The lamina affixa of the chorioid epithelial lamina which covers this outer area separates it from the ventricular cavity. A lamina of fibers, the external medullary lamina, forms the lateral surface of the thalamus and rests upon the superior lamina of the internal capsule. Its fibers are continuous with those of the capsule. The inferior surface blends with the superior surface of the tegmentum and substantia nigra, and forms the laminw and nuclei of the tegmental part of hypo thalamus (see below).
Tegmental Hypothalamic Region (Figs. 37 and 54).—The hypothalamus is divided into three parts, viz., the optic, the mammillary and the iegmental. The optic and the mammillary parts have been considered with the base of the fore-brain; the tegmental part is visible only in sections. The pars tegmentalis hypothalami is composed of three layers: (I) Stratum dorsale next the thalamus; (2) zona incerta, the middle; and (3) hypothalamic nucleus, the inferior. The nucleus hypo thalamicus (Luysi) is ventro-lateral in position and lies be tween the base of the internal capsule and the zona incerta. It has the shape of a flattened cylinder io mm. by 3 mm. and about r mm. in thickness. Like the substantia nigra just be low it, it is composed of pigmented gray matter. The reticular formation of the tegmentum continuing beneath the thalamus forms the zona incerta. The stratum dorsale is made up as follows: (a) Fibers from the medial longitudinal bundle (Meynert); (b) the brachium conjunctivum (Forel), in which is the upper end of the red nucleus of the tegmentum; (c) the medial fillet, which runs lateral and slightly ventral to the red nucleus; (d) the rubro-thalamic tract; (e) the strio-fugal tract's.
The red nucleus (nucleus ruber, Fig. 44) is an elongated ovoid mass of slightly pigmented neurone bodies, located near the me dian plane in the tegmental part of the hypothalamus. It measures about 10 mm. in length and 3 mm. in width and is al most round in cross section. Its anterior end is just behind the
mammillary bodies and its posterior end touches the transverse plane between the superior and inferior quadrigeminal bodies. It lies at the same horizontal level as the hypothalamic nucleus, but it is somewhat caudal to it: the two nuclei are of the same length and the red extends farther downward in the mid-brain and the hypothalamic farther forward in the inter-brain. The red nucleus is intermediate between the cerebellum and the thalamus and between the cerebellum and the spinal cord. It also receives axones from the cerebral cortex and the globus pallidus.
The lateral geniculate body (corpus geniculatum laterale, Fig. 55) forms a slight swelling at the lowest point of the thalamus. It marks the apparent end of the lateral root of the optic tract and is the terminal nucleus of 8o per cent. of its fibers. It is joined to the superior quadrigeminal emi nence by the brachium superius. In appearance it is dark colored and laminated; its gray matter, which contains pig mented multipolar cell-bodies, is divided into thick layers by thin laminae of fibers from the optic tract and radiation. The processes of the multipolar cell-bodies help to form the optic radiation.
The medial geniculate body (corpus geniculatum mediale), also belongs to the inter-brain and, together with the lateral geniculate body, constitutes the metathalamus (Fig. 55). It is placed at the end of the medial root, as the lateral geniculate is at the end of the outer root, of the optic tract. It rises up from the groove between the thalamus and corpora quadrigemina, and is joined to the inferior quadrigeminal eminence by the brachium inferius. The brachium superius sweeps around it in front. The medial geniculate body is gray in color and is not laminated. Its cell-bodies are small and fusiform in shape. They perhaps give origin to the intercerebral fibers (Guddeni) of the optic tract and to a large part of the acustic radiation.