UNIVERSITY

SUBTHALAMIC TEGMENTAL REGION

CALIFORNIA

113

thalamus is cut off from it by another septum of white matter. The anterior separated part is the anterior nucleus of the thalamus or nucleus of the anterior tubercle (a). It contains comparatively large nerve-cells and from its depth a number of fibres pass downwards and converge to form a well-marked bundle (bundle of Vicq-d'Azyr), which, entering the corpus albicans, forms within that tubercle a sharp bend, and appears to pass upwards again in the wall of the ventricle as the anterior pillar of the fornix. There would seem, however, to be no actual continuity between the two (see pp. 129, 130). The middle commissure unites the mesial nuclei across the third ventricle, and is also continuous below on each side with the grey matter of the cavity. It contains nerve-cells and transversely coursing fibres, but many of the fibres are said to loop backwards near the median plane in place of passing across to the opposite side.

In the trigonum habenula is a collection of nerve-cells termed by Meynert the ganglion of the habenula. To it, fibres pass from the peduncle of the pineal body, and from it a bundle of fibres (fasciculus retroflexus, Meynert's bundle (fig.

Fig. 84.-SECTION OF THE INTER-BRAIN NEAR THE JUNCTION WITH THE MID-BRAIN, SHOWING THE SUBTHALAMIC TEGMENTAL REGION. Magnified about 2 diameters. From a photograph (E. A. S.).

The section passes nearly horizontally, so as to cut the fibres of the internal capsule across transversely. It is just above and in front of the corpora mamillaria.

t., tænia (attachment of epithelial roof of ventricle); v. III., ventriculus tertius; str., stria pinealis; Th., thalamus; n.t., mesial nucleus of thalamus; opt., optic fibres passing to pulvinar; z.i., zona incerta: c.s., corpus subthalamicum; c.i., capsula interna; a.l., ansa lenticularis; f., anterior pillar of fornix passing backwards towards corpus albicans; V.A., bundle of Vicq d'Azyr, passing upwards and forwards from corpus albicans into thalamus; x., white bundle containing a central nucleus of grey matter (? the fasciculus retroflexus of Meynert); g, g', special groups of nerve-cells, of which g' is the hinder end of the ganglion habenula.

84, x)) passes with a curved course through the tegmentum on the mesial side of the red nucleus towards the place where the cerebral peduncles diverge; where,

VOL. III.

I

according to Forel, it passes to another collection of nerve-cells, better marked in most animals than in man, termed the interpeduncular ganglion (see p. 103).

Subthalamic tegmental region; transitional region. The prolongation of the tegmentum under the posterior part of the thalamus is divided by Forel into three layers, which are named respectively from above down, the stratum dorsale, the zona incerta, and the corpus subthalamicum or nucleus of Luys (fig. 83, 1, 2, 3). The latter has here taken the place of the substantia nigra, lying next to the prolongation of the crusta, the fibres of which are seen at the side of the subthalamic tegmental region forming the internal capsule. The stratum dorsale consist chiefly of fine longitudinal fibres-prolonged from the posterior longitudinal bundle according to Meynert, or from the fibres enclosing the tegmental nucleus according to Forel, possibly from both sources. The red nucleus of the tegmentum is prolonged into its posterior part, and from this a considerable number of fibres stream into the internal medullary lamina of the thalamus, and a well-marked bundle passes across the internal capsule to the lenticular nucleus. Some fibres of this layer, which are traceable downwards into the upper fillet (Wernicke), turn into the external medullary lamina of the thalamus, which lies along the mesial side of the internal capsule, and from here they probably diverge into the white matter of the hemisphere; whilst others, coursing through the zona incerta, and crossing the inner capsule, join a tract (ansa lenticularis, fig. 84, a.l. and p. 112) which leads to the lenticular nucleus of the corpus striatum. Finally another bundle of fibres is said to be traceable from the mesial nucleus of the thalamus backwards through the subthalamic region into the dorsal part of the posterior commissure, and thus over the Sylvian aqueduct into the tegmentum of the opposite side.

