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which pass directly to the mid-brain (fasciculus vestibulomesencephalicus), chiefly to the oculomotor and trochlear nuclei (Fig. 524, p. 590).
Central Connexions of the Cochlear Nerve. The cochlear nuclei are brought into connexion with the inferior colliculus and the medial geniculate body of the opposite side by the fibres of the corpus trapezoideum and the lateral lemniscus.
The fibres of the cochlear nerve end in the ventral cochlear nucleus and in the dorsal cochlear nucleus (tuberculum acusticum). From the cells of these nuclei two tracts arise, viz., a ventral tract, composed of the fibres of the corpus trapezoideum, and a dorsal tract, which is represented by the striæ medullares.
The corpus trapezoideum (Figs. 531 and 532) is formed of the axons of the cells of the ventral cochlear nucleus, as well as certain of the axons of the cells of the dorsal nucleus. In the midst of the corpus trapezoideum are lodged large cells which are known as the nucleus trapezoideus, and these give off axons which join the strand with which they are associated. Many of the fibres of the corpus trapezoideum end in a large mass of gray matter called the nucleus olivaris superior, which is placed immediately behind the
1. Ascending part of facial nerve.
2. Medial longitudinal bundle.
3. Descending root of vestibular nerve.
4. Radicular fibres of facial nerve.
5. Restiformr body.
6. Facial nucleus.
7. Spinal tract of trigeminal nerve.
8. Vestibular nerve.
9. Superior olive.
10. Lemniscus medialis.
FIG. 536.-SECTION THROUGH THE PONS OF THE ORANG.
The left side of the drawing is taken from a section at a level slightly inferior to the section
11. Pyramidal tract.
12. Transverse fibres of pons.
1. Ascending part of facial nerve.
2. Emergent portion of facial nerve.
3. Restiform body.
4. Nucleus of abducens nerve.
5. Abducens nerve.
6. Emergent part of facial nerve.
7. Peduncle of superior olive.
8. Superior olive.
9. Corpus trapezoideum.
10. Facial nerve.
11. Abducens nerve.
12. Pyramidal tract.
13. Transverse fibres of pons.
trapezoid body. The trapezial fibres cross the median plane and decussate with the corresponding fibres of the opposite side. Reaching the opposite superior olivary nucleus, more fibres leave the trapezoid body, and almost immediately after this the strand bends upwards and forms the lemniscus lateralis (Figs. 535, p. 605; 537, p. 607). But still another nucleus is interposed in its path, viz., the nucleus lemnisci lateralis. Here some fibres are dropped, whilst from the nuclear cells others are acquired, and the lateral lemniscus then proceeds upwards until it reaches the inferior colliculus and the medial geniculate body, in which its fibres end.
Other fibres arise from the cells of the dorsal cochlear nucleus, and arrange themselves in the conspicuous bundles which sweep round the dorsal aspect of the restiform body and proceed medially across the floor of the fourth ventricle, often immediately beneath the ependyma (Fig. 482, p. 550). Reaching the median plane they dip forwards into the substance of the pons, and, crossing the median plane, they join the lateral
lemniscus. It is important to remember that the striæ medullares are not always visible in the floor of the fourth ventricle (Fig. 535), but are often buried more or less deeply.
The connexion between the terminal cochlear nuclei and the inferior colliculus is not altogether with that of the opposite side, as the foregoing description and the diagram (Fig. 534) might lead one to infer. A few fibres pass directly to the inferior colliculus of the same side, but none to the corresponding medial geniculate body: the connexion with the latter is entirely crossed (Ferrier and Turner).
From the medial geniculate body there proceeds a tract to the cerebral cortex of the transverse temporal gyri (Heschl's). The whole nervous apparatus is thus linked on to the cerebral cortex, and the succession of neurones which build up the entire chain are therefore: (1) in the cochlea of the internal ear, the bipolar cells of the spiral
FIG. 537.-A VERTICAL TRANSVERSE SECTION OF THE BRAIN TO SHOW THE WHOLE OF THE CENTRAL ACOUSTIC PATH. The left hemisphere (right side of the figure) is cut on a plane posterior to that of the right. Motor fibres red. Sensory fibres blue. Acoustic fibres yellow.
ganglion emit axons that terminate in the brain; in (2) the cochlear nuclei, from the nerve-cells of which fibres arise and cross to the lateral lemniscus of the opposite side, proceeding to (3) the medial geniculate body, from which fibres pass to the cerebral cortex.
It must be borne in mind that all the axons of the cells of the superior olive do not join the trapezoid strand. Many leave its dorsal aspect and pass backwards in a group called the pedicle of the superior olive, to end in the nucleus of the abducens nerve, and, through the medial longitudinal bundle, in the nuclei of the trochlear and oculomotor nerves. In this way the organ of hearing is brought into connexion with the nuclei which preside over the movements of the eyeballs (Figs. 531, p. 599, and 536, p. 606).
