xii PHYLUM CHORDATA 509 



cerebro-spinal nervous system the brain and spinal cord. 

 The brain (Fig. 286) of the Rabbit consists of the same 

 principal parts as that of the Pigeon, with certain differences, 

 of which the following are the most important. 



The surface of the cerebral hemispheres (Fig. 286, /. b.\ 

 which are relatively long and narrow, presents certain de- 

 pressions or sulci, which, though few and indistinct, 

 yet mark out the surface into lobes not distinguish- 

 able in the case of the Pigeon or the Lizard. A slight 

 depression the Sylvian fissure at the side of the hemi- 

 sphere separates off a lateral portion, or temporal lobe, 

 from the rest. There are very large club-shaped olfac- 

 tory lobes at the anterior extremities of the cerebral hemi- 

 spheres. Connecting together the two hemispheres is a 

 commissural structure the corpus callosum not present in 

 the Pigeon \ this runs transversely, above the level of the 

 lateral ventricles. Below the corpus callosum is another 

 characteristic structure of a commissural nature ihefvrm'x 

 a narrow median strand of longitudinal fibres which 

 bifurcates both anteriorly and posteriorly. Below the corpus 

 callosum, between it and the fornix, the thin inner walls of 

 the hemispheres (septum luciduni) enclose a small, laterally 

 compressed cavity, the so-called fifth ventricle or pseudoccele ; 

 this is not a true ventricle, but merely a space between the 

 closely apposed hemispheres. 



The lateral ventricles of the cerebral hemispheres are 

 much more extensively developed than in the brain of 

 the Pigeon, and of somewhat complex shape. The floor 

 of the anterior portion of each is formed of an 

 eminence of gray matter the corpus striatum. The 

 right and left corpora striata are connected together by 

 a narrow transverse band of white fibres the anterior 

 commissure. 



