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Size distribution and number of fibres in the human Corpus Callosum.

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SIZE, DISTBIBUTIOS, AND NUllBER O F FlBlZES I N
THE H C l f A K CORPUS CALLOSUJI
JOSEPH TOMASCH
Depa~twientof Anatomy, Queen's Uwiversity, Kingston, Ontario
T W O FIGURES
011 attempting to find out the present status of exact knowledge concerning a structure of the size of the Corpus Calclosum,
(C.C.) in the human brain, one is amazed about the divergence
of opinions on most of the important questions and problems
related to it. Such questions include, for instance, the origin
and the exact courses and destinations of its fibres and also the
function of the whole structure. It is not so very long ago, according to Mingazzini's important monograph ('22) on the
C.C., that it was believed to be the situation of the s o d , a s
expressed by Lapeyronie (1740), Lancisi (1713) , Louis, Chopart and others. The progress of modern surgery brought the
C.C. within the interest of practical medicine, especially the
puncture of the C.C. as described by Elsherg ( '15), MarburgRanzi ('15) and many others and performcd in eases of hydrocephalus internus, a s a relief operation in brain tumors and in
certain cases of epilepsy. Total sectioning of the C.C. a s performed by Dandy and Foerster also elmucidated our knowledge
about this part of the brain. It was a slow development u p to
the recent investigations of Akelaites, Risteen, Herren and Van
Wageneii ( '42). They showed very clearly and in accordance
with some earlier authors like Dandy, Foerster, Meagher and
Barre, whose material however was not so extensive, that the
C.C. is hardly connected with psychological functions a t all.
This view is contrary to ideas expressed in many textbooks
* This investigation was carried out by aid of grants from the National Research
Council of Canada and the William Spankie Memorial Fund of Queen's University.
119
120
JOSEPH TOJIASCH
which assume that it has to do with orientation, speech aiid
even visual aiid auditory function.
However, interesting aiid important though these functional
aspects a r c for the justification of taking an interest in this
structure, we a r e here more concerned with the morphological^
aspects and 011 these also one finds the amount of unquestionable knowledge quite limited. The origin of the fibers, for instauce, might be consideiaed. Following Mingazzini in this
matter two possible SOII~CCS are to be mentioned.
( a ) Some fibres seem to be the direct axis-cylinders of cells
in tlie cortical g r a y matter. According to Ram6n y Cajal great
a i d small pyramidal celmlsare mother-cells of callosal fibres.
This opinion is however not shared by all investigators ; Jenschmidt (quoted after Miiigazzini) and De Vries ('11)011 experimental basis expressed their strong beliefs that the biggest
cells in the 4th layer of the cerebral cortex are the mother cells.
Elements of the deep parts of the cortex are in close connection with the white matter and apparently the source of
association- commissural- and projection-fibres. However,
Mingazziiii stated that the ixwilts of these investigations cannot be regarded as final.
(b) Other callosal fibres seem to be formed below the cortex
itself either as collaterals o r even direct brarichings of the descending projection-fibres or the association fibres in the centi-um ovale. Representing this opinion are also Ram611 y Cajal,
and (on account of physiological observations) Mott and
Scliaefer (1890) wlio came to the belief that the C.C. is formed
by dendrites of the cells in the motor areas.
The question of fibre-origin leads over to the equally important and equally debatable problem of their course. I n most
texts related to the matter, the C.C. is thought to be a strict
commissural system connectiiig corresponding parts of the cortex in both hemispheres. However, after some authors, callosal
fibres not only connect cortical areas of opposite sides, but also
coiinect those of one side with the internal aiid external capsules of the opposite side, and through these enter the
pyramidal-tract. Hamilton (1886), Ransom (1895), Gaddi
FIB1;ES O F H U M A N COLWG'S CALLOSUM
121
(ltlil),Biaiichi aiid I)'Abuiido (1886) regard the callosal systen1 as similar to the crossing system of the pyramidal tract.
