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Show 1892.] CEREBRAL HEMISPHERES OF RODENTS. 597 § 1. Description of the Cerebral Hemispheres in certain Genera. Castor canadensis1.-Total length 54 mm., breadth 41 mm., height 25 m m. 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 hemisphere ; 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 m m . long) longitudinal fissure. These two fissures are indicated by Leuret and Gratiolet as forming one continuous fissure. O n the under surface of the brain no trace was apparent of the Sylvian fissure. The rbinal 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 15 m m . from the upper surface of the hemispheres. The olfactory bulbs are large. Capromyspilorides.-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 m m . 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 m m . from the intercerebral sulcus and about 4 m m . 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 1 Figured by Leuret and Gratiolet (8, plate i'i. fig. 1). |
