OCR Text |
Show 1892.] CEREBRAL HEMISPHERES OF RODENTS. 601 hemisphere I could only recognize one of these. On the right half of the brain there is a marked indentation to the outside of the posterior extremity of the longitudinal furrow. On the under surface of the brain the rhinal fissure is seen to be very distinct and complete. The Sylvian fissure is not so distinct where it joins the rhinal fissure as it is laterally; just in front of the Sylvian fissure is a small fissure arising from the rhinal fissure, which appears to be the termination of the anterior transverse fissure of the dorsal surface of the brain. Sphingurusprehensilis.-Total length 36 mm., breadth 27 mm., height 19 m m. The cerebral hemispheres show the same peculiar shape that is seen in Hystrix; their contour is almost circular ; the posterior divergence of the two hemispheres is so nearly filled by the projecting cerebellum that the merest peep at the corpora quadrigemina is alone possible. Each hemisphere measures 24 m m . in length; the upper surface is smooth, but not so smooth as in the Beaver; it is marked by a good many meandering lines which are for the most part directed downwards. I do not describe these particularly as they do not appear to me to be of any morphological importance. In addition to these there are two dents on the upper surface of the brain. The posterior of these is placed at a distance of about 6 m m . from the hinder margin of the brain; the anterior mark is about tbe same distance in front. The rhinal fissure is not well marked, but it is complete. The Sylvian fissure was hardly apparent. Sphingurus villosus.-Total length 36 mm., breadth 30 mm., height 20 m m. This brain is almost exactly identical in size with that of the last species; nor is there any occasion to describe the general shape and the proportions of its various regions, for they are precisely as in Sphingurus prehensilis. On the other hand, the brain is deeply furrowed in comparison with that of the other Sphingurus. It appears to m e to be a very remarkable fact there should be this difference in the brain-surface of two animals of the same size and belonging to the same genus. The rhinal fissure is quite deep and perfectly complete. The Sylvian fossa is much deeper than in Sphingurus prehensilis. The Sylvian fissure is also quite conspicuous and passes on to the dorsal side of the brain, where it is deeper than at its origin; this region of the Sylvian fissure is represented by the posterior of the two dents upon the surface of the brain of Sphingurus prehensilis. In front of this fissure are two others equidistant from it and from each other, which run in a similar direction ; besides these principal fissures are numerous smaller onels which are principally branches of them. There was no longitudinal fissure. |