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The histology of the radix mesencephalica n. Trigemini in the dog

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The Departmmt of Anatomy, The Johns Hopliii~sMedical School
Studies of the cells and fibers of the midbrain root of the
n. trigeminus form a fascinating chapter in neurological research, illustrating an important principle of embryological
development somewhat at variance with older theories if the
present popular belief a s t o the function of this system is
Stilling, in 1846, first described the cells grouped around
the central gray matter of the cerebral aqueduct now recognized a s the cells of origin of the fibers of the mesencephalic
root of the trigeminal nerve. Meynert ('72) studied the
mesencephalic root, connected it with the trigeminal system,
and differentiated its cells from those of the substantia ferruginea, ascribing to them a sensory function, on the basis of
their morphology (size, shape, and lack of numerous processes). Later, when, as the result mainly of the introduction
of Golgi's silver technique, it was believed that primary
sensory neurones arose only from cells located outside the
central axis of the nervous system, this opinion of hleynert's
as to their function fell into disrepute. Henle ('79) thought
that the mesencephalic root was a part of the motor system
of the trigeminal. Van Gehuchten ('95) and Kijlliker ('96)
strongly favored this opinion, supporting it with the observation that the fibers of the mesencephalic root apparently
joined the motor nerve. Cajal ('96) was the first to describe
the important system of collaterals connecting the midbrain
root with the primary motor cells of the trigeminal. Merkel
and Krause, Held (’93), and Edinger ( ’11)also favored the
theory of its motor function.
Prior to the work of Johnston (’05 and ’OY), very little
evidence opposed the generally accepted view of the mesencephalic root as a motor structure; he, however, in a series
of very thorough and painstaking researches, definitely
established its sensory nature. H e pointed out that tlie
central location of the cells of origin of the fibers of the root
was not evidence that they were motor fibers: “The only
conclusive evidence would be to trace them to motor nerveendings within a muscle or to demonstrate their action physiologically. The origin of fibers from central cells is not even
presumptive evidence that the fibers are motor unless the
cells lie in the ventral (motor) zone of the brain or cord. . . .
The most natural assumption is that peripheral fibers arising
from cells in the brain roof would be sensory fibers as in all
lower vertebrates.” From evidence in the work of Cajal
on the mouse and Van Gehuchten on the trout, Johnston concluded that the collaterals from the mesencephalic fibers ending around the cells of the motor nucleus of the trigeminal
corresponded to the descending fiber arising from giant cells
in the cord of teleosts and suggested the hypothesis that the
primary axone has been reduced or is absent from the mesencephalic cells of most mammals and that the motor collaterals
conduct direct reflexes from the sensory surfaces about the
mouth t o the muscles controlled by the n. trigeminus.
Willems ( ’ll), in his exhaustive experimental work on the
whole trigeminal system, presented evidence in favor of the
sensory function of the midbrain root. H e believes with
Johnston that the midbrain root is a sensory structure, that
the cells of origin resemble closely spinal ganglion cells, and
that the collaterals to motor nuclei are histologically and
functionally axones. From tlie embryological point of view,
the midbrain nucleus is a true homologue of a spinal ganglion,
notwithstanding the fact that the neural-crest cells which
went t o form it were not separated off when the medullary
folds of the midbrain rolled up to form a closed tube.
Willems found, however, that the midbrain root fibers do
end in muscle, for chromatolysis occurred in the cells after
cutting the nerves t o the masseter, temporal, and to a less
extent the external and internal pterygoid. No chromatolysis
was seen after cutting the nerves t o the digastric and
mylohyoid. The mesencephalic cells are not arranged, therefore, in definite muscle groups.
May and Horsley ( Y O ) showed by methods of degeneration
that the mesencephalic root also contains a number of ascending fibers originating from cells in the gasserian ganglion
and distributing collaterals t o the motor trigeminal nucleus
and t o a small group of cells situated dorsal and medial to the
principal trigeminal sensory nucleus. Above the motor 1111cleus the mesencephalic root is composed almost entirely of
descending fibers.
Allen (’18) agreed with Johnston and Willems that the cells
of origin of the mesencephalic root were sensory cells, but
disagreed as to the course of the fibers, claiming that they
follow the motor root fibers into the masseteric, pterygoid,
and temporal branches of the n. masticatorius. “They are
not fibers of cutaneous sensibility else they would be distributed to the maxillary, ophthalmic and sensory branches of
the mandibular nerve, hence they must be muscle sense
fibers.’’ Incidentally, he confirmed the work of May and
Horsley a s to the origin of certain fibers of the root from
cells in the gasserian ganglion.
