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longer seen, but one or two small collections of grey matter {o.s.) repre-
sent it and constitute the superior olivary nucleus. These as well as a
collection of nerve-cells in the trapezium {nucleus of the trapezium)
are connected with fibres of the trapezium, while some of their axons
pass into the adjacent lateral part of the fillet.

The nucleus of Deiters (n. VIII., fig. 413) begins to appear in the
upper part of the bulb, and extends into the pons ; it lies near the
floor of the ventricle, a little mesial to the restiform body. The
nerve-fibres connected with its cells pass towards the middle line,
and enter the posterior longitudinal hundle, which is more clearly seen
in the two next sections (fig. 414). This bundle of fibres connects
Deiter's nucleus, the nucleus of the third and sixth nerves, and the
anterior horn cells of the spinal cord. The fibres which pass into it
from Deiters' nucleus bifurcate, one branch passing upwards to
arborise around the cells mainly of the oculo-motor nucleus of the
opposite side ; the other extends downwards through the bulb into
the cord, where they are found in the antero-lateral descending tract
of each side. They end by synapses around the anterior horn cells.

This bundle receives in addition to the fibres from Deiters' nucleus, other fibres
from the sensory nucleus of the fifth nerve, and from large cells in the reticular for-
mation of mid-brain, pons, and bulb.

The nerves which are connected with the grey matter of this
region of the pons are the sixth, seventh, and eighth, as shown in the
diagram. The nuclei in connection with the fifth nerve are higher
up, where the floor of the ventricle is again narrowing. At last, in
the region of the mid-brain, wo once more get a canal (Sylvian
aqueduct) which corresponds to the central canal of the spinal cord.

Fifth and Sixth sections are taken through the mid-brain, and
are drawn on a smaller scale than the others we have been examin-
ing ; they represent the actual size of the sections obtained from the
human subject.

Near the middle is the Sylvian aqueduct, with its lining of ciliated
epithelium. In the grey matter which surrounds it are large nerve-
cells from which the fourth nerve, and higher up the thii'd nerve,
originate ; the fibres of the third nerve are seen issuing from these in
fig. 414, B., III. The reticular formation of the pons is continued up
into the mid-brain, and is called the tegmentum. It is composed of
both longitudinal and transverse bundles of fibres intermingled with
grey matter. Its transverse fibres include those of the superior
peduncles of the cerebellum which decussate in the middle line (fig.
414, A., S.C.P.).



672 STRUCTURE OF THE BULB, PONS, AND MID-BRAIN [CH. XLVI

Another important longitudinal bundle in the tegmentum is the
fillet. This, we have seen, is the longitudinal continuation of the
internal arcuate libres, which, starting from the colls of the posterior
column nuclei of the opposite side, form the second relay on the
sensory path; to those libres others are added which originate from





Fig. 414. — Outline of two .sections across tlie mid-brain: A, through the middle of the inferior; B,
through the middle of the superior corpora quadrigemina, C.Q. Cr., crusta; S.N., substantia nigra
— shown only on one side ; T, tegmentum; S, Sylvian aqueduct, with its surrounding grey matter;
L.G., lateral groove; ji.l., i)ost«rior longitudinal bundle; d.V., descending root of tlie fifth nerve;
S.C.P., su[)erior cerebellar peduncle; P, fillet ; HI., third nerve. The dotted circle in 1! represents
the situation of the tegmental nucleus. In 15 the three divisions of the cnista are indicated on one
side. The pyramidal tilires (Pi/) are in the middle, and the fronto-cerebellar (F.C.) and temporo-
occipital cerebellar (T.O.C.) at the sides. (After Sch^ifer.)

other masses of grey matter in bulb and pons. In the mid-brain the
fillet splits into three bundles, termed the lateral, the upper, and the
mesial fillet.

