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tractus spinalis. This tract descends on the lateral side of the column of gray matter formed by the substantia gelatinosa, which constitutes its terminal nucleus, nucleus tractus spinalis nervi trigemini. Fibres constantly leave it to enter the nucleus, so that the lower it gets the smaller does the spinal tract become until, in the upper part of the spinal medulla, about the level of the first or second spinal nerve, it disappears altogether.

The large spinal tract of the trigeminal nerve is a conspicuous object in sections through the pons and medulla oblongata. In the pons it traverses the dorsal or tegmental part, first, between the emergent part of the facial nerve and the vestibular nerve; and then lower down, between the restiform body and the nucleus of the facial nerve (Fig. 498, p. 565). In cross sections it presents a well-defined semilunar or curved piriform outline. In the superior part of the medulla oblongata it lies on the ventral aspect of the restiform body, and therefore nearer the surface than in

Anterior medullary velum

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Brachium pontis

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Corpus trapezoideum

Transverse fibres of pons

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FIG. 532.-SECTION THROUGH THE PONS OF THE ORANG, AT THE LEVEL OF THE NUCLEI
OF THE TRIGEMINAL NERVE.

the pons (Fig. 495, p. 561). Here it is traversed and broken up into separate bundles by the olivo-cerebellar fibres and the roots of the glossopharyngeal and vagus nerves. Finally, it comes to the surface and its fibres are spread over the area on the side of the medulla oblongata known as the tuberculum cinereum of Rolando (Fig. 494, p. 561).

The small motor part of the trigeminal nerve is distributed chiefly to the muscles of mastication, and derives its fibres from the motor nucleus.

The motor nucleus (Fig. 532) lies in the lateral part of the tegmental portion of the pons, close to the medial side of the main sensory terminal nucleus, but somewhat nearer the floor of the fourth ventricle. It is serially homologous with the motor nuclei of the lateral somatic group, namely, the facial and nucleus ambiguus. It does not become displaced so far forwards as these nuclei, because its chief source of sensory impulses the terminal nucleus of the trigeminal afferent fibres-is placed alongside it, and there is no need for any definite migration (Fig. 532).

The mesencephalic root or radix descendens nervi trigemini takes origin from

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a column of loosely arranged pear-shaped unipolar cells which are placed in the extreme lateral part of the gray matter surrounding the aquæductus cerebri. As this root is traced downwards it gradually increases in size by the accession of new fibres, and it assumes a crescentic form in transverse section (Figs. 501, p. 569; 532, p. 601; 533; and 534, p. 603). In the inferior part of the mesencephalon it lies on the medial side of the brachium conjunctivum; and the trochlear nerve, on its way to the surface, runs downwards in its concavity and on its medial aspect. the superior part of the pons it continues its course downwards on the lateral and deep aspect of the gray matter in the floor of the fourth ventricle. Finally, reaching the level of the nuclei of the trigeminal nerve, the fibres of the mesencephalic root turn forwards and are said to join the sensory part (Johnston) of the trigeminal nerve. Otto May and Horsley, however, confirm the usual description, viz., that it passes into the motor root; but, according to them, it cannot be traced beyond the semilunar ganglion. It is customary to describe this mesencephalic root as belonging to the motor division of the trigeminal nerve; but Johnston has recently questioned this and claimed: (1) that its fibres become associated at their exit from the central nervous system with the sensory, and not with the motor, root; (2) that its nucleus develops in the alar and not in the basal lamina; and (3) that the pear-shaped unipolar cells, from which its fibres arise, conform to the sensory and not to the motor type.

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Decussation of lateral lemniscus fibres.

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The reason why its sensory nature has not been suspected hitherto is no doubt the fact that its fibres arise not in SOme ganglion outside the central nervous system, like but other sensory nerves, from cells in the tectum

mesencephali. If Johnston's view is correct, the neural crest in the mesencephalic region must have been drawn into the neural tube during development and given rise to this sensory nucleus of origin (not a terminal nucleus) within the central nervous system.