The zona incerta is a reticular formation prolonged from that of the tegmentum ; it passes anteriorly into the substantia interansalis.

The corpus subthalamicum, or nucleus of Luys (figs. 83, 84, c.s.), is a well-marked brown stratum of grey matter containing numerous nerve-cells, and a close plexus of very fine medullated fibres. It is lens-shaped in section, and has an enclosing envelope of white substance, through which strands of fine fibres pass from the interior of the body mesially towards the zona incerta, and outwards and downwards through the internal capsule. This stratum is distinct only in the Primates.

The pineal body or gland (conarium, epiphysis cerebri) (fig. 82, Cn, fig. 91), is a reddish body about the size of a small cherry-stone, and is named from its supposed resemblance in shape to a fir-cone. It is connected with the posterior part of the third ventricle, projecting backwards and downwards between the superior pair of corpora quadrigemina. It is attached on each side by a broad but flattened stalk of white fibres (pedunculus conarii) which is separated by the pineal recess of the ventricle (p. 97) into a dorsal and ventral portion. The ventral portion curves downwards; it belongs to the ventral portion of the posterior commissure. These fibres are said to be derived from the optic tract near the lateral geniculate body, and to go to the oculomotor nucleus of the opposite side (compare p. 109). The upper portion extends on either side along the ridgelike junction of the upper and mesial surfaces of the thalamus as the pineal stria or tænia fornicis (fig. 82, Tfo.). At the sides the stalk merges into the trigonum habenulæ. The pia mater which invests the mesencephalon, covers the pineal gland with a special investment before being prolonged as the velum interpositum over the third ventricle and thalamus; and the gland is liable to be torn away in removing the pia mater.

The pineal gland is composed of a number of hollow follicles generally spherical, but in some cases tubular, separated from one another by ingrowths of connective tissue. The follicles are almost filled with epithelial cells and often contain much gritty calcareous matter (acervulus cerebri, brain-sand), composed of microscopic

particles, aggregated into masses and formed of earthy salts (phosphate and carbonate of lime, with a little phosphate of magnesia and ammonia) combined with animal matter.

The same sandy matter is frequently found on the outside of the pineal body, or deposited upon its peduncles. It is found also in the choroid plexuses; and in a scattered form occurs in other parts of the membranes of the brain. It occurs at all ages, frequently in young children, and sometimes even in the foetus. It cannot, therefore be regarded as the product of disease. The pineal body is larger in the child and the female than in the adult male (Huschke). In the brains of other mammals it is proportionally larger than in the human subject, and less loaded with brain-sand.

The pineal body is developed originally as a hollow outgrowth from that part of the embryonic brain which afterwards forms the third ventricle; the diverticulum becomes subsequently cut off from the ventricle, and undergoes ramification to form tubes which are afterwards separated for the most part into isolated vesicles.

The pineal body is present in all vertebrates, Amphioxus only excepted. In elasmobranch fishes and in most reptiles, as the researches of de Graaf, Baldwin Spencer, and others, have shown, it is continued into a long tubular prolongation from the third ventricle, which passes through an aperture in the skull (parietal foramen) and ends under the skin in a small vesicle lined with ciliated epithelium. But in some reptiles (e.g., Hatteria, blind-worm, lizard) this vesicle becomes developed into a structure which bears a close resemblance to an invertebrate eye (pineal eye), the part nearest the surface becoming thickened to form a kind of lens, and the part connected with the stalk becoming pigmented and stratified like a retina, whilst the stalk itself becomes solid and has nerve-fibres developed in it. It is doubtful how far this structure serves as an eye in any living reptile, but in certain extinct forms it was probably more completely developed. In birds and mammals the pineal eye is not developed, but the organ is similar in structure to that of man.