Froin a study of 4-moiith-old human embryos, Hamilton described fibres arising in the cerebral cortex crossing through
the C.C. to the opposite hemisphere to end partly in the thalamus, partly in the cauclatum and in the gray centers of the poiis
and partly also to joiii with the pyramidal' tract. Also Van
Valkciiberg ('11)states, that p a r t of the C.C. fibres coiinect the
iieostriatum (caudatum and putamen) with cortex of opposite
side. llingazzini quotes here also Scliriopfhageii ( '09) arid
Bessos ~1-110on experimental basis confirm tlie pi*eseiicc of
callosal fibres in the external capsule and Arcliarribault ( '10)
who in cases of congenital absence of the C.C. fouiid also cornplete ageiisis of' tlie fibres of the external capsule. Considering
these opiiiioiis and fiiidings, includiiig the description of Ram&
y CajaE that callosal fibres give off collaterals before and after
crossing, it is understandable that some authors do not regard
the C.C. as a strict coniniissural system in the seiise of hleynert,
but as forming part of an inter- and intrahemispheric associat ion-sy st em.
Summing up the ioregoing remarks, oiie must say that we
a r e unable yet, in spite of esteiisive research by Morphologists
and Physiologists and Clinical study, to give ail unquestionable
account of the nature aiid function of tliis structure. However,
we kiiow now at least that tho C.C. does not function in various
ways formerly attributed to it, and me can assume that its
actual function and morphology can at some time be elucidated.
Nurncrical investigations aiid quantitative view-points have
given us in the recent past maiig iiew aspects of brain-anatomy
and it seemed well justified, to me, to investigate the human
C.C. froin such a n angle, especial3ly because in studying the
literature related to the matter I did not find such a contribution. Information as to the size of fibres which make up this
structure, their distribution in different parts o r their total
number, may help in locating inore exactly the cells of origin.
Since fibres of different sizes a r e included in the formation of
the C.C. - thin and thick ones perhaps derived from small and
122
JOSEPH T O M A S C H
large nerve-cells - the proportion and ratio of their distribution may, it is felt, play sonie rol,e in locating the sources of
cells corresponding to these fibres.
MATERTAL A X D M E T H O D
The brains of three iiien in their fifties, having died from
causes not involving the central nervous system, were fixed in
fo~*malin.After fixation was completed, the brains were sectioned in the median-sagittal plane and the so-obtained mediansection of the C.C. was photographed. The C.C. was then dissected off the hemispheres, a s a whole, about 2 cm lateral to the
midline. The so-obtained isolated corpora callosa were placed
in potassinni bichroniate solution fo r mordanting arid after
several weeks they were cut, each into 4 parts of about equal
size, a i d these parts imbedded in paraffin. The blocks were cut
so as to obtain the sections from as close to tlie former medianplane as possible; sections were made a t a thickness of 7 p.
F r o m sections of each part tm7o cliffereiit stainings were made.
111 order to h a r e all fibres stained, including the unmyelinated
ones, the modification of the Alzheimer-Mann glia staining
method as reconimended by H a e g g p i s t ( ’36) was employed.
This method rmiders the a x o m very distinctly visible but does
not show the myelin-sheaths too well. F o r showing the myeliiiated fibres only and the niyelin-slieatlis themselres the staining method of Weigert was used.
The fibre-distributioii curves (figs. 1 and 2) were obtained
from the photomicrographs on which they are drawn, by having counted and measured about 500 fibres for each, in the
usual manner for fibre analysis. Because of the small size of
tlie callosal fibres these curves are macle in steps of 3 p. The
fibre-size is plotted against the percentage of their presence in
a certain group.
F o r the estimate of the total nuniber of fibres, a Whippledisc was placed in an eye-piece and counts of fibre-density were
made at 6-10 parts of each section using a magnification of
approximately 1400 X. B y “fibre-density ” we mean the number of fibres per unit -in our case per square micron. The
number of fibers actually c*ounted in each of the three Ca.Ca.
was approximately 60,000. The total surface area of the C.C.
was then obtained by printing 011 photographic paper the sections from which the couiits had been made, with translucent
millimeter-paper insei*ted between, and by counting out the
number of sqi1wi.e niillimetci~scovered by the section. Finally,
4
Fig. 1 Fibre-distriI)utioii iii spleniuiii of corpus cnllosum. St:iiiied by W'eigert 's
method. Mngiiifieatioii 17.70 X .
124
J O S E P H TOAIASCH
the density-values liaviiig been calculated iii iiuniber oi' fibyes
per square-millimeter, this number was ninltipliecl b:- the number of square millimeters in the total surface area. In the seetioris stained by the Weigert method, the myelinatecl fibres oiily
were counted, but in the sectioiis stained by the Haeggquist
method in which the axons show np, all fibres conld he seen.
