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presence of a subjacent longitudinal strand of fibres called the stria medullaris. When these two structures, viz., the ependymal ridge and the subjacent stria, are traced backwards, they are seen to turn medially and become continuous with the stalk or peduncle of the pineal body. Behind the portion of the tænia thalami which turns medially towards the pineal body a small depressed triangular area, the trigonum habenulæ, situated in front of the superior colliculus, forms a very definite medial boundary for the posterior part of the superior surface of the thalamus.

The superior surface of the thalamus is slightly bulging or convex, and is of a whitish colour, owing to the presence of a thin superficial covering of nerve-fibres, termed the stratum zonale. It is divided into two areas by a faint oblique groove, which begins in front at the medial border, a short distance behind the anterior extremity of the thalamus, and extends laterally and backwards to the lateral part of the posterior extremity. This groove corresponds to the edge of the fornix. The two areas which are thus mapped out are very differently related to the ventricles of the brain, and also to the parts which lie above the thalamus. The lateral area, which includes the anterior extremity of the thalamus, forms a part of the floor of the lateral ventricle. It is covered with ependyma, overlapped by the chorioid plexus of this ventricle, and lies immediately subjacent to the corpus callosum. Along the line of the groove the epithelial lining of the lateral ventricle is reflected over the chorioid plexus of this cavity. The medial area, which includes the posterior end of the thalamus, intervenes between the lateral and third ventricles of the brain, and takes no part in the formation of the walls of either. It is covered by a fold of pia mater, termed the tela chorioidea of the third ventricle, above which is the fornix, and these two structures intervene between the thalamus and the corpus callosum.

The anterior extremity of the thalamus, called the tuberculum anterius thalami, forms a marked bulging. It projects into the lateral ventricle, behind and to the lateral side of the free portion of the column of the fornix. The foramen interventriculare, a narrow aperture of communication between the lateral and third ventricles of the brain, is bounded in front by the column of the fornix and behind by the anterior tubercle of the thalamus.

The posterior extremity of the thalamus is very prominent and forms a cushionlike projection, which overhangs the brachia of the corpora quadrigemina. This prominence is called the pulvinar. Another oval bulging on the posterior part of the thalamus receives the name of the corpus geniculatum laterale. It is situated below, and to the lateral side of, the pulvinar, and presents a very intimate connexion with the optic tract.

The medial surfaces of the two thalami are placed close together, and are covered not only by the lining ependyma of the third ventricle, but also by a moderately thick layer of gray matter, continuous below with the central gray substance which surrounds the aquæductus cerebri in the mesencephalon. A band of gray matter, termed the massa intermedia, crosses the third ventricle and joins the medial surfaces of the two thalami together.

Intimate Structure and Connexions of the Thalamus.-The upper surface of the thalamus is covered by the stratum zonale, a thin coating of white fibres derived to some extent from the optic tract, and probably also from the optic radiation. The medial surface has a thick coating of central gray matter, whilst intervening between the internal capsule and the lateral surface is the lamina medullaris externa. The lower surface merges into the hypothalamus.

The gray matter of the thalamus is marked off into three very apparent parts -termed the anterior, the medial, and the lateral thalamic nuclei-by a thin vertical sheet of white matter, continuous with the stratum zonale, termed the lamina medullaris interna. The lateral nucleus (nucleus lateralis thalami) is by far the largest of the three. It is placed between the medial and the lateral medullary laminæ, and it stretches backwards beyond the medial nucleus, and thus includes the whole of the pulvinar (Fig. 541). The medial nucleus (nucleus medialis thalami) reaches only as far back as the habenular region. It is placed between the central gray matter of the third ventricle and the internal medullary lamina. The lateral nucleus is more extensively pervaded by fibres than the medial nucleus.

From the lateral nucleus by far the greatest number of the fibres which form the radiatio thalami pass, and these are seen crossing it in various directions towards the lamina medullaris externa. The anterior nucleus (nucleus anterior thalami) is the smallest of the three thalamic nuclei. It forms the prominent anterior tubercle, and is prolonged in a wedge-shaped manner, for a short distance, downwards and backwards between the anterior parts of the medial and lateral nuclei. The internal medullary lamina splits into two parts and partially encloses the anterior nucleus. In connexion with its large cells a very conspicuous bundle of fibres, the fasciculus thalamomamillaris, comes to an end. [As this bundle arises in the corpus mamillare, it ought to be called "fasciculus mamillo-thalamicus."]

