more or less simple, yet elegant, curved lines, proceeding backwards from the outer side of the corresponding cerebral peduncle, an evidently homologous fissure, present in many, otherwise most varying, brains. This fissure is the fissure of the hippocampus. Its extension 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, in the form of two deep hidden sulci, 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 a rudimentary fissure occupying 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 hemispheres, 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 surface-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 recognisable 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 difference is not nearly so marked as in the ape. The average thickness of the grey matter is about 330ths of an inch, in the Chimpanzee, as compared with 430ths, 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 4⋅5 thirtieths of an inch; in man the corresponding quantities are 93, 16, 6 and 13. The sectional