4
Fig. 3 Fibre-distribution ill spleiiiiiin~of corpus callosuni stained 1 ) IIaegg~
quist 's method. Magnifieatioii 17.50 X .
Thus we were able, by taking tlie difference between the counts
by the two methods, to estimate the number of uiimyelinatecl
fibres. This procedure is similar to tlie one recommended by
Dalilstroeni and Swenson ( '42).
The values we obtained correspond very well on the thi*ee
braiiis investigated. Szeiitagothai-Schimert ( '42) stated that
indiridual variation in the normal composition of central patliways is very slight, in fact within tlie possible errors of our
methods. Wc can, therefore, assume that the values obtainecl
a r e acceptable f o r the average adult human brain.
Among the possible sources of error, one is that in a fell7
areas of the cross-sections investigated, a few bundles of callosal fibres 1'1111 a niore oblique conrse than the others. Since,
liomvcr, tlie nuinber aiid size of such areas i s extremely small,
tlie ralues obtained should be, I think, within about 255% above
o r liclon- the correct number.
liESULTS AKD C O J I M E S T S
It is generally accepted,
a s regards the peripheral nerres,
that tlie fibres of cliff'erent sizes halre different conduction
velocities, the large fibres conducting faster than sniall ones.
F o r fibres in tlie central-nervous system this relatioiiship is
also probable though not accepted a s proven. X a n y suggestions have been made a s to the possible significance of this
clifferelice in fibre-size. To inentioii some, the size of a cell is
thought to iiiflueiice, at least, the calibre of the fibre to which it
gives rise. It is ofteii stated that fibres running f o r long distaiiccs a r e larger tliaii short fibres and that fibre-tracts ~ h i c l i
show early myelination are made up of relatively large fibres.
It is certain, however, that the various fibre-systems have a
very coiistaiit composition. Such constancy is found, in the
central pathways, a s regards to diameter of the fibres themselves and also the thickness of the inpelin sheaths. Among
these fibre-systems the C.C. i s one composed of very fine fibres,
the myelinatecl ones having only thin myelin-sheaths. Szentagothai-Schiniert ( '42), in his article 011 the fibre-size and fibrecomposition of central tracts, devoted a few lilies to the de-
126
JOSEPH TOLMASCH
scription of the C.C. He observed that it is formed in most
parts by comparatively small, fibres, those of medium size being r a r e except in the body where they are more numerous.
The substance of the C.C. is composed almost exclusively of
nerve fibres, except for a few rudiments of gray matter belonging to the induseum griseum, some islands of which may be
imbedded in the white matter. In examining our three Ca.Ca.
such an island of g r a y matter was found in only one case.
Other components to be mentioned are of course neuroglia cells
and a certain number of blood vessels. I t might be mentioned
hei-e, that of those nerve fibres making u p the main mass of the
corpus callosum, a certain proportion can be expected to he
non-niyeEinated. This is not surprising if we consider that in
many parts of the central nervous system such fibres a r e rather
numerous (Bielschowsky, '28 ; Larsell, '51). As mentioned before, the myelinated fibres of the C.C. are very fine. This fact
is clearly visible from figure 1, in which are seen fibres from
the splenium of our brain no. 1. In this photomicrograph is
drawn the histogram, obtained by having measured the fibres
contained in this picture in the manner previously described.
The curve indicates the most nunierous fibres to be those of
1 p, this group making 42% of the total fibres measured. The
fibres of larger size are seen to be very few and only to reach
the size of 5 p. Their presence is indicated by the more gradual
sloping of the whole distribution curve towards the large fibresizes. I n our table 1, are given the numbers per square millimeter for each of the 4 parts of the C.C. into which me divided
the whole structure. Each of these 4 parts corresponds to
approximately one-fourth of the C.C., the first including rostrum and g e m , the second and third parts obtained by dividing
the body into two and the 4th part being the splenium portion.