A diffuse gray mass, imperfectly marked off from the inferior surface of the lateral nucleus, receives the name of the ventral nucleus. Its inferior part is composed of the central nucleus of Luys and the nucleus arcuatus. In section the former appears as a circular mass of gray matter, which comes into view immediately behind the point where the internal medullary lamina disappears. It would seem to be intimately connected with fibres which reach it from the red nucleus and from the posterior commissure. These fibres pass round it so as to mark it off from the rest of the thalamus, and in front of the nucleus many of them enter the internal medullary lamina. The nucleus arcuatus is a small semilunar mass of gray matter placed below and to the lateral side of the central nucleus of Luys.

The connexions of the thalamus are of an extremely intricate kind. It would appear to be a ganglionic mass interposed between the tegmental corticipetal tracts and the cerebral cortex. In its posterior part, and through its stratum zonale, it also has important connexions with the optic tract. The corticipetal tegmental tracts, which enter it from below, will be noticed in connexion with the hypothalamic

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region. Suffice it to say, for the present, that these fibres end in the midst of the thalamus in connexion with the thalamic cells. In addition to these, enormous numbers of fibres, arising within the thalamus as the axons of its cells, stream out from its lateral and inferior surfaces to form the thalamic radiation. These thalamo-cortical fibres pass to every part of the cortex; and although there is no separation of them into distinct groups as they leave the thalamus, it is customary to regard them as constituting a frontal stalk, a parietal stalk, an occipital stalk, and a ventral stalk. But fibres from the cortex, cortico-thalamic fibres, likewise stream into the thalamus in large numbers, and end in fine MEGNCEPHALON arborisations around its cells. A

CORP. CALLOSUM

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OPTIC RADIATIONS

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FIG. 540.-SCHEMA.

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The frontal stalk of the thalamic radiation emerges from the anterior part of the lateral surface of the thalamus and passes through the anterior limb of the internal capsule, to reach the cortex of the frontal lobe. Many of these fibres end in the caudate and lentiform nuclei, between which they proceed. The parietal stalk issues from the lateral surface of the thalamus, and, passing through the internal capsule (and to some extent, also, through the lentiform nucleus and the external capsule), gains the cortex of the posterior part of the frontal lobe and of the parietal lobe. The occipital stalk emerges from the lateral aspect of the pulvinar and constitutes the so-called optic radiation. These fibres sweep laterally and backwards round the lateral side of the posterior horn of the lateral ventricle to gain the cortex of the occipital lobe. The

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Founded on the observations of
Flechsig, and Ferrier and Turner.

double connexion with the cerebral
cortex is thus established by the
thalamus.

ventral stalk streams out from the under aspect of the anterior part of the thalamus, in front of the hypothalamic tegmental region and the corpus mamillare. Its fibres arise in both the medial and lateral nuclei, and sweep downwards and laterally to reach the region below the lentiform nucleus. One very distinct band which lies dorsal to the other fibres (ansa lenticularis) comes from the lentiform nucleus to the thalamus, whilst the remainder (ansa peduncularis) proceed in a lateral direction from the thalamus below the lentiform nucleus and gain the cortex of the temporal lobe and of the insula.

Flechsig divides the thalamo-cortical fibres of ordinary sensation into three sensory systems. These he has been able to distinguish by studying the order in which they assume their sheaths of myelin in the foetus and infant.

Ferrier and Turner, by the degenerative method of investigation, corroborate Flechsig's results. They confirm the observation of Flechsig that, while thalamic fibres are distributed to the several regions of the cerebral cortex to an almost equal extent, there is one district, viz., the frontal pole, to which the supply is scanty. Another very important result has been obtained by these authors. They have established the fact that many of the thalamic fibres cross the median plane in the corpus callosum, and thus gain the cortex of the opposite cerebral hemisphere. Hamilton's crossed callosal tract thus receives confirmation.

Intimate Structure of the Corpus Geniculatum Laterale.-Sections through the lateral geniculate body reveal the fact that it is composed of a series of alternately placed gray and white curved laminæ. This gives it a very characteristic appearance. The white laminæ are composed of fibres which enter the body from the optic tract. The connexions of the geniculate bodies

[graphic]

Caudate nucleus

Corpus callosum

Fornix

will be studied Anterior nucleus with the optic

of thalamus

Stria medullaris

tract.

Internal capsule
Medial nuclens
of thalamus

External capsule

Hypothalamic Region. The tegmental part of the pedunculus cerebri is prolonged upwards and assumes a position below the posterior part Basis pedunculi

of the thalamus.

