimulus (allor-nothing principle).
Illumination of the retina produces a
discharge of impulses which rises rapidly to a miximum frequency
and then declines, at first rapidly and then more slowly.
Both
the latent period and the maximum frequency of the discharge
are determined approximately by the quantity of light in unit
time (area X intensity), and the frequency is not directly prop­
ortional to the area of the retinal image.
Matthew made a
histological study of the eel's optic nerve (vide review of the
literature) and appended it to this study.
Just what correlat­
ions exist between their results and the anatomical findings are
not discussed.
Granit (1933) became interested in the relation of retinal
and optic nerve action currents and studied them in the cat.
He
found that of the three components of the retinal action potent­
ials, as outlined above, only one can be shown to be associated
with the discharge of impulses through the optic nerve.
That
component is P XI, which is associated in the retina with the
production of impulses.
Hartline (1938) pointed out that the work of Adrian and
Sdatthews and Granit is concerned with the simultaneous activity
of large numbers of optic nerve fibers;
then showed that the
work should be extended to include an analysis of the activity
of single optic nerve fibers.
This possibility of study of
3ingle fibers in the vertebrate optic nerve is based on the
157.
earlier work of Hartline and Graham (1932) on the optic nerve
fibers of Limulus*
Hartline*s procedure is briefly this:
after
excising the eye and pinning the fundus down in a moist chamber,
a wide V-shaped cut is made extending almost to the nerve head
to give access to the fundus and to permit the dissection of
small bundles of nerve fibers for a length of 1 to 2 millimeters.
These are then further dissected away until only one fiber remains
active.
Action potentials from such a single fiber are recorded
by means of an oscillograph.
Most of the work was done on Rana
catesbiana but a few experiments were run on similar preparations
of one shark, one Hecturus, a number of turtles and alligators,
one iguana and several varieties of snakes.
First using a bundle
of several of these intraocular optic nerve fibers, Hartline
obtained results similar to those found by Adrian and Matthews
in the eel and Granit in the cat optic nerve.
But when the
bundles were dissected down to a few or a single fiber, he found
some new and striking features:
not all of the optic nerve fibers
give the same kind of response to light, but these various res­
ponses fall into three categories.
First, about 20 per cent of
the fibers respond to illumination of the retina with a burst of
impulses at high frequency, followed by a steady discharge at
lower frequency which is maintained throughout illumination, and
stops when the light is turned off.
Second, about 50 per cent of
the fibers show only a burst of impulses in response to the onset
o f illumination, and another in response to its cessation;
no
Impulses are discharged during steady illumination of the retina.
Phird, about 30 per cent of the fibers show no discharge either
at the onset of illumination, or throughout its duration, but
158.
give a vigorous and prolonged discharge when the light is turn­
ed off.
The type of response in any given fiber does not depend
upon conditions of stimulation or adaptation of the eye, accord­
ing to Hartline, and even certain external agents (asphyxia,
carbon dioxide, ion unbalance, temperature), while affecting the
responses, do not alter their essential character.
Allthe
various forms studied give essentially the same results as re­
gards the types of responses found.
Further, the type of response
is not correlated with the location of the fiber's receptive
field in the retina.
Hartline speculated about possible explanations of these
rather surprising results.
First, he considered the retinal
percipient elements as the possible source for these three types
of response;
he found it difficult to reconcile the results with
the duplicity theory and found no evidence that would associate
any of the response types with either rod or cone function.
Still, the presence of different forms of rods and cones in the
cold-blooded vertebrates could be responsible for these variations
in type responses.
Or, on the other hand, he speculated about a
site of origin of the diversity of response in the layers of the
retina between the rod and cone layer and the ganglion cells.
Thus a given ganglion cell might be subjected to diverse and
rival influences, and its response thereby determined by the
relative amounts of each.
This, in turn, might be fixed in a
large measure by the anatomical connections between the ganglion
cell and its underlying neurones.
This is the site that Granit
and his co-workers have urged in interpreting their results
(vide summary of Granitfs researches).
Hartline offered still
159.
another explanation in considering these type responses:
functional differences may exist among the ganglion cells.
While subject to essentially the same influences from the
underlying retinaly layers, different ganglion cells may res­
pond differently to shifts in their equilibrium.
A final answer
to these puzzling results must wait for further experiments on
the subject.
Since Hartline used only intra-ocular optic nerve fibers in
his experiments, it is certain that his results were obtained on
only unmyelinated fibers.
Adrian and Matthews did their work on
the conger eel, with presumably all myelinated fibers (since all
the Teleosts studied had only myelinated fibers), and the frog,
with approximately 50 per cent unmyelination;
Granit worked on
the cat, this form having all myelinated fibers in its optio nerve.
