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Show 1892.] CEREBRAL HEMISPHERES OF RODENTS. 605 The brain of this animal is also figured by Leuret and Gratiolet (8, pi. iii.). The furrows are not, however, sufficiently marked in their drawing. Cavia porcellus.-Length 28 mm., breadth 20 mm., height 12 mm. The description of this brain need not occupy us long, since the convolutions upon the cerebral hemispheres are very greatly reduced as compared with allied forms. The outline of each hemisphere, which measures 20 m m . in extreme length, is roughly triangular; the widest point is not far in front of the posterior margin of the hemispheres ; thence the opposite margins converge to the anterior end of the brain, which is of slight diameter. The hemispheres are divaricated behind so as to display the corpora quadrigemina. On the under surface the rhinal fissure is well marked, entirely separating the rhinencephalon from the pallium ; this fissure is considerably deeper behind than in front. In the temporal region the rhinencephalon is very convex, a more or less strongly marked fissure (the Sylvian fossa) separating the convex posterior from the more flattened anterior part of the rhinencephalon. The rhinencephalon is visible when the brain is viewed laterally. On the left side there is a distinct Sylvian fissure, which extends for a distance of about 6 mm.; its direction is almost vertical, but it slopes backwards a little. On the right side of the brain there was no trace that I could see of this fissure. The upper surface of the brain is but little fissured. O n each hemisphere is a short fissure about 5 m m . long commencing a little way in front of the posterior margin of the brain and situated 3 m m. from the median interhemispheral fissure ; in addition to this another longitudinal fissure is present on each side 5 m m . away from the last, longer and commencing at the posterior margin. This fis-ure is, however, even shallower than the last, but in spite of this is perfectly evident. It is generally stated that the convolutions of the hemispheres bear some relation to the size of the animal; thus in relation to the Cervidae Sir W . H. Flower has pointed out1 " how closely the amount of convolution bears relation to the bulk of the hemisphere, the primitive pattern being exactly the same in all." Again, among the Primates the Marmoset has the tiniest brain, and this brain is quite smooth. Broadly speaking the rodents form no exception to this generalization ; its truth becomes more apparent when the comparisons of extent of complication of brain-surface are restricted to a family rather than when applied to the whole group. Otherwise the brain of the Beaver forms a marked exception; it is as large as any rodent brain excepting Hydrochcerus and is nearly perfectly smooth, while considerably smaller brains, such as Layo-stomus, are decidedly convoluted. The above-given account of the various brains which I have been 1 7, P- 174. PROC. ZOOL. Soc-1892, No. XLI. 41 |