Comparing these 4 parts of each of the three brains studied,
stained by Weigert a s well as by IIaeggquist methods, the number of fibres per square millimeter, can be seen to be highest in
the area of the genu and splenium. This might be due to a relatively greater density of fibre arrangement there, but tliis is
possible because there a r e more thin fibres in genu and sple-
TABLE 1
Brain no. 1
A . C0urLt.v carried out
PARTS O F CORP. CALLOSUM
011 sections
stained by Veiget’t ’s method
SURFACE-AREA
KO. O F F I B R E S
I N T H I S PART
N O . OF F I B R E S
P E R Mi#
-
~~
G e m and Rostrum
Eody, ant. half
Body,. post. half
Splenium
1
2
3
4
Total corp. callosuni
B. Counts ca,rzed owt
ON
mma
millions
142
107
119.5
156
31.08
23.19
22.43
35.09
218,873
216,728
187,698
224,936
624.5
111.79
213,136
sections stained by Haeggqnist’s method
1 G e m and Rostrum
2 Rody, ant. half
3 Body,, post. half
4 Spleniuni
142
107
119.5
156
54.87
36.16
36.08
49.86
386,408
337,943
301,924
319,615
Total corpus callosuni
524.5
176.9;
337,407
63.18
124,271
Difference between counts 011 sectioiis stained
h.; Haeggquist ’s and Weigert ’s niethods :
-
Brain no. 2
A . Counts carried out
Genu and Rostrum
Body, ant. half
Body, post. half
Spleniuni
011
sections stained by Weigert’s ?nethod
130.6
135
109
142
28.32
26.19
19.59
29.97
216,845
186,628
179,800
211,110
Total corpus callosum
516.6
103.07
199,516
B . Counts carried out o n sections stained by Haeggquist ’s m t h o d
Genu and Rostmni
Body, ant. half
Eody, post. half
Spleiiiuni
130.6
135
109
142
49.62
44.86
33.97
46.24
379,938
332,335
311,705
325,655
Total corpus eallosum
516.6
174.70
338,172
___
-
Difference between counts on sections stained
by Haeggquist’s and Weigert ’s methods:
-
~~~~
71.63
.
--_
138,656
.
-
__._
...-
Brain no. 3
A . Counts carried out on sections stained by Weigert’s niet7iod
____~____Geiiu and Rostriini
-028
45.94
301,605
Body, ant. half
88
13.73
156.091
91
16.32
179.34‘1
Eody, post. half
148
30.48
206,00:3
Splenium
Total corpus callosum
555
106.48
191,853
_ .~__
~~~~
B. Coicnts carried oatt on sections stained by Haeggquist ’s method
Genu and Rostrum
Eodp, ant. half
Rodp, post. half
Spleiiium
228
88
91
148
83.05
29.07
30.52
50.83
Total corpus callosum
555
193.47
Difference between counts on sections stained
b.; Haeggquist ’s and Weigert ’s methods:
-____
___
127
86.99
364,260
330,319
335,434
343,449
_ _
348,394
__
__
156,T39
~-
128
JOSEPH TO M A S CH
nium than in the area of the body. Though one really finds, in
sections through the posterior half of the body, many more
fibres of sizes up to 5 p , their number is not high enough to
cause significant changes in a distribution curve drawn from
this region. The general smallness of the callosal fibres is also
revealed by the high number of fibres per square millimeter.
The number is about 300,000 as seen in the Haeggquist-stained
sections. This represents a fibre-density three times that found
in the human pyramidal tract by Weil and Lassek ( ’as),their
estimate being approximately 100,000 per square millimeter.
We ourselves examined sections of the pyramidal tract a t the
same magnification as we used for the corpus callosuni and we
found in our Whipple-disc squares not nearly half as many
fibres as in our counts on the C.C. It has to be mentioned in
this connection, that the diameter-values of the fibres as well
as the total area of the cross-sections of the C.C. are affected
by the shrinkage involved in processing the sections. No attempt was made to eliminate this factor in our figures. This
shrinkage is the same for both staining methods used, since the
two procedures require a similar treatment. The amount of
this shrinkage has been estimated for the Haeggquist method
by Rexed and Swenson (’41) a t 2570. This figure gives us a
more exact idea of the actual intra-vitam size of these fibres.
As f a r as the myelinated fibres of the C.C. are concerned, our
observations agree with those of Szentagothai-Schimert whose
findings are summarized as follows (translated). “The corpus
callosum contains everywhere thin fibres in great abundance.
Only a small percentage of fibres with a calibre more than 4
microns is found. Fibres more than 6 microns thick are rarely
found. I t is interesting to note, however, that the percentage
of fibres of a calibre more than 4 microns varies characteristically in the different parts of the corpus callosum. It is lowest
in the genu where there are only 0.570, considerably higher in
the splenium - 2.570, and highest - 5% in the posterior third
of the body.” He preferred for his investigation the Weigert
method which shows the myelin-sheaths distinctly stained. He
made no attempt, however, to determine anything about the un-
FIBRES O F H U M A N C O R P U S C A L L O S U M
129
myelinated fibres present in the C.C. We have gone a step
further and have estimated the total number of fibres (myelinated and un-myelinated) by counting the axones, which are
stained by Haeggquist’s method. By subtracting from this
total the number of myelinated fibres, estimated from the
Weigert-stained sections, we have indirectly determined the
number of non-myelinated fibres. (It is quite possible, however, that a proportion of the fibres classified in this group
may be very thinly myelinated.)
