MARSHALL ON THE BRAIN OF A YOUNG CHIMPANZEE. 811 



more or less simple, yet elegant, curved lines, proceeding backwards 

 from the outer side of the corresponding cerebral peduncle, an evi- 

 dently homologous fissure, present in many, otherwise most varying, 

 brains. This fissure is the fissure of the hijipocampus. Its exten- 

 sion backwards to the tip of the occipital lobe is seen in all ; and it 

 serves at once to identify parts which, on the upper surface of the 

 hemisphere, cannot so easily be compared. It is at the bottom of 

 the middle half of this fissure, that the cerebral substance is tucked 

 %, in the form of two deep hidden sidci, to constitute the hippocampus 

 minor and eminentia collateralis, in the posterior cornu of the lateral 

 ventricle, where that prolongation of the great internal cavity of the 

 brain exists. But supposing that prolongation did not exist in any 

 particular brain, still the presence of even ^rudimentary fiysure occupy- 

 ing the above-described characteristic position, would suffice to 

 justify the conclusion that the surrounding parts of the cerebrum 

 were homologous parts. Now, a careful comparison of these parts in 

 the human brain, in the brain of our Chimpanzee, and in the brain 

 of a common Green Monkey, has satisfied me that the fissure of the 

 hippocampus and its two deep hidden sulci, are present in all three. 



Internal structure of the Brain. The cerebral convolutions of the 

 Chimpanzee's brain are very large on the outer surface of the hemi- 

 spheres, where indeed, as is seen in fig. 5, the sulci are, proportionately, 

 quite as deep as in the human brain. On the frontal lobe, they are also 

 bold ; but in the occipital lobe the convolutions are smaller, and the 

 sulci for the most part shallower, though both are still very numerous, 

 so that the smoothness of this part of the brain is not owing to an 

 absence of convolutions, but to their diminutive size and depth. The 

 superior occipital convolution is, however, almost devoid of any sur- 

 face-markings. This part of the brain is smoother than in the Orang. 

 It certainly would seem as if it were behind the rest in development, at 

 least in the young Chimpanzee. We may remark, as suggestive of a 

 similar idea, that these posterior convolutions were found to be 

 more tender than those of the parietal or frontal regions ; and, as is re- 

 cognisable in fig. 5, that the grey cortical layer is thinner in them than 

 elsewhere. In the human brain, also, the occipital convolutions are 

 not so bold as those on the sides and fore part of the hemispheres ; 

 but the difierence is not nearly so marked as in the ape. The aver- 

 age thickness of the grey matter is about -^-ths of an inch, hi the 

 Chimpanzee, as compared with -^ths, in man. In proportion to the 

 size of the brain, it is curious that the quantity of white matter in 

 the centre of the hemispheres seems smaller than in man. 



Of the various commissures of the cerebrum in the Chimpanzee, 

 we will speak first of the corpus callosum. This is both shorter and 

 thinner in proportion than in man, as the following measurements, in 

 30ths of an inch, taken in each case from the hardened brain, will show. 

 In the ape, the length, the greatest thickness, the least thickness, 

 and the average thickness of the corpus callosum divided along the 

 middle line, are respectively 51, 6, 2 and 45 thirtieths of an inch ; in 

 man the corresponding quantities are 93, 16, 6 and 13. The sectional 



vol. i. — n. h. r. 2 s 