Their results are all reported as comparable, and Hartline report­
ed that his results using bundles of intra-ocular fibers are the
same as those of the other workers who used the entire nerve*
This evidence would seem to indicate that the presence of absence
of a myelin sheath has no effect on the type of impulse that an
optic nerve fiber carries.
As pointed out elsewhere (vide
paragraphs on myelin), myelin sheaths do influence the speed of
conduction of an impulse in a fiber of a peripheral nerve and
there is a suggestion that even the type of impulse may be mod­
ified by the absence of such a sheath.
It is difficult to recon­
cile these two sets of experimental data;
perhaps the evidence
is far too incomplete to even attempt to do so at present.
But
Bishop's work on the optic nerve electrophysiology, approached
from a different type of experimental procedure, should be care-
160.
fully considered in relation to this question.
b.
Action currents of optic nerve
Bishop (1933) studied the action currents of the frog and
rabbit optio nerve elicited by direct stimulation of the nerve
itself.
His purpose was to describe visual sensations in terms
of action currents;
action currents of the retina, of the optio
nerve fibers, of the thalamus and cortex;
and in terms of
frequency of response, number and kinds of elements involved,
inhibition and facilitation of the pathway, et cetera*
Bishop
noted the presence of distinct functional fiber groupings in
peripheral nerves, e.g., in the saphenous nerve the group of
fibers with the fastest conduction Tnrtilrrlffi mediates touch and
pressure, a slower conducting group mediates pAin and temper­
ature, and a still slower group is motor (Heinbecker, Bishop
and O'Leary, 1933), so generally that the finding of such group­
ings in any nerve is circumstantial evidence at least that dif­
ferent functions are served by these groups.
Utilizing this
observation, Bishop studied the optic nerve, making his observ­
ations by means of a cathode ray oscillograph.
The frog optic
nerve was found to contain three main groups of fibers differing
in conduction rates and other physiological properties.
The
rabbit optic nerve was noted to contain two groups similar to
the firsttwo of the frog, and probably a third whose identificat­
ion was uncertain.
fibers.
These fiber groups Bishop termed A, B and 0
In assigning groups A and B a function, Bishop suggest­
ed (by analogy with peripheral nerves) that the large fibers (A)
mediate that aspect of vision concerned with spatial discriminat­
ion or form, while the more numerous small fibers (B) are
concerned with the quantitative factor of intensity.
In this
connection, it is of interest to point out that Gudden (1886)
noted the size variation in the optic nerve fibers and assigned
the fine fibers the function of production of pupillary reflexes
whereas he believed the coarse fibers carried visual impressions.
Group C fibers, occurring as a clear-cut group only in the frog,
Bishop presumed are non—myelinated axons (by analogy with such
nerves as the saphenous, vagus and sympathetics where C fibers
can be inferred to be the non—myelinated ones).
Further, the
area of the 0 wave in the frog is nearly equal to one—half the
total potential of the A and B waves, indicating the possibility
(if we accept this suggestion) of the presence of a large number
of unmyelinated fibers in the frog optio nerve.
The absence of
a clear-cut 0 wave in the rabbit, following through with this
line of reasoning, would similarly suggest the absence of un­
myelinated fibers in the rabbit optic nerve.
Bishop made no
suggestions as to the possible functions of these 0 fibers.
A comparison of these results of Bishop's and his speculat­
ions concerning them with the results of the enumerations made
in this research is of considerable interest.
Bishop's theory
postulates a large number of unmyelinated fibers in the optic
nerve of the frog;
this study shows that such a group of fibers
is actually present:
are unmyelinated;
unmyelinated.
in Rana pipiens, 47.6 per oent of the fibers
in Rana catesbiana (Howe), 57 per cent are
On the other hand, few or no unmyelinated fibers
should be present in the rabbit, according to Bishop's ideas;
this study shows the complete absence of unmyelinated fibers in
this rodent (Schaible found 18 per cent unmyelination but
162.
questioned his results because his osmic—stained sections were
poor).
Bishop's suggestion and A and B fibers are myelinated
fibers of various size ranges arouses one's interest in fiber
size, but as yet no measurements of fiber diameter and class­
ification into size categories has been made on the material
used in this study.
o«
Summary of retinal and ootio nerve aotion currents and
their possible relation to the structure of the optio
nerve
Two general approaches to the problem of action currents
in the optic nerve have been outlined:
the recording of action
currents brought about by stimulating the retina with a beam of
light (Adrian and Matthews, 1927;
Granit, 1933, 1935;
Hartline,
1932, 1938), and action currents arising from a stimulation of
the nerve itself (Bishop, 1933).