I n connection with the possible proprioceptive function of
the midbrain trigeminal nucleus it is interesting to note recent
work dealing with the function of the central secondary sensory nuclei of the fifth nerve. Gerard (’23) found that loss
of pain and temperature sensibility was correlated with the
destruction of the spinal root or its nucleus. Moreover, a
study of cases of syringomyelia has demonstrated that a loss
of tactile sensation follows injury of the main sensory nucleus. As no unbranched fibers to the spinal tract were found,
the author concluded that all the fibers bifurcated upon entering the medulla, one branch descending to the spinal nucleus,
the other ascending to the main sensory nucleus, and suggested the remarkable hypothesis that all types of impulses
might run through a single nerve fiber, a sifting occurring
at the synapses. Cajal (’96) believed that all the fibers
bifurcated, while Kolliker considered that some did not.
Windle ( % ) , using the Golgi technique, has traced nonbifurcating fibers into the spinal tract. He believes that these
form a pathway for pain impulses separate from that for
It is evident from a survey of recent and careful work
that the general opinion favors the view that those cells
giving origin t o the fibers which make up the mass of the
midbrain root of the trigeminal are sensory in function. It is
the purpose of this paper to present the results of a morphological study of the cells of the midbrain root and to draw
certain analogies between them and cells of known sensory
function; the primary sensory neurones found in spinal and
cranial ganglia.
The brain stem of a six-weeks-old puppy was fixed in
Carnoy’s fluid and impregnated with silver according to a
technique perfected by Miss Campbell of Washington. As
this method presents certain advantages over other silver
stains and is not well known, the procedure is given here.
This impregnation method for nerve cells and f o r medullated and non-medullated fibers has the advantage of allowing a counterstain for the differentiation of other tissue elements. It has been found satisfactory in staining the fibers
of both the central nervous system and peripheral nerves.
1. Fix pieces of tissue not over 3 mm. thick for three hours
in Carnoy’s fluid: absolute alcohol, 6 parts; chloroform, 3
parts; glacial acetic acid, 1part.
2. Wash in several changes of absolute alcohol for twentyfour hours.
3. Place in 50 per cent alcohol for six hours.
4. Place in ammoniated 50 per cent alcohol (5 drops of
NH,OH to 50 cc. 50 per cent alcohol) for twenty-four hours.
5. Rinse quickly in distilled water.
6. Impregnate in a 2 per cent aqueous solution of AgNO,
for five days.
7. Wash in frequently changed distilled water for one to
two hours.
8. Reduce for twenty-four hours in the following solution:
hydroquinone, 1gram; distilled water, 100 cc.; neutral 40 per
cent formalin, 15 cc.
9. Wash in frequently changed distilled water for two to
three hours.
10. Place in 80 per cent alcohol.
Steps 6, 7, and 8 should be carried out in the dark at 37°C.;
step 9, in the dark at room temperature. The Carnoy’s fluid,
silver solution, and reducing solution should be freshly made
up. It is essential to have clean glassware and to use distilled water throughout. After the tissue has been dehydrated,
embed, and cut according to the usual paraffin technique. The
sections may be counterstained with hematoxylin and eosin
and mounted in balsam.
Nerve cells stain brown; their nuclei, blue, and their processes, black. Non-medullated fibers stain intensely black and
medullated fibers brownish black. The myelin sheath appears
as a yellowish segmented cylinder surrounding a darker zone.
Where the neurilemma is present its oval blue nuclei are
easily distinguishable lying along the axis cylinder of the
medullary sheath.
A very successful impregnation was obtained. Serial sec~
The cells of
tions of the brain stem were cut 1 2 thick.
the mesencephalic trigeminal nucleus with their fibers and
related structures stood out especially well.