The red nucleus is

1

a conspicuous object in sections through the lower part of this region (Fig. 541). It presents the same appearance as lower down in the mesencephalon, and, gradually diminishing, it disappears before the level of the corpus mamillare is reached. Carried up around it are the same longitudinal tracts of fibres which have been studied in relation to it in the tegmental part of the mesencephalon. Certain of these fibres, placed in immediate relation to the red nucleus, form a coating or capsule for it. This coating is partly derived from those fibres of the brachium conjunctivum which pass directly up into the thalamus and also partly from fibres which issue from the nucleus itself. The medial lemniscus also, which in the superior part of the mesencephalon is observed to take up a position on the lateral and dorsal aspect of the red nucleus, maintains a similar position in the hypothalamic region. When the red nucleus comes to an end these various fibres are continued onwards and form, in the position previously occupied

Red nucleus Nucleus hypothalamicus

Substantia nigra

Caudate nucleus

FIG. 541.-FRONTAL SECTION THROUGH THE CEREBRUM OF AN ORANG PASSING
THROUGH THE HYPOTHALAMIC TEGMENTAL REGION.

by the nucleus, a very evident and dense mass of fibres. The fibres of the medial lemniscus, of the brachium conjunctivum, and of the red nucleus are prolonged upwards into the ventral part of the thalamus, where they end in connexion with the thalamic cells (ventro-lateral nucleus).

The substantia nigra is likewise carried into the hypothalamic region, where it maintains its original position on the dorsal aspect of the basis pedunculi. As it is traced upwards, it is seen gradually to diminish in amount. It shrinks from the medial to the lateral side, and finally disappears when the posterior part of the corpus mamillare is reached.

In frontal sections through the hypothalamic region, the most conspicuous object which comes into view is the nucleus hypothalamicus or the nucleus of Luys (Fig. 541). It is a small mass of gray matter, shaped like a biconvex lens, which makes its appearance on the dorsal aspect of the basis pedunculi immediately to the lateral side of the substantia nigra. At first it lies in an angle which is formed by the meeting of the cerebral peduncle and the internal capsule; but, rapidly enlarging in a medial direction, it takes the place of the diminishing substantia nigra on the dorsal surface of the basis pedunculi at the level of the inferior part of the corpus mamillare. The nucleus hypothalamicus is rendered all the more evident by the fact that it is sharply defined by a thin capsule of white fibres. On its medial aspect these fibres proceed medially and form a very evident decussation across the median plane in the floor of the third ventricle, immediately above the posterior ends of the corpora mamillaria.

The nucleus hypothalamicus, in the fresh condition, presents a brownish colour, partly from the fact that its cells are pigmented, and partly also on account of the numerous capillary blood-vessels which pervade its substance.

This

Corpus Pineale.—This is a small, dark, reddish body, about the size of a cherrystone and shaped after the fashion of a fir-cone. Placed between the posterior ends of the two thalami, it occupies the depression on the dorsal aspect of the mesencephalon, which intervenes between the two superior colliculi. Its base, which is directed upwards, is attached by a hollow stalk or peduncle. stalk is separated into a dorsal and a ventral part by the prolongation backwards into it of a small pointed recess of the cavity of the third ventricle. The dorsal part of the stalk curves laterally and forwards, and, on each thalamus, becomes continuous with the tænia thalami; the ventral part is folded round a narrow but conspicuous cord-like band of white matter, which crosses the median plane immediately below the base of the pineal body and receives the name of the posterior commissure of the cerebrum (Fig. 519, p. 585).

The pineal body is not composed of nervous elements. The only nerves in its midst are the sympathetic filaments which enter it, with its blood-vessels. It is composed of spherical and tubular follicles, filled with epithelial cells, and containing a variable amount of gritty, calcareous

matter.

The pineal body is a rudimentary structure, but in certain vertebrates it attains a much higher degree of development than in man. In the lamprey, lizard, etc., it is present in the form of the so-called pineal eye. In structure it resembles, in these animals, an invertebrate eye, and it possesses a long stalk, in which nerve-fibres are developed. Further, it is carried through an aperture in the cranial wall, and consequently lies close to the surface on the dorsum of the head between the parietal bones.

