THE BRAIN 83 



portion with faint lateral elevations appended to it. Collectively, 

 these ventral structures are considered to form a major division 

 of the fore-brain, the hypothalamus, the latter consisting of two 

 portions, namely, an optic portion, comprising the optic chiasma 

 and some adjacent tissue, and a mamillary portion, including the 

 mamillary bodies, the tuber cinereum, the infundibulum, and the 

 hypophysis. The optic portion belongs to the telencephalon and 

 is better termed telencephalon medium, while the mamillary 

 portion belongs to the diencephalon. 



The more dorsal portion of the diencephalon, containing the 

 major part of the third ventricle, is sometimes known as the 

 thalamencephalon, a term now falling into disuse. Its lateral 

 walls are greatly thickened, while its roof is extremely thin, es- 

 pecially in its anterior part. Here the actual roof of the ventricle is 

 formed of a layer of tissue only one cell in thickness, the epithelial 

 chorioid lamina, but the latter has associated with it a series of 

 vascular ingrowths of the investing pia mater, the latter being 

 described in this relation as the chorioid web (tela chorioidea). 

 The two structures together form a chorioid plexus. This extends 

 downward into the third ventricle, reaching out also into the lateral 

 ventricles. 



The dorsal portion of the diencephalon bears posteriorly the pineal 

 body or epiphysis cerebri, an endocrine gland borne upon a stalk which 

 is attached to certain other small dorsal parts of the brain, the habe-- 

 nulae and habenular commissure. These all together form the 

 epithalamus. The lateral wall of the third ventricle is formed by 

 the thalamus, which has become so massive in the mammal that 

 it bulges medially to fuse with that of the other side and thus to 

 produce a broad bridge across the middle of the ventricle, the 

 massa intermedia. In the brain of the rabbit it will be seen that 

 the thalamus is indicated externally chiefly by a rounded pro- 

 tuberance, the lateral thalamic tubercle. The latter is dorsal in 

 position and is imperfectly marked off from a second protuberance, 

 the lateral geniculate body, lying on its postero-lateral side. Postero- 

 medial to this is a third protuberance, the medial geniculate body. 

 The medial and lateral geniculate bodies as thus defined constitute 

 the metathalamus (Fig. 116). 



The second of the primary divisions, the mesencephalon, or 