The posterior perforated space (locus perforatus posticus) (fig. 32, ×), lies in a deep fossa (fossa Tarini, His) at the base of the brain, at the bottom of which is greyish matter, connecting the diverging crura. It is perforated by numerous small openings for the passage of blood-vessels; and some horizontal white striæ usually pass out of the grey matter and turn round the peduncles close to the upper border of the pons, entering which they reach eventually the medullary centre of the cerebellum (tania pontis). It corresponds posteriorly, as far as a line joining the anterior borders of the third nerves, to the floor of the aqueduct of Sylvius, but in front of those nerves to the posterior part of the floor of the third ventricle. In the grey matter over the space are a few scattered nerve-cells.

The corpora albicantia or mamillaria (fig. 32, a; fig. 86) are two round white eminences in front of this space, each about the size of a small pea, connected together across the middle line. Each corpus albicans contains grey matter concealed within its superficial white fibres, the nerve-cells being arranged in two groups, the lateral and mesial (nuclei of the corpus albicans); of these the lateral contains larger nerve-cells than the mesial.

The white matter of the corpora albicantia is formed by the anterior pillars of the fornix: hence they have also been named bulbs of the fornix; and by the bundle of Vicq d'Azyr, which enters the anterior part of each tubercle at the dorso-mesial aspect. Posteriorly each corpus albicans receives a bundle of nerve-fibres, which is termed its peduncle. This, which in man is concealed within the grey matter of the floor of the third ventricle, but which is seen at the base of the brain in many animals, and is connected with the lateral nucleus of the body, is traceable to the tegmentum and ultimately to the mesial part of the crusta (v. Gudden). In most vertebrates there is but one (median) corpus albicans in place of two.

An isolated bundle of one of the anterior pillars of the fornix is sometimes visible at the base of the brain passing to the corpus albicans (stria alba tuberis, Lenhossék).

The tuber cinereum (fig. 86; fig. 32, t.c.) is a lamina of grey matter extending forwards from the corpora albicantia to the optic commissure, to which it is attached. It forms part of the floor of the third ventricle. In the middle it is prolonged forwards and downwards into a hollow conical process, the infundibulum (fig. 85, i), to the extremity of which is fixed the pituitary body. On its outer side close to the optic tract is a tract of grey matter with nerve-cells, termed by Meynert the basal optic ganglion (see p. 119). According to Lenhossék this is distinctly subdivided into three successive groups of nerve-cells, the most anterior being just above the chiasma, the most posterior near the corpus albicans.

The pituitary body or hypophysis cerebri (figs. 32, 39, h, and fig. 85), formerly called pituitary gland, from its being erroneously supposed to discharge pituita into the nostrils, is a small reddish grey mass, of a somewhat flattened oval shape, widest in the transverse direction, and occupying the sella turcica of the sphenoid bone. The pituitary body has a special prolongation of the dura mater completely enclosing it, except above where there is a small aperture for the passage of the infundibulum (see p. 182). The body consists of two lobes, of which the anterior is the larger, and is concave behind, where it embraces the smaller posterior lobe. The two lobes are entirely different, both in their structure and development; and it is only in mammals that they come into close connexion with one another.

In the

The posterior lobe is developed as a hollow downgrowth of the part of that cavity of the embryonic brain, which afterwards becomes the third ventricle. lower vertebrates, and especially in fishes, the cells which compose its walls become converted into nerve-cells and fibres, and as the lobus infundibuli it becomes an integral part of the brain. But in the higher vertebrates it remains small and almost undeveloped; its cavity is obliterated, and all nervous structure becomes obscured by the ingrowth of vessels and of connective tissue into the now solid organ. The connective tissue forms reticulating bundles, between which occur

Fig. 85. SAGITTAL SECTION OF THE PITUITARY BODY AND INFUNDI

BULUM WITH THE ADJOINING FART OF THE THIRD VENTRICLE

(Schwalbe).

a, anterior lobe; a', a projection from it towards the front of the infundibulum, i; b, posterior lobe connected by a solid stalk with the infundibulum; l.c., lamina cinerea; o, right optic nerve; ch, section of chiasma; 7.0., recess of the ventricle above the chiasma; c.m., corpus mamillare.

numerous spindle-shaped and branched cells, as well as a few larger corpuscles containing pigment-granules in their protoplasm. Sometimes remains of the original hollow are seen in the form of a cavity lined by columnar ciliated cells.