The cells of origin of the midbrain root of the 11. trigeminus
lying in the outer edge of the central gray matter around the
cerebral aqueduct will first be described, then their fibers,
and, finally, characteristic end bulbs terminating upon the
surface of the large cells. The cells are sparse in sections
through the upper end of the superior colliculi, but immediately increase in number below; eight to twenty-four cells
are seen in a single section. Many of the larger may be
followed through two or three transverse sections. The cells
are arranged in groups or nests (figs. 11 and 14) of two to
five; generally these nests are composed of units of almost
identical size and structure. For the purpose of description,
it may be well to divide the cells into two classes on the basis
of size, one of large, the other of medium-sized and small
cells. The large cells (figs. 1, 5, and 8) are pear-shaped,
deeply staining, usually unipolar, though occasionally bipolar
or multipolar, occupying, in general, the ventral portion of
the nucleus. The medium-sized and small cells (figs. 3, 4,
and 10) are more irregular in shape, less intensely stained,
many unipolar, though more frequently bipolar and multipolar than those of the first group. They are found in pure
culture in the dorsal porti6n of the nucleus, but also are
scattered among the large cells. Among these two groups a
few cells may be seen of entirely different appearance (fig.
15). These are small fusiform cells similar to those found in
small numbers scattered in the reticular formation of the
The first group, then, are large, round, or oval cells containing a nucleus eccentrically placed, a definite nucleolus,
and usually giving rise to a single process which runs laterally
to meet the mesencephalic tract, and then turns sharply to
join it (fig. 1). The outline of the cells is not always regular,
but is often sharply indented by small neuroglia cells which
lie in a depression that they have formed in the surface of
the cytoplasm (fig. 2). The large cell is the classical type
described as composing the mesencephalic trigeminal nucleus.
Bipolar cells of this same size and structure are relatively
common and some multipolar cells are seen (figs. 6 and 7 ) ,
with their principal process running to join the descending
root. Several smaller processes, morphologically of the structure of dendrites, leave the cell to run toward the colliculi,
reticular formation, or the central gray substance. A few
turn dorsalward and probably in a cephalic direction. There
are also unipolar cells whose processes branch just before
or a t the time of entering the tract (fig. 5 ) . Both branches
are sometimes of equal size; one joins the descending tract,
the other, usually the smaller, turns dorsally and probably
cephalad. This type of branching is similar to that occurring
in the fibers of the spinal ganglia. As a rule, the process
of a unipolar cell from its origin to the point where it joins
the mesencephalic root runs a fairly straight course. There
are processes, however, which are undulatory and others
that are definitely curled (fig. S), again suggesting a parallel
with the coiled axone given off from the spinal ganglion cell.
The second group of medium-sized and small cells are
somewhat more numerous than the large (figs. 3 , 4 , 9 , and 10).
They occur in groups of three or four in the dorsal portion
of the nucleus, and in the ventral portion are found scattered
among the large cells. They all present the morphological
characteristics of sensory cells, having a round or oval contour with the nucleus placed a little to one side of the center
of the cell (fig. 3). The surface is often indented by neuroglia
cells (fig. 10). A majority are unipolar, the axone running
to join the main tract (fig. 3 ) , but a branching of the process
often occurs at a short distance from the cell (fig. 10). Bipolar cells are seen with branched and unbranched processes
(figs. 4 and 12) and many multipolar cells (figs. 9 and 13).
The easily recognizable principal process is seen to join the
midbrain root, while the smaller dendrites run toward the
colliculi, the formatio reticularis or the central gray matter
around the aqueduct. It would appear that most of these
are short fibers for local reflexes ending near the cell and
never uniting to form a central common pathway.
A great number of the large cells occurring in the ventral
portion of the nucleus are seen to have ending upon and
about them curious, bulb-like structures, all staining an
equally intense black with the silver. They are more common
at the cephalic end of the nucleus and are not found in its
dorsal portion nor ending around the smaller cells. I n form
and mode of ending they show such wide variations that many
of them are pictured in drawings carefully traced with the
camera lucida (figs. 16 to 32). Usually one or two terminate
about a single cell (fig. 17), but sometimes they are so numerous as to obscure to some degree the outline of the cell about
which they end.
The fibers giving rise to these endings are of variable thickness. Often very fine fibers expand into a large bulb which
is closely applied to the surface of the cell (fig. 20) ; sometimes
the transition from fiber to ending is more gradual (fig. 24).
The fiber when first seen may be thick only to divide and
produce several bulbs (.fig. 19) which may end upon one, two,
or more cells. Other fibers develop bulb-like expansions along
their course over the surface of a cell (fig. 22).
The bulbs themselves with this stain show no differentiation
of structure, being entirely homogeneous throughout. They
vary in size from mere varicosities upon the fibers t o about
one-fifth the size of the nerve cells (fig. 16). In the majority
of cases there springs from the pole of the bulb opposite the
main fiber of origin a very short and tenuous filament. This
is so characteristic in size and length that it can scarcely be
considered as the continuation of t,he main fiber to another
h majority of the bulbs are closely applied to the surface
of the cell bodies, though in profile a definite plane of separation can always be seen between cell and ending (fig. 25).
