xii PHYLUM CHORDATA 515 



Fig. 310, c. h), which are relatively long and narrow, presents 

 certain depressions or sulci, which, though few and indis- 

 tinctly marked, yet divide the surface into lobes or con- 

 volutions not distinguishable in the case of the pigeon or 

 the lizard. A slight depression, the Sylvian fissure, at 

 the side of the hemisphere separates off a lateral portion or 

 temporal lobe (Fig. 311, ch-) from the rest. There are 

 very large club-shaped olfactory lobes at the anterior ex- 

 tremities of the cerebral hemispheres. Connecting together 

 the two hemispheres is a commissural structure, the corpus 

 callosum (Fig. 310, cp. el), 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, the fornix (b. fo), 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 

 lucidum) (sp. lu.) enclose a small, laterally compressed cav- 

 ity, 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 the lateral ventricle 

 is formed of an eminence of gray matter, the corpus 

 striatum {cp. s). The right and left corpora striata are 

 connected together by a narrow transverse band of white 

 fibres, the anterior commissure (a. co). 



The diaccele (z* 3 ) is a laterally compressed cavity. From 

 the posterior part of the roof of the diacoele arises the 

 peduncles of the pineal body, and just beyond their point of 

 origin is the posterior commissure (p. co), a delicate trans- 