The anterior lobe, darker in colour than the posterior, is developed as a tubular prolongation from the epiblast of the buccal cavity, with which it is therefore originally in connection, although it soon becomes separated by the growth of intervening tissue. In the adult it is constituted by a large number of slightly convoluted tubules or alveoli, similar to those of a secreting gland, and in like manner lined by epithelium, which in some cases fills up the tubule. The tubules are united by connective tissue, which is especially abundant in the neighbourhood of the larger blood-vessels, and also forms a sort of capsule to the organ. Moreover, portions of the tubules are frequently cut off by the connective tissue so as to form isolated vesicles. The outer layer of epithelium is columnar; and in some of the larger tubes, especially those next to the posterior lobe, cilia may be detected on the cells. The blood-vessels are numerous, and the capillaries form a close network

around the walls of the tubules. The lymphatics of the organ originate in cleftlike spaces between the tubules and pass to a network in the capsule. In its microscopic structure the anterior lobe of the pituitary body bears a resemblance to the thyroid body, the vesicles of which are also originally a network of anastomosing tubules, and in some animals remain throughout life in this condition. Moreover, a colloid substance like that in the thyroid vesicles, is found sometimes in the alveoli of the anterior lobe of the hypophysis.

In the middle line of the base of the brain, in front of the optic commissure, is the anterior portion of the great longitudinal fissure, which separates the two hemispheres. At a short distance in front of the chiasma, this fissure is crossed transversely by the anterior recurved extremity of the corpus callosum. On gently

post.perf. sp. -corp. alb.

turning back the optic commissure, a thin connecting layer of grey substance, the lamina cinerea, is seen occupying the space between the corpus callosum and the chiasma, and continuous above the chiasma with the tuber cinereum. It is connected at the sides with the grey substance of the anterior perforated space, and forms part of the anterior boundary of the third ventricle (fig. 57, p. 70): it is somewhat liable to be torn in removing the brain from the skull; and, in that case, an aperture is made into the fore part of the third ventricle.

The optic tracts and optic commissure or chiasma. The optic tracts curve round the crusta on each side to unite with one another immediately in front of the tuber cinereum, where they form the X-shaped commissure which is known as the chiasma. The optic tracts form the posterior limbs of the X; the optic nerves, passing into the optic foramina, the anterior limbs (fig. 86).

Each tract arises posteriorly by a broad root, which is divided by a longitudinal groove into two distinct parts, a lateral and a mesial. The lateral root is the larger. It is connected with and emerges from the posterior and ventral part of the thalamus (lateral geniculate body and pulvinar), and is partly continuous with the brachium of the superior quadrigeminal body. The mesial root, when traced backwards, is seen to curve round the crusta and then to lose itself beneath the mesial geniculate body, in which it appears to end, although it may perhaps ultimately pass into the inferior quadrigeminal body as its brachium. Although this root appears connected with the internal geniculate body, it is doubtful if there is any functional connexion between this body and the optic nerve, the part of the tract which enters the mesial geniculate body being Gudden's commissure, which joins the mesial geniculate bodies. of the two sides (see below). Before reaching the chiasma the optic tract lies postero-mesially to the anterior perforated space.

Darkschewitsch describes the passage of some of the fibres of the optic tract to the ganglion habenula and pineal peduncle, and ultimately by the posterior commissure to the oculomotor nucleus (compare pp. 109 and 114). He regards these as fibres subserving the reflex changes of the pupil.

As for the course of the optic fibres in the tract and chiasma it may be stated