Some bulbs lie against the axone of unipolar cells (fig. 21) ;
others end free without any,relation to the cells, but in the
region of the mesencephalic nucleus (figs. 16 and 31). The
cells of the nucleus lie medially to the ventrally coursing
fibers; occasional endings are found lying free in the reticular
formation on the lateral side of the midbrain root (fig. 29).
I n the silver preparations studied, it has been impossible to
trace the origin of the fibers giving rise to the end bulbs.
Since such nerve terminations have been described only in
one other place (i.e., upon the cells of the sensory ganglia
of the cranial and spinal nerves), and since the cells of the
mesencephalic trigeminal nucleus seem to have a similar
origin and function, it may be justifiable to draw certain
analogies between the two.
The central cells giving rise to the fibers of the radix mesencephalica trigemini vary greatly in size ; many are large, while
medium and small cells are even more numerous. This
divergency in size is also evident in the sensory ganglia of
the cranial and spinal nerves. Ranson ( '20), seeking to correlate this morphological fact with the function of the cells,
suggested that many of the small cells of the ganglia give rise
to fibers conveying pain and probably also temperature sensation; some are also interoceptive cells. No method has been
found to separate groups of cells in the ganglia giving rise
to fibers of one physiological type. Warrington and Griffith
('04) cut the nerve to a striated muscle and found that the
cells in the posterior root ganglion showing a reaction to
the injury were the largest cells in the ganglion; they concluded then that these large cells were proprioceptive cells.
They also found that the cells whose peripheral fibers ended
in smooth muscle and glandular tissue (interoceptive cells)
were relatively few, composing only about 2 per cent of the
cells of the ganglion and that most of these cells were of small
There are few places, however, where a group of sensory
cells having a particular function can be isolated in pure
culture. But, if the evidence of Willems ('ll), Korsaka ( '12),
and Allen ('18) is accepted, the mesencephalic trigeminal
cells give origin to fibers all or most of which end in the
masticatory muscles ; the cells are, therefore, probably proprioceptive cells.
At least one other place where proprioceptive cells are
found to the exclusion of other physiological types is known.
Laiigworthy ( '24 a and b) proved by morphological methods
that iieuromuscular spindles occur in the intrinsic musculature of the tongue, aiid by means of degeneration experiments
showed that these proprioceptive fibers run with the hypoglossal nerve. I n searching for the cells of origin of these
fibers he noticed that a small ganglion is often found upon the
n. hypoglossus in the cat aiid other mammals analogous to
Froriep's ganglion described in the human embryo, but not
commonly found in the adult brain of man. The ganglion
when sec1,ioned was seen to contain both large and small cells
of the sensory type. When time was allowed for degeneration after cutting the n. hypoglossus, chromatolysis was observed i n all these cells. Their fibers, therefore, run in the
hypoglossal nerve and a r e probably all proprioceptive in function. Langworthy did not believe that this inconstant ganglion was the source of all the proprioceptive fibers to the
intrinsic tongue musculature, but suggested other places
where the cells could be found.
Hatai ( ' O 2 ) , as a result of a study of the proportion of
large to small cells and of myelinated to non-myelinated fibers
in the second and third posterior cervical roots of rats of
different ages, concluded that the small cells in the ganglia
were arrested in development and that throughout life some
of them were continually growing to large cells and their
fibers gaining a myelin sheath. I n young rats the proportion
of large to small cells is ten to twenty-four, in the adult
it becomes ten to fourteen. The diverse size of cells, all,
apparently having a proprioceptive function, might be
accounted for in this way.
It has been seen that the great majority of the cells of all
sizes in the mesencephalic trigeminal nucleus are unipolar,
aiid whereas Willems ('11) considered that all the axones
were straight, definite coils and twists have been noted in the
preparations just described, suggesting a structure analogous
to spinal-ganglion cells. The axoiie runs lateralward to meet
the descending fibers aiid then turns sharply to join them.
Often, however, just before joiniiig the mesencephalic root,
the axone divides into two branches, sometimes of equal, but
often of disproportionate size. Both branches may descend
in the tract or one, usually the smaller, may turn dorsalward
and probably cephalad.