Trigonum Habenulæ.-The small, triangular, depressed area which receives this name is placed immediately in front of the superior colliculus in the interval between the peduncle of the pineal body and the posterior end of the thalamus (Fig. 539, p. 610). It marks the position of an important collection of nerve-cells, which constitute the ganglion habenulæ. The axons of these cells are collected on the ventral aspect of the ganglion into a bundle, called the fasciculus retroflexus, which takes a curved course downwards and forwards in the tegmentum of the mesencephalon. The fasciculus retroflexus lies close to the medial side of the red nucleus, and finally comes to an end in a group of cells termed the ganglion interpedunculare, situated in the inferior part of the substantia perforata posterior (see p. 591).

The ganglion habenula is likewise intimately connected with the stria medullares (tania thalami) and the dorsal part of the stalk of the pineal body.

As previously stated, the stria medullaris-a very evident band of white matter-lies on the thalamus, subjacent to the ependymal ridge termed the tænia thalami. When traced backwards, many of the fibres of the stria medullaris are observed to end amongst the cells of the ganglion habenula, whilst others are continued past the ganglion to enter the peduncle of the pineal body, and, through it, to reach the ganglion habenula of the opposite side, in connexion with the cells of which they terminate. The stria medullaris, therefore, ends partly in the ganglion habenula of their own side and partly in the corresponding ganglion of the opposite side. The decussation of fibres across the median plane forms the dorsal part of the pineal stalk or peduncle, and is termed the commissura habenularum.

When the stria medullaris is traced in the opposite direction, it is noticed to split into dorsal and ventral parts near the column of the fornix. The dorsal part arises from cells in the hippocampus: these fibres pass into the fornix and when they reach its column they turn abruptly backwards to enter the stria medullaris. The ventral part springs from a collection of cells in the gray matter on the base of the brain close to the optic chiasma. The striæ medullares are believed to form a part of the olfactory apparatus.

Commissura Posterior.-The posterior commissure is a slender band of white matter, which crosses the median plane under cover of the stalk of the pineal body and overlies the entrance of the aqueduct of the brain into the third ventricle. The fibres which enter into the formation of the posterior commissure are believed to arise in a special nucleus, which is placed in the central gray matter immediately above the oculo-motor nucleus. They decussate with each other across the median plane and thus the commissure is formed. The other connexions of this little band are not satisfactorily established, but Held believes that some of its ventral fibres pass downwards into the medial longitudinal bundle.

Substantia Perforata Posterior.-This has already been described on p. 542. Some delicate bands of white matter, termed the tænia pontis, may frequently be seen emerging from the gray matter of this region; they then curve round the pedunculus cerebri in close relation to the superior border of the pons, with which they enter the cerebellum to end in the nucleus dentatus (Horsley).

Corpora Mamillaria.-The corpora mamillaria are two round white bodies, each about the size of a pea, which lie side by side in the interpeduncular fossa on the base of the brain, immediately in front of the substantia perforata posterior.

Each corpus mamillare is coated on the outside by white matter derived from the column of the fornix, and contains, in its interior, a composite gray nucleus with numerous nerve-cells. Several important strands of fibres are connected with the corpus mamillare: (1) The column of the fornix curves downwards in the side wall of the third ventricle to reach the corpus mamillare, and their fibres end amidst the cells of that body. (2) A bundle of fibres, the fasciculus mamillo-thalamicus, takes origin in the midst of each corpus mamillare and extends upwards into the thalamus, to end in fine arborisations around the large cells in the anterior thalamic nucleus. (3) Another bundle of fibres, the pedunculus corporis mamillaris, takes form within the corpus mamillare and extends downwards in the gray matter of the floor of the third ventricle, to reach the tegmentum of the mesencephalon. These tracts, together with the striæ medullares (thalami) and the fasciculi retroflexi, are amongst the most ancient fibre-systems in the brain. They represent the paths by which olfactory impulses may reach the brain-stem, and perhaps the spinal medulla also, and so influence the muscles of the body.

Tuber Cinereum and Infundibulum. The tuber cinereum is a small, slightly prominent field of gray matter, which occupies the anterior part of the interpeduncular fossa between the corpora mamillaria behind and the optic chiasma in front. From its anterior part the infundibulum, or stalk of the hypophysis, projects downwards and connects the hypophysis with the base of the brain. In its upper part the infundibulum is hollow, a small, funnel-shaped diverticulum of the cavity of the third ventricle being prolonged downwards into it.

Hypophysis (O.T. Pituitary Body).-This is a small oval structure, flattened from above downwards, and with its long axis directed transversely, which

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