The number and presence of the un-myelinated fibres as revealed by the method of counting axones and myelin-sheaths
on differently stained sections is indicated also by the difference in the curves on our figures 1and 2. In comparing the two
curves one easily notices that the peak in the Weigert stained
section is situated a t the fibre size of 19 p and reaches in this
fibre group the proportion of 43% of the total fibres. I n tlie
curve obtained from the same part stained after Haeggquist
the spike is situated at the fibre-size of 1p and their proportion
is 51%. This difference between the two curves clearly indicates that a great number of very small fibres escape staining
and therefore, are not seen in the Weigert-stained sections.
The effect is the same as obtained by Ranson and Davenport
( ’31) in comparing osmic acid-stained with silver-impregnated
sectioiis of spinal-cord roots. Their observation that these
fine un-myelinated fibres are grouped together in small^ bundles
between the larger fibres conforms closely t o our findings regarding these fibres in the C.C. Sections stained by Haeggquist’s and Weigert’s methods are even more directly comparable than the ones prepared by the osmic acid and silver
methods. The reason for this is that the difference in shrinkage as between osmic-acid and silver-stained sections is very
large, but between the two methods employed by us no difference in shrinkage is present. The diff erence-values obtained,
therefore, represent the actual difference and do not require
any adjustment in magnification or calculation. As shown in
our tablce 2, the number of non-myelinated fibres in certain
areas is found to be as high as 50@, the average-value, how-
130
JOSEPH TOMASCH
ever, being about 40% in most parts. The percentage of these
fibres is found in all three brains to be lom7est in the area of the
splenium. With the exception of brain no. 3, the highest percentage is found in the anterior 4th of the C.C. This indicates
that although the fibre density was found to be highest in both
the anterior and posterior 4ths, the genu-area contains the
TABLE 2
Brain no. 1
YARTS O F C. C.
DIFFERENCE
BETWEEN
COUNTS FROM
HAEGGQUIST AND
\VEIGERT STAINED
SECTIONS
PERCENTAGE
OF U N -
MYELINATED OR
THINLY
MYELINATED
FIBRES I N THIS
SECTION
RATIO OF
3lTELINATED TO
US-YYELINATED
FIBRES
%
Geiiu and R,ostrum
Body, ant. half
Body, post. half
Spleniuni
167,535
121,215
114,226
94,679
43
35
37
29
1.3: 1
1.7: 1
1.6:1
2.3:1
Total eorp. call.
124,271
36
1.7: 1
163,093
145,707
131,905
114,545
43
43
42
35
1.3:l
1.3: 1
1.311
L8:l
Total corp. call.
138,656
41
1.4:1
Genu aiid Rostruni
Body, ant. half
Kody, post. half
Splenium
162,755
174,228
156,092
137,446
44
52
46
40
1.2: 1
0.8: 1
1.1:1
1.4: 1
Total corp. call.
156,739
44
1.2:1
...
.
__
Brain no. 2
Genu and Rostrum
Body, ant. half
Body, post. half
Spleniuni
~
Brain no. 3
__
~
larger proportioil of lion-myelinated fibres, while in the splenial area, though there are found more fibres per square millimeter than in the body, they are apparently small fibres, with
thin myelin-sheaths. Davenport and R.anson found in the dorsal, roots of dogs 59% of non-myelinated fibres. The existence
of un-myelinated fibres in the cortex and the striatum and in
pathways like the pyramidal tract is nom almost generally
FIBRES O F H U M A N CORPUS CALLOSUiM
131
accepted (Larsell, ’51). However, information as to their
numbers is scarce and to some extent controversial. The existence of a t least a thin film of myelin sheath is, for instance,
postulated by physiologists in accordance with conduction
theories. However, as polarization-microscopy and other
methods indicate (Bejdl, ’50), the term “un-myelinated” is
for many fibres in the central and peripheral nervous system
well justified. As stated previously, we are aware. that a iiuniber of the fibres counted as un-myelinated might have lost thin
myelin-sheaths in the differentiation process of the Weigertstaining. This possibility seems to be indicated by some variation in the values obtained from the corresponding parts of
our sections of the three brains. On the average we found the
Ca.Ca. to contain myelinated and un-myelinated fibres in a
ratio of 1.4: 1, a proportion which is comparable with the figures given by Davenport and Ranson for the posterior roots in
dogs, their ratio being 1:1.4.