PROSENCEPHALON OR FORE-BRAIN.
The fore-brain vesicle in the embryo has been subdivided, somewhat arbitrarily,
into two parts-an anterior, termed the telencephalon, and a posterior, called the diencephalon, which forms the greater part of the walls of the third ventricle. The extreme anterior part of the third ventricle belongs to the telencephalon, and this includes the anterior wall of the neural tube, which is known as the lamina terminalis.
DEVELOPMENT OF PARTS DERIVED FROM FORE-BRAIN.
The alar part of each side wall of the telencephalon is pushed out to form a diverticulum, which ultimately constitutes the cerebral hemisphere, and thus, from a very early period, the primitive position of this part of the side wall is indicated by the wide foramen interventriculare, or aperture of communication between the cavity of the cerebral hemisphere and the third ventricle (Fig. 538).
The alar part of the side wall of the diencephalon is utilised for the development of the thalamus, the epithalamus, and the metathalamus. Of these the thalamus is derived from the anterior and by far the greatest part of the alar wall. It arises as a large oval swelling, which gradually approaches its fellow of the opposite side, and thus diminishes the width of the third ventricle. Finally, the two bodies sometimes come into contact in the median plane and cohere over an area corresponding to the massa intermedia. This may occur about the end of the second month.
From that section of the side wall to which the name of metathalamus is given the two geniculate bodies arise. Each of these shows, in the first place, as a depression on the inside, and a slight elevation on the outside, of the wall of the diencephalon. As the thalamus grows backwards, it encroaches greatly upon the territory occupied by the geniculate bodies. It thus comes about that in the adult brain the medial geniculate body seems to hold a position on the lateral aspect of the mesencephalon, whilst the lateral geniculate body, viewed from the surface, appears to be a part of the thalamus.
From the epithalamic region of the wall of the diencephalon are developed the pineal body, its peduncle, and the habenular region. These parts are relatively much more evident in the embryonic than in the adult brain. The pineal body appears to be developed as a diverticulum of the posterior part of the roof of the diencephalon, but in reality it is a derivative of the alar lamina. Viewed from the dorsal aspect of the braintube, this diverticulum shows in the first instance, as a rounded elevation, from each side of which a broad ridge runs forwards. This ridge becomes the tænia thalami, whilst in the region of its junction with the pineal elevation the trigonum habenulæ takes shape. The pineal diverticulum ultimately becomes solid, but a small portion of the original cavity is retained as the recessus pinealis of the third ventricle.
The part of the diencephalon and telencephalon which represents the basal lamina (ie. lies below the level of the sulcus hypothalamicus) retains its primitive form, and undergoes only slight change. Consequently, when this region in the adult brain is compared with the corresponding region in the embryonic brain, the resemblance between the two is very striking.
In the fore-brain, therefore, it is the alar lamina which plays the predominant part in the formation of the cerebrum. The value, also, of the basal part of the wall of this portion of the neural tube is still further reduced by the fact that it no longer contains the nuclei of origin of efferent nerves. The highest of these nuclei (the oculomotor) is placed in the mesencephalon. [Johnston has recently announced the discovery of a sensory nerve (nervus terminalis) attached to the fore-brain in human embryos; and of course the optic and olfactory nerves enter the fore-brain.]
The region of the fore-brain which lies below the sulcus hypothalamicus is termed the hypothalamus. The part of this which corresponds to the diencephalon is called the pars mamillaris hypothalami, whilst the part in front, which belongs to the telencephalon, receives the name of pars optica hypothalami. From the pars mamillaris hypothalami are derived the corpus mamillare and a portion of the tuber cinereum. With the pars optica hypothalami are associated the following parts, viz., the tuber cinereum, with the infundibulum and the cerebral part of the hypophysis, the optic chiasma, the optic recess, and the lamina terminalis. The corpora mamillaria form, in the first instance, a relatively large ventral bulging of the floor of the brain-tube. As development goes on this bulging becomes relatively small, and about the fourth. month the single projection becomes divided into the two tubercles. The infundibulum and posterior or cerebral lobe of the hypophysis are developed as a hollow downward diverticulum of the floor of the telencephalon (Fig. 538). A portion of the original cavity is retained in the upper part of the infundibulum, and constitutes the infundibular recess in the floor of the third ventricle.