One gives rise to a few concepts
concerning the nature of the impulses that arise in the retina
and which of these passes on to the nerve;
the other gives some
idea of the manner in which the optio nerve transmits and in­
fluences a non-specific sort of impulse, lie., simple electrical
stimulation of the nerve itself.
That the results are so differ­
ent as even to defy comparison is not surprising, but there is
every indication that both types of approach must be more thor­
oughly studied.
Proper anatomical correlation is nothing more
than pure speculation, as already pointed out, yet such speculat­
ion may be worthwhile in eventually leading to the establishment
of laws concerning the visual processes.
163.
VIII.
1*
CONCLUSIONS
The difference in the total number of fibers between the
right and left nerves of a pair of optic nerves is so small,
under normal conditions, that if falls within the experimental
error factor.
2*
Staining with osmic acid is a difficult procedure but
chances for successful preparations are improved by the use of
the vapor.
Axis cylinders of the optic nerve fibers are con­
sistently well-demonstrated in sections stained by the rapid
silver nitrate-protargol technique.
3.
The strip method of enumerating nerve fibers is a rapid
procedure, adjustable to any size of nerve, and is reasonably
accurate.
4.
Fishes show an optic nerve composed of either all "un­
myelinated fibers (Cyclostomata) or of all myelinated fibers
(Pisces).
The following averages of total fiber estimates
(obtained from silver preparations) were secured:
1,579;
brook lamprey, 5,317;
guitar fish, 74,636;
(hackleback), 13,555;
52,955;
5.
dogfish shark pup, 113,817;
sting ray, 39,663;
goldfish,
bullhead, 26,636.
The amphibian optic nerve reveals various degrees of un-
(Rana pipiens), 47.6 per cent.
6.
shovel-nosed sturgeon
bowfin (dogfish), 113,735;
myelination of its nerve fibers:
are:
hagfish,
toad, 15,433;
toad, 33.9 per cent;
frog,
Average total fiber estimates
Rana pipiens, 28,912.
The reptilian optic nerve also shows some unmyelination of
its nerve fibers:
37 per cent.
baby alligator, 22 per cent;
horned toad,
The averages of the total fiber estimates are:
164.
alligator, 105,398;
7.
horned toad, 128,780.
The bird optic nerve shows complete myelination of all
its fibers.
These large nerves show characteristically high
fiber estimates with exceedingly small fibers.
total fiber estimates were secured:
chick, 413,948;
8.
pigeon, 986,471;
These average
duckling, 408,632;
canary, 427,516.
The mammalian optic nerve reveals great variation in
the unmyelination of its fibers:
rat, 20.8 per cent.
bat, 43.7 per cent;
white
All the other forms show complete myelin—
ation of their fibers.
Great differences in the total number
of fibers contained within the mammalian optic nerve can be
noted in these total fiber estimates:
153,713;
cat, 118,871;
hooded rat, 80,133;
bat, 6,934;
dog,
rabbit, 364,611;
white rat, 74,337;
guinea pig, 136,367;
monkey, 1,208,120;
man, 1,009,423.
9.
As a group, the amphibians have the smallest average optic
nerve size with a fiber concentration of about 390,000 fibers
per square millimeter.
The fish optic nerve is the next smallest
but it has a mtigh higher fiber concentration:
380.000 fibers per square millimeter.
approximately
The reptilian nerve is
considerably larger than that of fishes and amphibians and it
has a higher fiber concentration:
square millimeter#.
around 495,000 fibers per
The bird optic nerve is markedly larger in
size than that of fishes, amphibians and reptiles, and it has
the remarkably high average fiber concentration of about
565.000 fibers per square millimeter.
The mammalian nerve is
the largest of all the classes, being more than twice the area
of that of the bird, but it has the lowest fiber concentration
of any of the vertebrate classes:
about 280,000 fibers per
165.
square millimeter.
10.
The number of fibers at the bulbar end of the optio
nerve is essentially the same as that at the chiasmal end;
therefore, enumerations made at any level of the optic nerve
respresent the total fibers contained in the nerve*
11.
The results on the rat hint at the possibility of the
existence of some relationship between albinism and unmyelin—
ation of the optic nerve fibers:
per cent fibers unmyelinated;
no unmyelinated fibers.
white rat (albino eye), 30.8
hooded rat (pigmented eye),
More investigation is required before
a conclusion that there is a relationship is warranted.
13.
The number of fibers in the optic nerve may increase
in the postnatal period.
Some evidence in favor of such an
increase is presented but it is too incomplete to permit more
than the suggestion that such an increase may occur.
13.