A few of the large cells and many of the medium and small
are bipolar or multipolar. That these cells are part of the
midbrain trigeminal nucleus seems certain, since they have
the general conformation of sensory cells and their axones
join the descending root. They have none of the characteristics of the motor cells seen in other midbrain nuclei, and
it is easily possible to distinguish them from cells of the
reticular formation that by chance lie in t,his region. The
dendritic branches run toward the central gray, formatio
reticularis, and corpora quadrigemina, or follow the midbrain
root fibers dorsalward and probably cephalad. Many of these
fibers no doubt establish local reflex connections. However,
it seems probable that a central pathway for the proprioceptive trigeminal area exists connecting with the thalamus
and eventually with the cerebral hemispheres in a manner
similar to that followed by the proprioceptive fibers from the
axial and limb musculature, many of which run to the
thalamus in the median lemniscus. The only important reflex
path now known, however, is the direct one formed by collaterals of the mesencephalic root fibers ending upon cells
in the primary motor nucleus of the n. trigeminus.
Johnston ( '09) believed that the trigeminal mesencephalic
cells in mammals were all unipolar and bipolar, the latter
giving rise to coarse peripheral and slender central processes.
I n the toad the cells are unipolar, bipolar, and multipolar,
the multipolar cells having one large process and small unimportant dendrites. ,Johnston thought with Cajal that these
dendrites were embryonic and transitory, having greater importance in lower than in higher forms. The large process
in selachians and toads has occasionally been seen to divide
at a short distance from the cell into two branches, one thick,
the other finer. I n the rat, mouse, and rabbit no branching
occurs near the cell body. The presence of bipolar cells with
an ascending axone was described by Merkel and Krause,
but Cajal ('96) saw no thick ascending processes. The number of multipolar cells in the present preparations seems
larger than that described by other authors for mammals.
The puppy from which these sections were made was only six
weeks old. It may be possible that, before adult life is attained, the dendrites tend to atrophy and disappear.
The conspicuous bulb-like nerve endings upon the large
cells of the mesencephalic nucleus have been described, as
well as their variations in size and shape and their uniform,
intense staining reaction t o the silver. Seen in profile, they
appear to present a cup-shape surface to the cell; a plane of
separation between ending and cell is always marked. Willems ('11) mentioned the presence of these bulbs, but did
not describe them. I n the present work the origin of the
fibers giving rise to the terminations has not been seen. The
one other place in the body where such endings are found
is in the spinal and cranial ganglia.
Dogie1 ('96) found, in addition to unipolar, both bipolar
and multipolar cells in spinal ganglia. The bipolar cells
were very few in number. The multipolar cells gave rise to
two principal processes, one centrally, the other peripherally
directed. I n addition, many dendritic processes arose from
the angles of the irregularly shaped cell body; these penetrated the capsule and ended in bulb-like end-plates upon
other cells within the ganglion. Cajal ('05) described these
multipolar cells and thought them the most unique and
strange elements in nerve centers. He observed that they
were rare in most mammals (cats, dogs, and mouse), but
common in man, particularly in the plexiform ganglion of
the vague. He found that the dendrite might end in three
different places. I n the first and most numerous group the
dendrite never left the cell capsule, but ended upon the same
cell from which it arose. These delicate appendices, always
few in number, arise from the cell body, glomerulus, or proximal portion of the axone and are made up of one or a group of
neurofibrils. The fibers increase gradually in diameter with
distance from their origin and terminate after describing one
or two twists in a bulb or occasionally in a row of bulbs which
usually lie against the cell body or in a depression of the
same. The second type of collateral pierces the capsule or is
given off from the axone some distance from the cell of origin
and penetrates the capsule of another cell to terminate in
end bulbs upon its surface. The third type of dendrite runs
out among the fibers of the ganglion and ends free, without
contact with a cell in a n end bulb surrounded by a definite
capsule. Sometimes two or three bulbs end in the same feltwork of fibers, and the whole is surrounded by a capsule.
Unfortunately, due to the thin sections necessary in studying silver preparations, it was not possible to follow the
dendrites from their origin to terminations. All of the end
bulbs occur on the cells of large size. If one accepts the
evidence of Warrington and Griffith ( '04), that the functional
proprioceptive cells are large cells, one must either assume
some other function for the small cells or agree with Hatai
that the small cells have been retarded in their development
and that at least a portion of them, before adult life is reached,
will have assumed the proportions of large cells. It is interesting to note that a number of the end bulbs were not in
contact with cells, but ended free in the neighborhood of
the fibers of the tract or even in the reticular formation.