Otto May and Sir Victor Horsley have shown that the mesencephalic root is a mixture of ascending and descending fibres, but there is no evidence to show that the latter may not be sensory like the former. Nothing is known of their peripheral distribution.

Aquæductus cerebri.

Central gray matter

Nucleus of inferior. colliculus

Inferior brachiumMesencephalic root of trigeminal nerve

Nucleus of trochlear.

nerve

Medial longitudinal bundle

Lateral lemniscus

Decussation of the brachia conjunctiva

Medial lemniscus

FIG. 533.-SECTION THROUGH THE INFERIOR COLLICULUS AND THE
TEGMENTUM OF THE MESENCEPHALON AT THE LEVEL OF THE

MIDDLE PART OF THE NUCLEUS OF THE TROCHLEAR NERVE
(Orang).

Nervus Trochlearis.-The trochlear nerve supplies the superior oblique muscle of the eyeball. It emerges from the brain, on its dorsal aspect, at the superior part of the anterior medullary velum, immediately below the lower border of the inferior colliculus (Fig. 517, p. 583). The nucleus from which it arises is a small oval mass of gray matter, placed in the ventral part of the central gray matter, at the level of the superior part of the inferior colliculus. The close association of this nucleus with the medial longitudinal bundle has already been referred to. It is sunk deeply in a bay which is hollowed out on the dorsal and medial aspect of that tract. The nerve has a course of some length within the mesencephalon. The axons of the cells leave the lateral aspect of the nuclear mass, and curve backwards and laterally in the central gray matter until they reach the concave medial surface mesencephalic root of the trigeminal nerve. Here they are gathered together

into one or two round bundles, which, bending sharply, turn downwards at a right angle and descend on the medial side of the trigeminal root. When the region below the inferior colliculus is reached, the nerve makes another sharp bend. This time it turns medially, enters the superior end of the anterior medullary velum, in which it decussates with its fellow of the opposite side. Having thus crossed the median plane, the trochlear nerve emerges at the medial border of the brachium conjunctivum. The course pursued by the trochlear nerve within the central gray matter may be traced by examining in succession Fig. 533; Fig. 534; Fig. 502, p. 570; and Fig. 512, p. 577.

Nervus Oculomotorius.-The oculomotor nerve supplies the levator palpebræ superioris, all the ocular muscles, with the exception of the superior oblique and the lateral rectus, and also two muscles within the eyeball, viz., the sphincter iridis and the musculus ciliaris. The nucleus of origin is placed in the ventral part of the central gray matter subjacent to the superior colliculus (Fig. 521, p. 587). In length it measures from 5 to 6 mm. Its inferior end is partially continuous with the nucleus of the trochlear nerve, whilst its superior end extends upwards for a short distance beyond the mesencephalon into the gray matter on the side wall of the third ventricle. Its relation to the medial longitudinal bundle is even more intimate than that of the trochlear nucleus. It is closely applied to the dorsal and medial aspect of this strand; many of its cells occupy a position in the intervals between the nervebundles of the tract, and some even are seen on its ventral or tegmental aspect. The axons of the nuclear cells leave the nucleus in numerous bundles, which describe a series of curves as they proceed forwards through the medial longitudinal bundle, the tegmentum, red nucleus, and medial margin of the substantia nigra, to emerge finally from the brain-stem along the bottom of the sulcus oculo-motorius on the medial aspect of the basis pedunculi.

The cells of the oculomotor nucleus are not uniformly distributed throughout it. They are grouped into several more or less distinct collections or clumps, some of which possess cells which differ in size and appearance from the others. These cell-clusters are very generally believed to possess a definite relation to the several branches of the nerve and the muscles which they supply. Perlia recognises no less than seven such cell-clusters in each nucleus, with a small median nucleus placed accurately on the median plane, and from which fibres for both nerves spring. Whilst the majority of the fibres in the oculomotor nerve arise from the cell-groups which lie on its own side of the median plane, it has been satisfactorily established that a certain proportion of its fibres are derived from the nucleus of the opposite side, thus forming a crossed connexion and giving rise to a median decussation. These crossed fibres are supposed by some to supply the medial rectus muscle; and we have seen that there is reason to believe that the part of the nucleus from which these fibres are derived stands in connexion through the medial longitudinal fasciculus with the abducens nucleus from which proceeds the nerve of supply for the lateral rectus muscle. The harmonious action of the medial and lateral recti in producing the conjugate movements of the eyeballs is thus explained.