In general, correlations between retinal specializations,
central optic connections and the visual habits of the species,
and the structure of the optic nerve fibers do occur in all
the forms studied, but these agreements are less clear-cut in
reptiles than in the other vertebrate classes.
14.
The role of myelin in optic nerve function is admittedly
obscure at present but it does appear to be concerned with
e
functional specialization: oompljely myelinated optic nerve
fibers are associated with visual systems subserving a higher
type of visual activity than those containing unmyelinated
fibers.
A more complete knowledge of myelinogenesis might be
of aid in clarifying the functional role of myelin:
much work
needs to be done on this aspect of the development of the
visual system.
166.
15.
The eleotrophysiology of the retina and optic nerve is
still too incompletely known to permit useful anatomical cor­
relations.
tore work on both the physiology and anatomy of
these structures must be done before these necessary correlat­
ions can become more than mere theoretical considerations.
%
167
IX.
ACKNOWLEDGMENTS
From the inception of this research, several persons
have contributed to it in the way of helpfdl suggestions and
encouragement*
These contributions are greatly appreciated
and it is hoped that these brief words of acknowledgment will
convey, to a certain extent, the depth of this appreciation*
Dr* L*B* Arey— the author of the plan of this research,
who has always been willing to devote time to discussion of
the difficulties of technique and the possible significance
of the findings;
an admirable director of a research because
of his attitude of cooperation rather than of direction*
Dr* H.A* Davenport~the authority always willing to help
unravel difficulties in staining technique; wlthbut his cooper­
ation the successful staining of many of the optic nerves would
have been very difficult, if not impossible*
Dr* W*F. Windle— the adviser who was always interested in
the progress of the research and who often made valuable sug­
gestions for improvements of technique and possible interpre­
tations of the results.
Dr. Frank Queen— the pathologist whose cooperation made
possible the securing of an abundance of human material*
168,
X.
APPENDIX
Statistical methods used in this study and their
interpretations
(Summarized from Davenport and Ekas, 1936)
Arithmetical Mean (M):
The sum of the separate variates divided by the number of
variates.
It is computed as follows:
Mr £ V
N
M,
£,
v,
N»
mean of distribution
the sum of
individual variates
number of variates
Standard Deviation (<Q;
The square root of the mean of the squares of the deviat­
ions from the mean*
It is computed as follows:
6
£ » standard deviation of the distribution
£(v - M)a , sum of the squared deviations from the mean
N, number of cases
The standard deviation of a normal distribution marks the
limit* of the middle 67 per cent of the measures.
Coefficient of Variation (CV):
The value obtained by reducing a standard deviation to an
abstract number which then allows this particular standard
deviation to be compared directly with any other standard
deviation treated similarly.
It is obtained by dividing
the standard deviation by the mean and multiplying the
quotient by 100.
Thus it expresses relative variability
in terms of per cent of the mean.
ov - —
169 ♦
Probable Error of Standard Deviation (PE*):
The probable error gives the measure of unreliability
of any determination#
It is a pair of values, one lying
above and the other below the determination being tested#
There is an even chance that the true value lies between
these limits, provided the distribution for which the
constant was obtained is normal or nearly normal#
chances that the true value lies within
within
3PE they are 32:1,
The
2PE are 4,6:1,
4PE are 142:1, et cetera.
The probable error of the standard deviation is obtained
by multiplying the standard deviation by 0.6745 and
dividing by the square root of twice the number of
variates:
PE
■■ 4=0.6745 6
J2H
6
Probable Error of the Coefficient of Variation (PEgy):
This is obtained by multiplying the coefficient of
variation by 0.6745 and dividing by the square root
of twice the number of variates:
PE™
ov
s
Of6745 0V
,--
JW
170
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XXI.
VITA
Name
Rulin Bruesch
Date of Birth
July 7, 1914
Academic Career
Bonita Union High School
La Verne, California 1937-1931
La Verne College
La Verne, California
1931-1935
Northwestern University Medical
School, Chicago, Illinois
1935-1940
Degrees Received
Positions Held
B.A.
La Verne College, 1935
M.S.
Northwestern University,
1939
M.B.
Northwestern University
Medical School, 1940
Assistant in Anatomy,
Northwestern University Medical
School, 1937-1938
Elizabeth J. Ward Fellow in
Anatomy, Northwestern University
Medical School, 1938-1939
Robert Laughlin Rea Fellow in
Anatomy, Northwestern University
Medical School, 1939-1940
Publications:
Staining paraffin sections with
protargol: 3. The optimum pH
for reduction. 4. A two-hour
staining method. Stain Techn.,
14: 20-38, 1939. (With H.A.
Davenport and Janet McArthur).
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