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 mm. 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 rhinence- 

 phalon 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 back- 

 wards 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. On each 

 hemisphere is a short fissure about 5 mm. long commencing a little 

 way in front of the posterior margin of the brain and situated 3 mm. 

 from the median interhemispheral fissure ; in addition to this 

 another longitudinal fissure is present on each side 5 mm. away 

 from the last, longer and commencing at the posterior margin. 

 This fissure 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 Cervidse Sir W. H. Flower has pointed out ^ " how closely the 

 amount of convolution bears relation to tlie 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 Hydrochmrus and is nearly 

 perfectly smooth, while considerably smaller brains, such as Lago- 

 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 



