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 
consisting 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 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 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. 