(1) The lateral fillet is chiefly formed by fibres derived from the accessory audi-
tory, the inferior olivary, and trapezoid nuclei of the opposite side. Some of its
fibres terminate by synapses around a new collection of cells (tiie lateral fillet
nucleus) ; their axons pass inwards towards the raphe. The rest of its fibres can be
traced to the grey matter of the inferior corpora quadrigemina.

(2) The upper fillet consists of fibres which go to the superior corpora quadri-
gemina and partly to the tegmental region of the mid-brain and optic thalamus.

(3) The mesial fillet goes on through tlie tegmentum of tlie crus cerebri, and its
fibres terminate around the cells of the optic thalamus, and the subthalamic region.
From here fresh axons forming a new relay continue the afferent impulses to the
cortex of the cerebrum.

The mesial fillet is the important link in this region between
the sensory spinal nerves and the part of the brain which is the seat
of those processes we call sensations. But most of the fibres which
continue the sensory path of the cranial nerves form another less
well-defined tract {the central tract of the sensory cranial nerves) which
Ues dorsal to the fillet, but terminates like it in the subthalamic
region and optic thalamus, whence a new relay carries on the
impulses to the cortex.

Ventral to the tegmentum is a layer of grey matter, of which
the cells are deeply pigmented ; hence it is called the substantia
nigra (S.N.). This receives many collaterals from the pyramidal
bundles.

The white matter on the ventral side of this is known as the



ClI. XLVI.] ORIGINS AND FUNCTIONS OF CRANIAL NERVES 673

criLsta (Or) or pes. It is here that the pyramidal bundles are situated ;
these occupy its middle three-fifths (Py). The mesial fifth is occupied
by fibres passing from the frontal region of the cerebrum to the pons,
and thence to the cerebellum ; hence they are called fronto-cerebellar
fibres. The fibres occupying the lateral fifth are usually spoken of
as temporo-occipital cerehdlar fibres, but there is no certainty as yet
regarding their origin or functions.

The corpora quadrigemina are formed mainly of grey matter ;
from each superior corpus a bundle of white fibres passes upwards
and forwards to the geniculate bodies, eventually joining the optic
tract of the same side. The white layer on the surface of the grey
matter of the C. quadrigemina is derived from the optic tract ; these
fibres come from the retina, and terminate by arborising around the
cells of the grey matter of the C. quadrigemina.

The cells of the grey matter of the corpora quadrigemina differ
greatly in form and size ; the destination of their axons is not pre-
cisely known, but some pass ventralwards, cross at the raphe, and
constitute the fountain decussation of Meynert ; after decussation
they form the main mass of the ventral longi-
tudinal bundle ; this gives off collaterals to the x X ^v^'
nuclei of the three nerves that supply the eye
muscles, and then runs ventro-laterally to the
posterior longitudinal bundle, with which its
fibres ultimately mix in the antero-lateral
descending tract of the spinal cord. ^^^_ 4i5.-section through

Seventh section. — This is through the crus of cerebrum, cr,

T. • 1 i> j/i'ia_- crusta ; S.N., substantia

crus. it IS made up OI crusta (which contains nigra; T, tegmentum.

the motor fibres), tegmentum (which contains

the sensory fibres, especially the bundle called the mesial fillet), and

the substantia nigra, the grey matter which separates them.

Origins and Functions of the Cranial Nerves,

Having now studied the internal construction of these parts, we
can take up more fully the origins and functions of the cranial nerves
which originate there. The olfactory nerve is connected to the
cerebrum, and will be considered with the sense of smell. The
optic nerve will be studied with vision, though it is, as we have seen,
immediately connected with the mid-brain.

The third, fourth, and sixth nerves supply the muscles of the eye.
Gaskell discovered among the rootlets of the third and fourth nerves
the vestiges of a degenerated and functionless ganglion, which indicates
the previous existence of a separate sensory root. Sherrington has
shown that in these three nerves sensory fibres are present which are
connected to the sensorial nerve endings (muscle-spindles).

