470 ZOOLOGY SECT. 



forming the splenium, which passes forwards and is united, 

 below the corpus callosum, with another characteristic struc- 

 ture of a commissural nature the fornix (b.fo) a narrow median 

 strand of longitudinal fibres, which bifurcates both anteriorly and 

 posteriorly to form the so-called pillars of the fornix (anterior and 

 posterior, a. fo., p. /<?.). 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 cavity, the 

 so-called fifth ventricle or pseudoccele ; 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 



FIG. 1090. Lepus cuniculus. Longitudinal vertical section of the brain (nat. size). Letters 

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

 c. ft 1 , parencephalon ; c. 1ft. temporal lobe ; c. ma. corpus mammillare ; /. in. foramen of 

 Monvo ; inf. infundibulum ; ly. psalterium or lyra ; m. o. medulla oblongata ; o. ch. optic 

 chiasma ; olf. olfactory lobe ; pty. pituitary body ; vl. ip. velum iuterpositum ; v. vn. valve 

 of Vieussens; II, optic nerve. (From Pai-ker's Zootomy.) 



prolongation, or anterior cornu, 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 tranverse section the 

 hippocampus (hp. in.) ; this corresponds to a groove, the hippocampal 

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

 two hippocampi merge in a median commissural area the 

 psalterium or 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 tsenias forms a 

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

 explained, lies below the corpus callosum, continuous with the 



