The Nervous System. 73 



thickened, forming externally a pair of rounded protuberances, 

 the mamillary bodies. In the brain of the rabbit this structure 

 consists superficially of a larger median portion with faint lateral 

 elevations appended to it. Collectively, these structures are 

 considered to form a major division, the hypothalamus, the latter 

 consiscing of two portions, namely, an optic portion, comprising 

 the infundibulum, tuber cinereum, and the optic chiasma, and a 

 mamillary portion, including the mamillary bodies. The two 

 portions are commonly classified as belonging respectively to the 

 telencephalon and the diencephalon, but embryological study places 

 the boundary of these of the latter divisions at the optic recess or 

 roughly at the point of the optic chiasma. 



The more dorsal portion of the diencephalon, containing the 

 major part of the third ventricle, is known as the thalamence- 

 phalon. Its lateral walls are greatly thickened, while its roof 

 is extremely thin, especially in its anterior part. Here the actual 

 roof of the ventricle is formed only of a thin layer of tissue, 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 chorioidweb (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 thalamencephalon bears posteriorly 

 the pineal body, the latter together with certain related structures, 

 the habenulae and habenular commissure, forming the 

 epithalamus. The general portion of the thalamencephalon 

 bordering the third ventricle, and broadly connected across the 

 latter by the massa intermedia, is the thalamus. In the brain 

 of the rabbit it will be seen that the thalamus is chiefly indicated 

 externally by a rounded protuberance, the pulvinar. 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. To the medial side of this is a third protuberance, the 

 medial geniculate body. The medial and lateral geniculate 

 bodies as thus defined constitute the metathalamus (Fig. 84). 



The second of the primary divisions, the mesencephalon, or 

 mid-brain, is noteworthy in a mammal as lacking a ventricle. 



