Anterior Wall.—The anterior commissure (commissztra an terior cerebri, Figs. 35, 5o and 96) is a very distinct round bundle of white fibers about 3 mm. (.12 in.)in diameter. It is seen in the anterior wall of the third ventricle supporting the roof epithelium. The epithelium there bends down between the columna of the fornix and invests the ventricular surface of the commissure. The columna of the fornix and the corn missure bound the recessus triangularis, in which the roof and anterior wall of the third ventricle meet. The anterior corn missure rests upon the upper extremity f the lamina terminalis, between the columna fornicis, behind, and the lamina rostralis of the corpus callosum, in front. With the last two structures it is developed in the lamina terminalis. It is the most impor tant connecting link between the hemispheres in vertebrates without a corpus callosum (all below mammals). Bending sharply backward in the cerebral hemisphere the anterior commissure pierces the inferior part of the globus pallidus and then radiates toward the cortex, some of its fibers- entering the external capsule. It contains two group's of fibers: (r) The anterior group, which is the commissure of the rhinen cephalon, called the pars olfactoria; and (2) the posterior group, the pars occipito-temporalis. The pars olfactoria probably contains two fasciculi: (a) A commissural bundle which rises in the cortex of the olfactory tract, ascends vertically to the commissure, traverses it and bends sharply downward to the opposite tract; then its fibers run forward to the olfactory bulb and terminate in the granular and glomerular layers. (b) A projection bundle which rises in the anterior perforated substance and septum pellucidum. Ascending to the com missure it bifurcates into a direct and a crossed fasciculus. The direct fasciculus joins the crossed fasciculus from the opposite side, forming the stria terminalis whose course has already been traced to the nucleus amygdalx. This is the fasciculus 'olfacto-amygdalaris. The pars occipito-tempora/is connects the tentorial areas and the hippocampal formations of the two hemispheres together, regions which are not con nected by the corpus callosum. In man it is larger than the pars olfactoria. It bends horizontally backward under the head of the caudate nucleus and runs longitudinally through the inferior part of the globus pallidus. Backward to the frontal plane cutting the mammillary bodies it is clearly visible to the naked eye; then it radiates to the temporo-occipital and to the hippocampal cortex. A thin transverse sheet of gray matter, called the lamina terminalis, extends downward and forward from the anterior commissure to the optic chiasma and completes the anterior wall of the ventricle (Figs. 17, 34 and 96). Between the chiasma and the lamina terminalis is a sharp angle which terminates on either side in a small pit, called the optic recess.
The floor of the third ventricle is very narrow (Figs. 21 and 37). It is formed by the interpeduncular structures plus the tegmenta, namely: optic chiasma, tuber cinereum and in fundibulum, corpora mammillaria, posterior perforated sub stance and the tegmenta. The last two are portions of the mid-brain; the others belong to the fore-brain with the surface of which we have already studied them, and all extend laterally beneath the thalami.
The third ventricle has its lateral wall formed chiefly by the thalamus and the columna of the fornix (Figs. 33 and 44). Below a slight longitudinal groove, extending from the optic recess to the cerebral aqueduct and called the sulcus hypo thalamicus, the thalamus is covered by upturned hypothalamic gray matter and by the upper part of the central gray sub stance of the mid-brain. The thalamus forms the immediate
lateral wall above this hypothalamic groove. The sulcus hypothalamicus is believed to represent the sulcus limitans separating the ventral and dorsal laminae of the embryo. This places the origin of the thalamus in the dorsal lamina (the afferent lamina).
The columna of the fornix diverging from its fellow proceeds downward and backward to the corpus mammillare through the medial part of the thalamus. In the ventricle, the pars libera of the columna fornicis is covered by the ependymal epithelium. It bounds the interventricular foramen in front.
Thalamus (Thalamus—a bed, Figs. 5o, 54, 55 and 56).— It is the great ganglion of the inter-brain. The thalamus is an important sensory relay station. Its medial part is concerned with smell (E. Sachs) and its lateral part with common sensa tion and taste. According to Head and Holmes, it is also an organ of consciousness for impulses of pain and temperature. The third ventricle separates the thalami from each other, except at the mid-point where they are joined by the massa intermedia. The thalamus is situated behind and medial to the corpus striatum, and projects backward over the mid brain. Laterally, it rests against the superior lamina of the internal capsule, which separates it from the lentiform nucleus. The thalamus is shaped like an egg, with the small end directed forward. It measures 4 cm. or about one and a half inches in length and 2.5 cm. or one inch in width and thickness. It has an anterior and a posterior extremity and four surfaces: inferior, medial and lateral.
Extremities.—The anterior extremity of the thalamus is lost in a large group of fibers (frontal stalk) which runs through the frontal part of the internal capsule. The free posterior end (Fig. 56) presents a large pillow-like prominence the pulvinar and beneath it are two smaller swellings; the outer one which forms the lowest point of the thalamus is the lateral geniculate body; the medial geniculate body is the other. The two geniculate bodies constitute the metathalamus (Fig. 55).
Surfaces.—The lateral and inferior surfaces of the thalamus are attached so that they can be seen only in section; but the medial and superior surfaces are almost entirely free. A thin layer of medullated fibers called the stratum zonale forms the free surfaces. The medial surface of the thalamus forms the immediate lateral wall of the third ventricle as far down as the sulcus hypothalamicus (Fig. 34). It is joined to the medial surface of the opposite thalamus by the massa intermedia. It is bounded above by the medullary stria. The superior surface of the thalamus is divided by an oblique groove, the chorioidal groove, lying just medial to the twnia chorioidea, into two areas—a medial and lateral (Fig. 56). The medial area is covered by the chorioid tela of the third ventricle and the fornix. Medially, it is bounded by the medul lary stria of the thalamus. Posteriorly next the stria is a tri angular depression bounded behind by a transverse groove in front of the corpora quadrigemina and by a slight groove, the sulcus habenula', laterally. That depressed surface is called the triangle of the habenula (trigonum habenula;). Beneath the triangle is one of the thalamic nuclei, the nucleus habenulce. The lateral area of the superior surface covered by the lamina affixa is seen in the floor of the lateral ventricle. It presents an anterior elevation, the anterior tubercle (tuberculum anterius thalami), beneath which is the anterior nucleus of the thalamus.