| OCR Text |
Show 686 MR. F. G. PARSONS ON THE [Nov. 20, lie Cowper's glands. The penis is 4 inches long, and has embedded in the dorsum of the glans a triangular os penis, into the base of which the tendons of the levatores penis are inserted. THE NERVOUS SYSTEM. The BRAIN of Atherura (figs. 5, 6), when viewed from above, differs very much in its general appearance from the brains of other Porcupines. Beddard says ' that " the Hystricidae form a perfectly natural family. Their brain is characterized by its peculiar shape, rounded in front, and by the fact that the convolutions for the most part are transverse and not longitudinal in direction." This description, although it is true for every other Porcupine I have seen, unfortunately would not give a good idea of the brain of Atherura, for in this animal the frontal part of the cerebral hemispheres is quite narrow and the transverse width increases to nearly the posterior end. At the posterior end of the central longitudinal fissure the hemispheres form a deep notch, in winch the corpora quadrigemina would be exposed were it not for the great development of the anterior part of the central lobe of the cerebellum. Two fissures are marked, but not deeply; they are the posterior part of the longitudinal and the Sylvian. The Sylvian fissure does not reach the rhinal fissure below. The convolutions and fissures of the brain of Atherura are less well marked than those of Hystrix, but better than those of Erethizon or, apparently, Sphingurus. On the base of the brain the chief points of interest are, the large size of the tuber cinereum, from which the pituitary body depends, and the great development of the external arcuate fibres, which run just behind the pons to join the anterior pyramids at right angles. The same points are present in Hystrix and, according to Mivart's figures2, in Erethizon. In a sagittal section of the brain, the chief point that attracts attention is the peculiar funnel-shape of the aqueduct of Sylvius; this is of very small calibre where it leaves the third ventricle, but rapidly dilates so that the posterior end of the lamina quadrigemina which forms its roof, and on which the corpora quadrigemina are situated, is tilted up. THE SPINAL NERVES. Owing to injuries inflicted on the head in removing the brain and tongue before the animal came into my possession, I was unable to make a complete dissection of the cranial nerves. I have therefore contented myself with a description of the nerves supplying the extremities. 1 P.Z.S. 1892, p. 596. 8 P.Z.S. 1882, p. 278. |