The optic nerve is formed chiefly by the passage of fibres backwards from the retina in the wall of the original optic stalk, whilst the chiasma takes form by the transit of fibres across the median plane in front of the infundibulum and behind the optic recess. To a large extent these fibres are derived from the optic The optic recess of the third ventricle marks the spot where the hollow optic vesicle was originally attached to the inferior and lateral part of the fore-brain, and in the adult it therefore represents a portion of the primitive cavity of the tubular stalk of the optic vesicle. In the course of development the optic nerve fibres, which appear in the stalk of the optic vesicle to form the optic nerve, seek an attachment much further back, and through the optic tract they are even carried as far as the mesencephalon.
The roof of the forebrain remains thin, and does not proceed to the development of nervous elements, although its posterior part becomes invaded by nervous tissue to form the pineal body and the posterior commissure. In front of these structures the roof of the fore-brain is epithelial, and remains so during life. It constitutes the epithelial roof of the third ventricle, and it becomes involuted along the median plane into the cavity to form the chorioid plexuses of the ven
tricle (Fig. 549, p. 622). The posterior commissure appears as a transverse thickening at the bottom of a transverse groove which appears in the roof of the early brain-tube, behind the pineal diverticulum.
FIG. 538.-Two DRAWINGS OF THE EMBRYONIC BRAIN (by His). Reconstruction of the fore-brain and mid-brain of His's embryo KO; profile view. B, Same brain as A, divided along the median plane and viewed upon its inner aspect.
Mamillary eminence; Tc, Tuber cinereum; Hp, Hypophysis (hypophyseal diverticulum from buccal cavity); Opt, Optic stalk; TH, Thalamus; Tg, Tegmental part of mesencephalon; Ps, Pars hypothalamica; Cs, Corpus striatum; FM, Foramen interventriculare; L, Lamina terminalis; RO, Recessus opticus; Ri, Recessus infundibuli.
PARTS DERIVED FROM THE DIENCEPHALON.
Under this heading we have to consider: (1) the thalamus; (2) the epithalamus, which comprises the pineal body and the habenular region; (3) the metathalamus, or the corpora geniculata; and (4) the hypothalamus.
The hypothalamus consists of two portions, viz., the pars mamillaris hypothalami, which comprises the corpus mamillare and the portion of the central gray matter which forms the floor of the third ventricle in its immediate vicinity; and the pars optica hypothalami, which embraces the tuber cinereum, the infundibulum, the hypophysis (O.T. pituitary body), and the lamina terminalis. Strictly speaking, the optic part of the hypothalamus does not belong to the diencephalon, but it is convenient to study the parts which it represents at this stage.
The original cavity of that part of the brain-tube which forms the diencephalon is represented by the greater part of the third ventricle of the brain. Thalamus.-The thalamus is the principal object in this section of the brain
(Fig. 538). It is a large ovoid mass of gray matter, which lies obliquely across the path of the cerebral peduncle as it descends from the cerebral hemisphere. The smaller anterior end of the thalamus lies close to the median plane, and is separated from the corresponding part of the opposite side only by a very narrow interval. The enlarged posterior ends of the two thalami are placed more widely apart, and in the interval between them the corpora quadrigemina are situated.
The two thalami, in their anterior two-thirds, lie close together, one on each side of the deep median cleft which receives the name of the third ventricle of the brain. The inferior and lateral aspects are in apposition with, and, indeed, directly connected with, adjacent parts of the brain, and on this account it is customary to study them by means of sections through the brain. The superior and medial surfaces are free.
The lateral surface of the thalamus is applied to a thick layer of white matter
interposed between it and the lentiform nucleus, called the internal capsule, and composed of fibres passing both upwards towards and downwards from the cerebral cortex. A large proporAnterior commissure tion of these fibres
descend to form
Stalk of pineal body bers of fibres
stream out and enter the internal capsule, to reach the cerebral cortex. They constitute what is termed the thalamic radiation. As the fibres leave the FIG. 539.-THE THALAMI AND THE PARTS OF THE BRAIN SURROUNDING THEM. thalamus, over Superior aspect. the whole of the lateral surface of the ganglionic mass they form a very distinct reticulated zone or stratum, which is termed the external medullary lamina.
The inferior surface of the thalamus rests on the hypothalamus. From the latter region many fibres enter the thalamus on its inferior aspect, whilst other fibres leave this surface of the thalamus to take part in the thalamic radiation.
The superior surface of the thalamus is free. Laterally it is bounded by a groove, which traverses the floor of the lateral ventricle of the brain and intervenes between the thalamus and the caudate nucleus. In this groove are placed a slender band of longitudinal fibres, termed the stria terminalis, and in its forepart the vena terminalis. Medially, the superior surface of the thalamus is separated from the medial surface in its anterior half by a sharp edge or prominent ledge of the ependyma of the third ventricle. This is termed the tænia thalami, and the ridge which it forms is accentuated by the