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FIG. 534.-SECTION THROUGH THE INFERIOR COLLICULUS AND THE
TEGMENTUM OF THE MESENCEPHALON AT THE LEVEL OF THE
INFERIOR PART OF THE NUCLEUS OF THE TROCHLEAR NERVE
(Orang).

The oculomotor nucleus is connected-(1) with the occipital part of the cerebral cortex by fibres which reach it through the optic radiation; (2) with the vestibular, trochlear and abducent nuclei (and probably with other nuclei) by fibres which come to it through the medial longitudinal bundle; (3) possibly with the facial nerve by fibres which pass out from it into the medial longitudinal bundle (p. 589); (4) with the visual system by fibres which enter it from the cells of the superior colliculus.

It is important to recognise that although the main part of the oculomotor nucleus belongs to the medial somatic group, which also includes the trochlear, abducent and hypoglossal nuclei, it also includes a representative (the EdingerWestphal group of small cells) of the column of splanchnic efferent nuclei in series with those of the facial, glossopharyngeal, and vagus nerves. Its axons pass out along with the other fibres of the oculomotor nerve and enter the ciliary ganglion, where they end in relationship with the cells that innervate the ciliary muscle and the circular muscle of the iris.

Nervus Acusticus.-This large nerve enters the brain at the inferior border of the pons. Its fibres spring from bipolar ganglionic cells in the immediate neighbourhood of the labyrinth or internal ear (see section dealing with the Organs of Sense). One group of these forms the spiral ganglion, the peripheral branches of which are distributed to the organ of Corti in the cochlea: another group constitutes the vestibular ganglion (often called Scarpa's), which distributes fibres to the ampullæ of the semicircular ducts, the utricle, and the saccule. Although the central processes of the cells in these two ganglia accompany one another and are known collectively as the acoustic nerve they really remain distinct throughout, in their mode of termination in the brain as well as in their peripheral distribution. Reaching the brain the acoustic nerve divides into two parts, viz., the nervus cochlearis and the nervus vestibularis, which present totally different connexions, corresponding to their distinct functions. In their further course these two divisions deviate from each other so as to embrace the restiform body — the vestibular part entering the pons on the medial aspect of the restiform body, whilst the cochlear part sweeps round its lateral surface. Special nuclei of termination require to be studied in connexion with each part of the nerve.

The cochlear nerve is composed of finer fibres than the vestibular nerve, and its fibres acquire their medullary sheaths at a later period. It is the true nerve of hearing, and its fibres end in a nucleus which lies in intimate relation to the restiform body. It may be described as consisting of two parts. Of these one, called the dorsal cochlear nucleus, is a piriform mass which is placed on the dorsal aspect of the restiform body-between it and the flocculus of the cerebellum. The second part, termed the ventral cochlear nucleus, is placed on the ventral aspect of the restiform body in the interval between the cochlear and vestibular divisions of the acoustic nerve, after they have separated from each other. The fibres of the cochlear nerve enter these two ganglia and end around the cells in arborisations, which are finer, closer, and more intricate than those met with in any other nerve-ending in the brain.

The vestibular nerve enters the brain at a slightly higher level than the cochlear nerve and on the medial aspect of the ventral cochlear nucleus. It proceeds backwards through the pons between the restiform body, which lies on its lateral side, and the spinal tract of the trigeminal nerve, which is placed on its medial side. Its fibres end in a series of terminal nuclei (Fig. 530, p. 597), viz.: (1) the nucleus vestibularis dorsalis, often known as the principal nucleus, (2) its inferior prolongation, nucleus tractus descendentis, (3) the nucleus vestibularis lateralis (Deiters'), (4) the nucleus vestibularis superior (Bechterew's), and (5) the cerebellar cortex.

