THE THIRD VENTRICLE AND INTER-BRAIN. 143 



area and 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 tegmen- 

 tum and substantia nigra, and forms the laminae and nuclei of the 

 tegmental hypothalamic region. (See below.) 



Tegmental Hypothalamic Region (Figs. 30 and 42). The 

 regio tegmentalis hypothalami is composed of three layers: (i) 

 Stratum dorsale next the thalamus; (2) zona incerta, the mid- 

 dle; and (3) hypothalamic nucleus, the inferior. The nucleus 

 hypothalamicus (Luysi) is ventro-lateral in position and lies 

 between the base of the internal capsule and the zona incerta. 

 Like the substantia nigra just below it, it is composed of pigmented 

 gray matter. The reticular formation of the tegmentum, con- 

 tinuing 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 con- 

 junctivum (Forel), in which is the upper end of the red nucleus 

 of the tegmentum; and (c) the medial fillet, which runs lateral 

 and slightly ventral to the red nucleus. 



The lateral geniculate body (corpus geniculatum laterale, Fig. 

 43) 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 eighty per cent, of its fibers. It is 

 joined to the superior quadrigeminal eminence by the brachium 

 superius. In appearance it is dark colored and laminated; its 

 gray matter, which contains pigmented 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 mediate), 

 also belongs to the inter-brain and, together with the lateral 

 geniculate body, constitutes the metathalamus (Fig. 43). 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, 



