36 Anatomy of the Rabbit. 



ventral portions of the optic tracts join to form the optic chiasma, and 

 immediately behind it the floor is thickened, forming. externally a pair 

 of rounded protuberances, the mammillary 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 con- 

 sisting of two portions, namely, an optic portion, comprising the in- 

 fundibulum, tuber cinereum, and the optic chiasma, and a mammillary 

 portion, including the mammillary bodies. 



The more dorsal portion of the diencephalon, containing the major 

 part of the third ventricle, is known as the thalamencephalon. 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. 53). 



The second of the primary divisions, the mesencephalon, or mid-brain, 

 is noteworthy in a mammal as lacking a ventricle. Its cavity is a 

 narrow canal, the cerebral aqueduct, leading from the third, ventricle 

 backward to the fourth ventricle, or cavity of the rhombencephalon. 

 Externally, its roof is differentiated into four rounded elevations, the 

 corpora quadrigemina, of which the anterior pair are much larger than 

 the posterior ones. Its floor is chiefly formed by a pair of divergent 

 cords, the cerebral peduncles. 



The parts of the mesencephalon and prosencephalon together con- 

 stitute the large brain, or cerebrum. 



The third primary division, the rhombencephalon, or primary bind- 

 brain, is a greatly elaborated portion from which arise the majority of the 

 cranial nerves. The constricted area joining it with the mesencephalon 

 is known as the isthmus rhombencephali. It includes the anterior 

 medullary velum and brachia conjunctiva (Fig. 55) . The rhombencephalon 

 itself is divisible into two portions, especially well defined in the 

 mammalia, namely, the metencephalon, or hind-brain', and the myelence- 

 phalon, or after-brain. The former includes the small brain, or cerebellum, 



