1892.] CEREBRAL, HEMISPHERES OF RODENTS. o97 



§ 1. Description of the Cerebral Hemispheres in certain Genera. 



Castor canadensis^. — Total length 54 mm., breadth 41 mm., 

 height 25 mm. 



The cerebral hemispheres trend away from each other posteriorly, 

 permitting the corpora quadrigemina to be seen ; they are closely 

 applied anteriorly. The length of each hemisphere is 38 mm. ; the 

 hemispheres are wider behind than in front; the width gradually 

 increases up to a point a little in front of the middle of the hemi- 

 sphere ; thence the two margins are approximately parallel to each 

 other. 



The upper surface of the cerebral hemispheres is nearly completely 

 smooth ; at about the end of the first half of the hemisphere is a 

 short fissure shaped like the Greek letter y ; behind this, separated 

 from it by a considerable interval and further from the middle line, 

 is a very short (3 mm. long) longitudinal fissure. These two 

 fissures are indicated by Leuret and Gratiolet as forming one 

 continuous fissure. On the under surface of the brain no trace was 

 apparent of the Sylvian fissure. The rhinal fissure is by no means 

 clearly marked ; it is only obvious anteriorly and again just before 

 the posterior margin of the hemispheres ; it ends at a distance of 

 1.5 mm. from the upper surface of the hemispheres. The olfactory 

 bulbs are large. 



Capromys pilorides. — Length 38 mm., breadth 26 mm., height 

 16 mm. 



The cerebral hemispheres (fig. 1, p. 598) show the same rounded 

 oval contour that distinguishes the brains of the Porcupines. The 

 extreme diameter of the hemispheres is reached a very short distance 

 behind the anterior end of the brain. The posterior divergence of 

 the two hemispheres partly displays the corpora quadrigemina. 

 Each hemisphere measures 25 mm. in length. The surface is faintly 

 marked by a few furrows. A longitudinal furrow (see woodcut, 

 fig. 1) on each side starts from the inner angle of the hemisphere ; 

 it is altogether 9 millimetres long ; it passes at first forwards and 

 outwards, and then at the middle of its length changes its direction 

 and runs straight forwards parallel to the long axis of the brain ; a 

 very short branch is given off at this point which continues the 

 straight line of the first part of the furrow ; another short branch 

 parallel to this is given off on the side just before the end of the 

 furrow. These furrows are more strongly marked on the right 

 half of the brain. In front of this posterior longitudinal furrow 

 is a short C-shaped furrow not connected with it ; the concavity of 

 this furrow is directed inwards. There are indications of a very 

 faint transverse furrow at a distance of 10 mm, from the intercerebral 

 sulcus and about 4 mm. from the posterior margin of the brain. A 

 diagonal furrow about 8 mm. in length is partly visible on the 

 sides of the hemispheres when the brain is viewed from above ; it 



^ Figured by Leuret and Gratiolet (8, plate i'i. fig. 1). 