The third nerve (motor octdi) arises in a group of nerve-cells in

2 u




674 STRUCTURE: Otr THE BULB, PONS, AND MtD-BRAIN [cn. XLVt,

the 'Trey matter on tlio side of the Sylvian aqueduct underneath the
superior corpus quadrigeminum, and close to the middle line. The
anterior part of this nucleus is composed of small cells from which
small nerve-fibres originate for the ciliary muscle and sphincter of
the iris (intrinsic muscles of the eyeball). These fibres correspond
to the visceral libres of a spinal nerve, and, like them, have a cell
station, namely, in the ciliary ganglion. The posterior part of the
nucleus is composed of larger cells, and these give rise to larger
fibres which supply the following extrinsic eye-muscles: — superior
rectus, inferior rectus, internal rectus, inferior oblique and levator
palpebrse.

The fourth nerve (trochlear) takes origin from the grey matter
immediately below the centre of the third, but slightly more lateral
in position. It is underneath the inferior corpus quadrigeminum.
It supplies the superior oblique muscle of the opposite eyeball.

The sixth nerve (ahducens) arises from a centre beneath the
eminentia teres in the upper part of the floor of the fourth ventricle
near the middle line. It supplies the external rectus.

It is obviously necessary that the eye-muscles should work
tor^ethor harmoniously, that the two eyeballs should also be moved
simultaneously and in corresponding directions, and that such move-
ments should take place in accordance with the necessities of vision.
This is provided for in the shape of association fibres which link the
centres of the eye-muscles together. The principal association tracts
are the posterior longitudinal bundle, which connects the nuclei of
the third and sixth nerves, and the ventral longitudinal bundle
which unites the optic nerves through the intermediation of the cells
of the C. quadrigemina, with the nuclei of all these nerves. It should
also be remembered tliat all the fibres of the fourth, and some of
those of the third nerve, decussate in the middle line.

The fifth nerve (trigeminal) is a mixed nerve ; it leaves the side of
the pons in a smaller motor, and a larger sensory division. The
former supplies the muscles of mastication, the tensors of the palate
and tympanum, the mylo-hyoid, and the anterior belly of the
digastric; the sensory division has upon it a ganglion called the
Gasserian ganglion; it is the groat sensory nerve of the face and
head. The motor fibres arise from the motor nucleus (V^n, fig. 408),
which lies at the lateral edge of the upper part of the floor of the
fourth ventricle, but a certain number of its filjres arise from cells
in the lower part of the mid-brain and upper part of the pons;
this long stretch of nerve-cells, indicated by the long blue tail in the
diagram, is called the accessory or siqjerior motor nucleus of the fifth.
The sensory fibres arise from the cells of the Gasserian ganghon,
which resemble in structure those of a spinal ganghon ; one branch
of each passes to the periphery in the skin of the head and face, and



CH. XLVI.] THE SEVENTH NERVE 675

the other grows centralwards ; or reaching the pons these bifurcate,
the ascending branches arborise around the 2^'^'''ncipal sensory nucleus
of the fifth (Vd, fig. 408), which lies just lateral to the motor nucleus,
while the descending branches pass down into the bulb, where they
form the descending root of the fifth, and some reach as far down
in the spinal cord as the second cervical nerve. Mingled with these
descending fibres are numerous nerve-cells, many of which are grouped
in clusters (islands of Calleja), and the descending fibres form synapses
around them. The new axons arising from the cells of the sensory
nuclei pass upwards in three principal tracts: — (1) The greater
number cross the raphe and join the mesial fillet ; (2) some ascend
the fillet of the same side ; and (3) others pass into a special ascending
bundle which lies near the ventricular floor (the central tract of the
cranial sensory nerves).