The principal nucleus (Figs. 498, p. 565, and 535, p. 605) is a large diffuse nuclear mass, which lies in the floor of the fourth ventricle subjacent to the surface district known as the area acustica (Fig. 482, p. 550). It is situated, therefore, in both the pons and the medulla oblongata to the lateral side of the fovea superior and the fovea inferior. In transverse section it is prismatic in outline, and crossing the surface of its upper or pontine part immediately under the ependyma of the ventricle are the striæ medullares.

When the nervus vestibularis, as it traverses the brain, reaches the medial aspect of the dorsal portion of the restiform body, its fibres bifurcate to form ascending and descending tracts. The latter pass vertically downwards in separate bundles and form the descending tract of the vestibular nerve (Figs. 498, p. 565; 495, p. 561; and 530, p. 597). This proceeds through the inferior part of the pons into the medulla oblongata, in which it may be traced as far as the level of the decussation of the medial lemniscus. Associated with the descending tract there is a column of gray matter, with nerve-cells strewn sparsely throughout it. This is the nucleus of the descending tract, and the fibres end in fine arborisations around these nerve-cells.

Some of the ascending fibres of the vestibular nerve end in the nucleus lateralis. This nucleus is composed of a number of large and conspicuous multipolar nerve cells, which are scattered amidst the

Nucleus fastigii

bundles of the

vestibular

nerve. As it is Anterior Insula

transverse

traced upwards temporal

gyrus

into the pons the nucleus gradually inclines backwards, and finally it occupies a place in the side wall of the fourth ventricle. It attains its greatest development at the level of the emerging part of the facial nerve and this upper part is sometimes termed the nucleus superior.

AUDITORY
RADIATION

Vermis cerebelli

Vestibular fibre __ passing co vermis

LEMNISCUS
LATERALIS

Principal
Vestibular nucleus

Corpus geniculatum mediale

Nucleus lemniscus lateralis

Inferior colliculus

PONS

Restiform body

Striae medullares

Superior
Olive

Nucleus emboliformis

Nucleus dentatus

cerebelli

Dorsal cochlear nucleus
Ventral cochlear nucleus

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Semicircular
ducts

MEDULLA OBLONCATA

Vestibular fibre ending in lateral cerebellar

cortex

Cochlea

Corpus trapezoideum

FIG. 535.-CENTRAL CONNEXIONS OF THE COCHLEAR AND VESTIBULAR
DIVISIONS OF THE ACOUSTIC NERVE.

Vestibular fibres green. Cochlear fibres yellow.

Other ascending fibres pass without interruption into the cerebellum to terminate in the cortex of the vermis and hemisphere. In their course many of these fibres pass through the nucleus fastigii, and many writers describe them as terminating in this nucleus; but according to Ramon y Cajal they merely traverse it on their way to the cerebellar cortex.

From the large-celled nucleus lateralis-best known as Deiters' nucleus-a strand of fibres passes medially to reach the medial longitudinal fasciculus, of which it forms one of the most important constituent elements. Some of these fibres pass upwards to the nuclei of the oculomotor, trochlear, and abducens nerves; others downwards, probably to the nucleus of the accessory nerve, which are concerned in regulating the movements of the eyes and the head respectively, because they need to be thus closely linked to the receptive nucleus of the nerve (vestibular), which is concerned with the appreciation of movements of the head and the position of the body in space. Other fibres arise from the lateral nucleus and pass directly to the spinal medulla without passing through the medial longitudinal bundle; they form the fasciculus vestibulospinalis, which passes downwards in the funiculus anterior and distributes fibres to the various motor nuclei in the anterior column of the spinal medulla (Fig. 524, p. 590, and Fig. 473, p. 534).

The nucleus superior (Bechterew's nucleus) likewise emits a group of fibres

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