PKOSENCEPHALON OF MAMMALS. 



105 



73 



B 



Didelphys Virginian;!. 



has the anterior apex of the hemisphere marked off by a deeper 



transverse fissure, extending to the inner surface. In the Her- 



bivorous Marsupials the fissures 



are more definite, deeper, and 



rather more numerous in the 



larger (Macropus major, fig. 74) 



than in -the smaller species 



(Hypsiprymnus). All Marsupials 



have the hippocampal fissure, 



fig. 46, 4, fig. 73, z, coextensive 



with the antero-posterior range of the prosencephalic cavity, and 



arching over all the commissural apparatus of the hemispheres. 



The concomitant extent of the convolution (hippocampus major) 



is shown in LXX'. pi. vii. figs. 3 (JDidelpliys) and 4 (Macropus), 



in the exposure of the ventricle from the outer side. In 



Didelphys, fig. 73, the surface of the hemisphere above the 



fissure is feebly impressed by blood-vessels ; in Tltylacinus there 



is a short fissure above the back part of the hippocampal 



one ; in Phascolomys and Macropus there is also an anterior one 



which bends or bifurcates at its fore part. 1 These fissures mark 



the level of the roof of the 



lateral ventricle ; the surface 



below forming the thin mesial 



o 



wall of the cavity, fig. 75, q, 

 which in the higher Pla- 

 centals is defined, as the ' sep- 

 tum lucidum,' by a corpus 

 callosum from the part above, 

 On the upper surface of the 

 hemisphere, in Macropus ma- 

 jor, a longitudinal part of the 

 fissure, fig. 74, s, marks off a 

 medial convolution, /, at the 

 anterior half, and occasion- 

 ally it is prolonged backward 

 by the fissure, 10, as in the 

 left hemisphere of fig. 74. 

 But there is continued from 

 8, in both hemispheres, a 

 fissure extending outward, 

 which bounds behind the 

 part of the hemisphere impressed by the ( sylvian fissure,' 5. The 



74 



1-2 



Brain of Macropus major. 



1 LXX'. pi. vi. figs. 4 and 6, </, q. 



