area of the longitudinally divided corpus callosum in the Chimpanzee, is therefore 230900ths of a square inch; whilst in man it is 1209900ths of a square inch. Comparing these numbers with the area of the internal surface of one of the cerebral hemispheres, in the Chimpanzee's and in the human brain, we find them to be as 1 to 28⋅5 in the ape, and 1 to 12⋅5 in man; so that the corpus callosum is more than twice as large, proportionally to the size of the brain, in man, as it is in the Chimpanzee. We may add, that the corpus callosum in our specimen is exactly of the same length as in Schroeder van der Kolk's figure, whilst the brain itself is a little longer. As in man, the corpus callosum of the ape, is thickest behind. The section of the anterior commissure is not so round as in the human brain, but it is proportionally as large. The posterior commissure also exists, but it is small. The so-called soft commissure is large. On the whole then, the system of transverse commissural fibres is defective in the Chimpanzee, as compared with man; and as the section of the medulla oblongata, in the former, is even larger in proportion to the cerebrum, than in the latter, it would seem as if the relative deficiency of white substance within the hemispheres, already noticed, is, to a great degree, owing to the fewness of these, as well as other, commissural fibres. Of the longitudinal system of commissures, the fornix is thin; the tænia semicircularis is only just recognisable; and the striæ longitudinales are slender.
Of the middle and fifth ventricles, nothing is to be remarked. The fourth is very wide, corresponding in this respect with the cerebellum. The lateral ventricle, examined on the right hemisphere, proved to be a very large cavity. It consisted, as in man, of a body (fig. 5), ** and three cornua; an anterior cornu *, a descending cornu (of which only the commencement is seen); and a very obvious, posterior cornu. * * * The body measured 12/10ths of an inch long, the anterior cornu 6/10ths, the posterior cornu nearly 5/10ths, and the descending cornu 20/10ths; whereas in the human brain, these parts measured respectively, 21/10ths, 14/10ths, 12/10ths and 26/10ths of an inch. Comparing these dimensions with the lengths of the two brains, (44/10ths, and 65/10ths of an inch) we get as ratios for the Chimpanzee, ⋅207, ⋅103, ⋅18 and ⋅45 to 1, and for man, ⋅32, ⋅21, ⋅184 and ⋅4 to 1. From this we perceive that the lateral ventricle was proportionally longer in the human brain, except as regards the descending cornu; and that the posterior cornu was only fractionally longer. It is worthy of note, as may be seen by comparing the dissected with the undissected side of fig. 5, that, in the ape, the body of the lateral ventricle corresponds almost exactly with the parietal lobe of the hemisphere, P, whilst the anterior cornu projects into the frontal lobe, F, and the posterior runs, beyond the vertical fissure, into the occipital lobe, O: the descending cornu of course occupies the temporo-sphenoidal lobe, Fig. 4, T. In the human brain, the same relations are observed, together with a coincidence in the measurements of the parts. In our Chimpanzee's brain, the posterior cornu begins at a line, midway