The seventh nerve (facial) is the great motor nerve of the face
muscles. It also supphes the platysma, the stapedius, stylo-hyoid,
and posterior belly of the digastric. When it is paralysed, the
muscles of the face being all powerless, the countenance acquires on
the paralysed side a characteristic, vacant look, from the absence of all
expression : the angle of the mouth is lower, and the paralysed half
of the mouth looks longer than that on the other side ; the eye has
an unmeaning stare, owing to the paralysis of the orbicularis palpe-
brarum. All these peculiarities are exaggerated when at any time
the muscles of the opposite side of the face are made active in any
expression, or in any of their ordinary functions. In an attempt to
blow or whistle, one side of the mouth and cheeks acts properly, but
the other side is motionless, or flaps loosely at the impulse of the
expired air ; in trying to suck, one side only of the mouth acts ;
in feeding, on account of paralysis of the buccinator muscle, food
lodges between the cheek and gums.

The motor fibres originate from a nucleus in the ventricular floor
below that of the fifth and to the outer side of that of the sixth
nerve. As they curve over the nucleus of the sixth, they give ofi" a
bundle of fine fibres which cross the raphe, but their destination is
unknown. The facial nucleus receives collaterals from the sensory
tracts in the reticular formation.

The seventh nerve, however, is not wholly motor. The geniculate
ganghon on it is of spinal type ; the fibres which arise from it pass
centrally into the pars intermedia of Wrisberg, which enters the pons
between the seventh and eighth nerves ; these, like other sensory fibres,
divide into ascending and descending branches ; the latter have been
traced down to the sensory nucleus of the glosso-pharyngeal nerve.
The peripheral branches of the geniculate ganglion cells pass into the
large superficial petrosal and chorda tympani, the gustatory fibres of
which they probably furnish. The secretory fibres of the chorda



676



STRUCTURE OF THE BULB, PONS, AND MID-BRAIN [CH. XLVI.



tympani are effereat fibres which reach it from the facial nucleus
via the pars intermedia.

The eighth nerve (auditory) runs into the hinder margin of the
pons by two roots. One winds round the restiform body dorsal to
it, and is known as the dorsal or cochlear division ; the other passes
ventro-mesially on the other side of the restiform body, and is known
as the ventral or vestibular division.

We will take these two parts separately. The fibres of the
cochlear nerve take origin from the bipolar nerve-cells of the spiral
gangUon of the cochlea ; the peripheral axons ramify among the
hair cells of the organ of Corti, and the central axons pass towards
the pons ; as they enter they bifurcate, and some pass to and arborise



NUCL LEMNISCI
JAORiGEMINA




NERVE-ENDINGS

IN ORGAN OF CORTi

Fig. 41tj. — Cochlear division of tlie auditory nerve, r, Restiform body ; V, descending root of the fifth
nerve; tub.ac, acoustic tubercle; n.acc, accessory nucleus; .1.0., superior olive; n.tr., trapezoid
nucleus; 71. 1'/., nucleus of the sixth nerve ; I'/., issuing fibre of sixth nen-e. (Schafer.)

around a collection of nerve-cells situated between the two roots and
the restiform body, called the accessory auditory nucleus ; the remain-
ing fibres terminate similarly in a collection of cells in the grey matter
overlying the restiform body, and extending into the ventricular
floor in its widest part. This is called the ganglion of the root, and
the mass of grey matter is termed the acoustic tubercle. The auditory
path is continued by new axons that arise from these cells. Those
from the accessory nucleus enter the trapezium, and pass in it partly
to the superior olive and trapezoid nucleus of the same side, but
mainly to the corresponding nuclei of the opposite side ; some fibres
end here, others traverse the nuclei, and merely give off collaterals to
them ; they then turn upwards in the lateral fillet, and so reach the
inferior C. quadrigemina. The fibres which arise in the acoustic



CH. XLVI.]



THE Eir.HTH NEKVE



677



tubercle pass superficially over the floor of the ventricle, forming the
strice acousticcc; having crossed the raphe, they join the fibres from the
accessory nucleus in their course to the superior olive and fillet.
Here again, however, a few fibres pass to the fillet of the same side.

