xiii PHYLUM CHORDATA 441 



lucidum) (sp. In.) enclose a small, laterally compressed cavity, the so- 

 called fifth ventricle or pseudocode ; this is not a true brain- 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. Each consists of a middle portion 

 or body roofed over by the corpus callosum, a narrow anterior 

 prolongation, or anterior cornn, a posterior cornu, which runs back- 

 wards and inwards, and a descending cornu, which passes at first 

 almost directly outwards, then downwards, and finally inwards 

 and forwards. On the floor of the body of the ventricle, and 

 continued along the whole extent of the descending cornu, is a 

 prominent ridge of nearly semicircular transverse section the 

 hippocampus major (hp.m.)', this corresponds to a groove, the 



Jf -l.t-z 



FIG. 1033. Lepus cuniculus. Longitudinal vertical section of the brain (nat. size). Letters- 

 as in preceding figure ; in addition, cb. cerebellum, showing arbor vitse ; c. c. cms cerebri ; 

 c. &1. pareiicephalon ; c. /<-. temporal lobe ; c ma. corpus mammillare ; /. m. foramen of 

 Monro ; 'inf. infiindibulum ; h/. lyra ; m. o. medulla obloiigata ; o. ch. optic chiasma ; olf. 

 olfactory lobe ; pti/. pituitary body ; vl. ip. velum interpositum ; v. vn. valve of Vieussens ;. 

 //, optic nerve. (From Parker's Zootomy.) 



hippocampal sulcus, on the inner surface of the temporal lobe. 

 Internally the two hippocampi merge in a median area the 

 lyra (ly.). 



Running along the anterior edge of the hippocampus is a ridge 

 of fibres the tcenia hippocampi or fimbria which passes down 

 into the descending cornu. The union of the two tsenise forms a 

 median longitudinal strand, the body of the fornix, which, as already 

 explained, lies below the corpus callosum, continuous with the 

 splenium of the latter behind, but diverging from it anteriorly by 

 dipping down towards the base of the brain. In the angular space 

 between the corpus callosum above and the fornix below is the 

 septum lucidum with the fifth ventricle. The tsenise hippocampi 

 are the posterior pillars of the fornix (p.fo.) ; the anterior pillars 

 (a.fo.) are a pair of vertical bands which pass from the anterior 

 end of the body downwa,rds to the corpus mammillare at the 

 base of the diencephalon. 