The function of these curious endings is certainly not understood and the one explanation thus f a r advanced seems far
from satisfactory. Nageotte ('96) combats the idea that these
are sensory receptors or receptive organs and points out their
analogy or even identity with regenerating afferent fibers as
described by him more particularly in cases of tabes. He
believes that they represent the attempt of injured cells to
replace their axones. It is his belief that these endings are
more common in old age or in morbid conditions where the
axones undergo destructive changes. This formation is a
normal process, however, for it is found even in young healthy
animals, as the present report amply illustrates.
The cells of origin of the mesencephalic root of the 11.
trigeminus differ as to size, and on this basis the large cells
may form one group; the medium-sized and small cells,
another. The large cells are found in the ventral portion
of the nucleus, the small cells and medium-sized dorsally,
but also scattered among the large cells. Both groups have
the general form of sensory cells and both send fibers to run
in the mesencephalic root. A majority of the cells are unipolar in the dog; a few of the large cells are bipolar and multipolar. Among the small cells bipolar and multipolar forms
are common. End bulbs staining a uniform, intense black
with the silver stain terminate close to the surface of the
large cells; a few end free in the region of the nucleus or
even on the lateral side of the tract in the reticular formation.
Former work has clearly demonstrated that the fibers from
the trigsminal mesencephalic cells are sensory and run in the
n. masticatorius to the muscles innervated by the trigeminal
nerve. It may be justifiable to conclude that the cells described are all proprioceptive cells, varying greatly as t o
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Camera-lucida drawings of cells, fibers, and nerve endings in the mesencephalic
trigeminal nucleus of the dog. A full description of the individual figures will
be found in the text. The magnification of all the drawings, except figures 29
and 31, is 325 times. Drawings 29 and 31 are magnified 110 times.
The cells of the mesencephalic trigeminal nucleus vary much as to size. Thus,
large cells are seen in figures 1 and 2, mediuni-sized and small cells in figures 3,
4, 9, and 10. The cells, particularly the large ones, often occur in clusters of
two to five of almost uniform size (fig. 11). Often neuroglia cells lie inside of
the capsule and markedly indent the nerve cell (fig. 2). The typical components
of the midbrain nucleus may be easily distinguished from the multipolar cells
common to the reticular formation that often are found close at hand (fig. 15).
While a majority of the large cells are unipolar (fig. l ) , the single fiber running laterally and turning sharply to join the mesenrephalie root, bipolar (fig. 5 )
and multipolar cells are often seen (figs. 6 and 7 ) . Both of the fibers of the
bipolar cells may join the mesencephalic root, or one, usually the smaller, turn
cephalad. The fine dendrites of the multipolar cells run toward the colliculi,
reticular formation, or the central gray substance. Among the medium-sized and
small cells bipolar (figs. 4 and 12) and particularly multipolar forms (figs. 9
and 13) are even more abundant. The process of the unipolar cell is often
undulatory or dcfinitely curled (fig. 8). A group of the large end-bulbs that
terminate around the mesencephalic trigeminal cells are seen in figure 16.
33 7
The bulb-like nerve endings found close to the niesencephalic trigeminal cell$
are so unique and show such wide variations in form and mode of ending that
numbers of them, carefully traced with the camera lucida, a r e shown here.
Usually one or two terminate about a single cell (fig. 17), but sometimes they
are so numerous as t o obscure t o some degree the outline of the cell about which
they end. Often very fine fibers expand into a large bulb, which is closely applied
t o the surface of the cell (fig. 20) ; sometimes the transition from fiber to ending
is more gradual (fig. 24). The fiber when first seen may be thick, only to divide
and produce several bulbs (fig. 19), which may end upon one, two, or more cells.
Other fibers develop bulb-like expansions along their course over the surface of
a cell (fig. 32).
A majority of the bulbs are closely applied t o the surface of the cell bodies,
though in profile a definite plane of separation can always be seen between cell
and ending (fig. 25). Some bulbs terminate against the axone of unipolar cells
(fig. 2 1 ) ; others end free without any relation to the cells, but in the region of
the mesencephalic nucleus (figs. 16 and 31). The cells of the nucleus lie medially
t o the ventrally coursing fibers; occasional endings are found lying free in the
reticular formation on the lateral side of the midbrain root (fig. 29).
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histology, mesencephalic, radio, trigeminal, dog
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