The vestibular nerve arises from the bipolar cells of the gan(jlion
of Scarpa, which is situated in the internal auditory meatus. The
peripheral axons ramify among the hair cells of the epithelium in the
utricle, saccule, and semi-circular canals. The central axons enter a
collection of small nerve-cells between the restiform body and the
descending root of the fifth ; this is termed the principal nucleus ; here



TO VERMI



FIBRES O

VESTIBULA

ROOT



NERVE
ENDINGS
IN MACUL/E
& AMPULL/E




Fig. 417. — Vestibular division of the auditory nerve, r, Bestiform body ; V, descending root of the fifth
nerve ; d, fibres of descending vestibular root ; n.d., cell of descending vestibular nucleus ; D, nucleus
of Deiters ; B, nucleus of Becbterew; n.t., nucleus tecti of cerebellum : p.l.h., posterior longitudinal
bundle. (Schafer.)

they bifurcate ; the descending branches run towards the lower part of
the bulb, and arborise around the cells of the neighbouring grey matter
(descending vestibular nucleus). The ascending branches pass upwards
in the restiform body to the cerebellum, in their course giving off many
collaterals which form synapses with the large cells of two nuclei
near the outer angle of the ventricular floor, and known as the
nucleus of Deiters and nucleus of Bechterew respectively. The fibres
which arise from Deiters' nucleus pass into the posterior longitudinal
bundles of both sides (see p. 674) ; those which start in Bechterew' s
nuclenis become longitudinal, but their destination is uncertain.

The accompanying diagrams (figs. 416 and 417) will serve to render
these complex relationships clearer.



678 STRUCTURE OF THE nULB, PONS, AND MID-BRAIN [CII. XLVI.

The ninth nerve (glosso-pharyngeal) gives filaments through its '
tympanic branch (Jacoljson's nerve) to parts of the middle ear;
also, to the carotid plexus, and through the great superficial petrosal
nerve to the sphono-palatine (Meckel's) ganglion. After communi-
cating, either within or without the cranium, with the vagus, it leaves
the cranium, divides into the two principal divisions indicated by
its name, and supplies the mucous membrane of the posterior and
lateral walls of the upper part of the pharynx, the Eustachian tube,
the arches of the palate, the tonsils and their mucous membrane,
and the tongue as far forwards as the foramen caecum in the middle
line, and to near the tip at the sides and inferior part.

It contains motor fibres to the stylo-pharyngeus, the constrictors
of the pharynx, and probably to the levator palati and other muscles
of the palate, except the tensor, which is supplied by the fifth nerve.
The nerve also contains fibres concerned in common sensation, and
the sense of taste, and secretory fibres for the parotid gland.

The cells from which the motor fibres originate are situated in a
special nucleus, which is a continuation upwards of the nucleus
ambiguus (the chief motor nucleus of the tenth or vagus nerve). The
sensory fibres arise in the jugular and petrosal ganglia from cells of
the spinal ganglion type. When the central axons reach the bulb
they bifurcate as usual; the descending branches pass down the
funiculus solitarius and terminate in synapses around the cells
scattered among its fibres. The ascending branches pass almost
horizontally to arborise around the cells of the principal nucleus
(IX. in fig. 408). The arrangement, in fact, is very like that of the
tenth nerve now to be described.