The values of the numbers of fibres per square millimeter as
shown in table 1 enabled us to gain an estimate of the total
number of fibres after having determined the total, surface-area
of the C.C. The total-surface area was investigated bT- Spitzka
(’05) from the viewpoint of individual differences. I n comparing the cross-section areas of the corpora callosa of eminent
men, he found the average higher than in ordinary men. He
found also, that the brain-weight and the cross-sectional area
of the C.C. seem to correspond insofar as a brain of more than
average weight can be expected to have a more than average
size C.C. These views are supported by the u7ork of Bean (’06)
who studied the C.C. cross-sections from the view-point of
racial differences. It is to be mentioned that in many cases of
congenital absence of the C.C. no detrimental effect on the intelligence was found. In the greater number of such cases,
however, reduced mental power is reported, frequently combined with other morphological anomalies of the brain.
The cross-sectional areas of the Ca.Ca. of our three cases are
524.5 mm2,516.6 mm2, and 555 mm2 respectively. These values
are obtained from the histological sections which, as mentioned
132
JOSEPH TOMASCB
previously, have undergone the amourit of shrinkage iiivolved
in the fixation and imbedding process. Considering this shrinkage as about 25%, our three brains represent an average crosssection of Caucasian male brains, estimated by Bean, in the
untreated condition, at 702 mm2. The total number of fibres in
each of our three cases, obtained by the multiplication of number of fibres per square millimeter with cross-section area, expressed in square millimeters, is contained in table 1. Brains
no. 1and no. 2 with 176.97 million and 174.70 million fibres are
seen to correspond very closely, the C.C. of brain no. 1having
a slightly larger cross-section and a slightly larger number of
fibres. I n brain no. 3 we found not only the highest crosssection area, but also a slightly higher fibre density than in
the other two brains. The total fibre number is 193.47 millions.
We do not attempt to draw any conclusions from this single
case as to whether large cross-section area and high fibredensity might be associated with each other. The difference
might well' be within the limits of error of our method, but
study of this view-point on a larger amount of material could
be justified. Table 1gives also the total number of myelinated
fibres, their numbers being for the three brains 111.79 millions,
103.07 millions, and 106.48 millions, respectively. The unmyelinated fibres number 65.18 millions, 71.63 millions, and
86.99 millions. For the third brain the number of myelinated
fibres is relatively low and the number of un-myelinated fibres
relatively high.
Not very much quantitative information on central fibretracts is available for comparison. The number of about 190
million fibres for only this one portion of the central-nervous
system seems, at first thought, to be quite high. However, if one
compares this number with the estimates of the total number
of nerve-cells in the cerebral cortex, the figure of 190 millions
is but a small fraction. Thompson (1899) using the estimation
method of Hammarberg on the human cerebral cortex found
9,200 million nerve-cells in it. This figure does not include the
number of cells present in the basal ganglia. I f nearly 200
millions of these cells give rise to callosal fibres, them still re-
FIBRES O F HUMAN CORPUS CALLOSUM
133
main 9,000 millions to be attributed to other fibre tracts. Weil
and Lassek found tm7o million fibres in the human pyramidal
tracts of both sides. Another two inillion a r e present in the
optic nerves (Chacko, '48). We feel that further information
concerning the numerical composition of central pathways may
assist in the way of a new approach towards the better understanding of these systems and the central nervous system as a
whole .
SUMMARY
Using sections made from three Corpora Callosa, estimates
have been made of the total number of fibres and the numbers
in each case of myelinated arid un-myelinated fibres. The total
iiuniber was estimated from counts on Haeggquist-stained sections, in which the axons can be seen. Weigert-stained sections
provided the basis for the estimate of the nuniber of myelinated fibres. The number of un-myelinated fibres was obtained
by subtraction of the latter from the total number.
ACKNOWLEDGMENT
The author wishes to express his thanks to the head of the
Department, Professor D. C. Matheson, for helpful assistance
in the writing of this paper.
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