TEE THALAMUS OPTICUS:—The inferior internal ganglion, or thalamus opticus, in general outline is fusiform in shape. Its body is somewhat elongated, and a transverse section shows that it is prismatic. Its upper surface is divided into two portions by an oblique depression which begins in front behind the anterior pillar of the fornix, and extends backward and outward to its external margin at the genu of the internal capsule. The surface above and external to this depression forms a part of the floor of the lateral ventricle. The por tion below or internal to the depression is covered by the fornix, and is continuous behind and below the crus cerebri with the outer division of the optic truct. The internal sur face of the thalamus forms the lateral wall of the third ven tricle, and in the middle of the ventricle, is united to the thal amus of the opposite side by the middle or gray commissure of the third ventricle. The inferior surface of the thalainus looks outwards and downwards and is attached to the internal capsule to which it gives off a great mass of fibres, before and behind, forming the anterior and the posterior divis ions of the internal capsule. A tract of longitudinal fibres, called the fillet, lies external and beneath the lower portion of the inferior surface of the thalamus.
Upon closer inspection the thalamus will be seen to con sist of several smaller masses, more or less similar in shape to that of the caudate nucleus. The anterior extremities of these masses, composing the thalamus, are large, and their posterior portions taper into tracts, which pass backwards around the genu of the internal capsule. These bodies are, from within outwards: first, a small body situated at the side of the posterior extremity of the third ventricle, between the pineal gland and the posterior extremity of the thalamus, and connected with the anterior pillar of the pineal gland, called the pison (pea). This body has no posterior exten sion. Second, a fusiform body, beginning in front by a pointed extremity above the middle commissure, and rapidly enlarging forms a prominence upon the posterior portion of the thalamus, after which it gradually contracts and is continuous with the external geniculate body and the superior division of the optic tract. This body is called the pulvinar
and is probably the visual centre of the thalamus. Third, a prominence upon the anterior extremity of the thalamus, in front of the apex of the pulvinar, behind the anterior pillars of the fornix and above the foramen Monro, called the tuber culum of the thalamus. Extending backward and slightly outward from this tubercle is a tract of white fibres which follows the groove, before mentioned, on the upper sur face of the thalamus as far as the genu of the internal cap sule, among the fibres of which it is lost. The tuberculum thalami is supposed to be the basal centre of taste and smell. Fourth, external to the tuberculum and the tract just de scribed, and more distinctly seen if the latter is removed, is the largest body composing the thalamus. It is broad in the centre, pointed at each extremity, fusiform in shape, reaching across the upper surface of the internal capsule, and separ ated from the caudate nucleus by the tnia semicircularis. This body is supposed to be the thalamic centre of common sensation. Fifth, underneath the central part of the thala mus, and imbedded in the teginentum of the crus cerebri, is a round body, somewhat larger than a pea, called the sub thalmic ganglion, or red nucleus. This body does not prop erly belong to the thalamus, but is closely associated with it. It is the termination of the anterior extremity of the processus, or superior peduucle of the cerebellum, and from it are distributed fibres which pass to the bodies composing the thalamus under which it lies. The red nucleus is in relation externally with the fillet, and internally with the continuation of the gray matter which surrounds the iter and lines the wall of the third ventricle.