The tenth nerve {vagus or pneumo-gastric) has so many and
important functions that I shall not attempt to describe them here ;
it would mean rewriting a great deal of what we have already learnt
in connection with heart, respiration, digestion, etc. It is sufficient
to say that it contains both efferent and afferent fibres. The efferent
fibres arise partly from the upper part of the combined nucleus, which
lower down gives origin to the spinal accessory nerve (fig. 408, X.) but
mainly from the nucleus ambiguus, the position of which is shown in
fig. 408, coloured blue, and also in transverse section in figs. 412 and
418. The afferent fibres originate from the cells of the ganglion of
the trunk and of the root ; they enter the bulb and bifurcate ; the
ascending branches are short and arborise around the cells of the
principal nucleus (X. in fig. 408) ; the descending fibres, together with
similar ones derived from the glosso-pharyngeal nerve, and pars
intermedia, pass down in the descending root of vagus and glosso-
pharyngeal, which is also known as the funiculus solitarius. These
fibres terminate by arborising around the cells of the grey matter
that lies along its mesial border (descending nucleus of vagv^ and



Cir. XLVI.] TIIK KLEVKNTII AND TWELFTH NKRVKS



679



glosso-pharyiKjeal). This approaches tho middle lino as it descends,
and finally joins that of the opposite side over the central canal
{commissural nucleus).

The eleventh nerve {spinal accessory) is wholly efferent : it arises
by two distinct origins — one from a centre in tho floor of the fourth
ventricle, and connected with the glosso-pharyngeal-vagus-nuclous ;
the other, from the outer side of the anterior cornu of the spinal cord
as low down as the fourth cervical nerve. The fibres from the two
origins come together at the jugular foramen, but separate again into
two branches, outer and inner. The outer, consisting of large
medullated fibres from the spinal origin, supplies the trapezius and




FiQ. 418. — The tenth and twelfth nerves, pyr, Pyramid ; n.XlI., nucleus of hypoglossal ; XH., fibre of
hypoglossal; d.n.X.Xl., combined nucleus of vagus and spinal accessory; n.amli., nucleus
ambiguus; f.s., fasciculus solitarius, descending fibre.s of vagus and glosso-pharyngeal; f.s.n., its
nucleus; A'., motor fi bre of vagus ; g, ganglion cell in vagus trunk giving rise to a sensory fibre;
d.y., descending root of the fifth nerve ; r, restifoim body. (Schafer.)



sterno-mastoid muscles. The inner branch, consisting of small
medullated fibres from the medulla, supplies chiefly viscoro-motor
and cardio-inhibitory filaments to the vagus. The muscles of tho
larynx, all of which are supplied by branches of the vagus, derive
their motor nerves from the accessory; Vrolik states that in the
■ chimpanzee the internal branch of the accessory does not join the
vagus at all, but goes direct to the larynx. The crico-thyroid, how-
over, receives fibres which leave the bulb by glosso-pharyngeal
rootlets.

The twelfth nerve {hypoglossal) is also entirely efferent. It sup-
plies the muscles of the tongue. It arises from a large celled and long
nucleus in the bulb, close to the middle line (see figs 408 and 418).



CHAPTER XLVri

STRUCTURE OF THE CEREBELLUM

The cerebellum is composed of an elongated central portion or lobe,
called the vermis or vermiform process, and two hemispheres. Each
hemisphere is connected with its fellow by means of the vermiform
process.

The cerebellum is composed of white and grey matter, the latter




Fio. 419. — Cerebellum in section .iti'l fourth ventricle, with the nei;,'hbourin^' parts. 1, median Lrroove
of fourth ventricle, emling below iu the cidntims scriptorins, with the loii;_'ituiiinal eminences formed
by the fasciculi rcriYc, - ', one on each side; '2, the same i;roove, at the place where the white streaks
of the auditorj' nerve emerge from it to cnjss the floor of the ventricle ; 3, inferior peduncle of the
cerebellum, formed by the restiform body; 4, funiculus gracilis; above this is the calamus scrip-
tonus ; 5, superior i)eiluncle of cerebellum ; G, (>, fillet to the side of the crura cerebri ; 7, 7, lateral
grooves of the crura cerebri ; 8, corpora (luadrigeniina. (From Sappey, after llirschfeld and



Online LibraryWilliam Senhouse KirkesHandbook of physiology.. → online text (page 67 of 93)