Page:Natural History Review (1861).djvu/319

Rh margins of those bones, we find that the proportionate dimensions in the Chimpanzee would be 46, 28, 26, instead of 54, 23, 23, out of 100, as in man.

Turning next to the outer side of the cerebral hemisphere, fig. 4, the so-called parallel fissure, situated parallel with and behind, the Sylvian fissure, is rather more complicated in our specimen than in M. Gratiolet's figure. On the inner surface of the hemisphere, besides the internal perpendicular fissure, there is seen a longitudinal fissure, surmounting the convolution of the corpus callosum. And lastly, on the under surface, rather than on the internal surface, of the hinder part of the hemisphere, is seen, very well marked, the fissure of the hippocampus, commencing, as described by Gratiolet, along the outer or lower border of the fimbriated convolution, and passing backwards in a curved direction, towards the hinder extremity of the hemisphere. The corresponding fissures plainly exist in the human brain dissected by us, pari passu with that of the Chimpanzee.

Now, whatever grounds of definition as to the leading sub-divisions of the cerebral hemispheres may be adopted, it is at once apparent that all those sub-divisions of the human cerebrum, called lobes, are present in the Chimpanzee. In the phraseology of the older anatomists, the anterior and middle lobes are well distinguished by the fissure of Sylvius, which, however, is comparatively not quite so deep as in man. At the bottom of this fissure, is plainly seen the insula, or island of Reil. Looking at the Chimpanzee's brain, it is quite indifferent whether we choose the usual arbitrary definition of the limits between the middle and posterior lobe, viz., a line drawn in front of the cerebellum, or whether we select the one more recently laid down, according to which the posterior lobe signifies that part "which covers the posterior third of the cerebellum and extends beyond it"; for, in either sense, the posterior lobe exists in our Chimpanzee's brain, inasmuch as the cerebrum projects half an inch beyond the cerebellum in its natural and undisturbed position, whilst the human cerebrum, under the same conditions, projects only a tenth of an inch more.

If, however, we reject these arbitrary modes of distinguishing the various lobes, and follow a more philosophical method, for example, the one suggested by Gratiolet, a corresponding conclusion is forced upon us, viz., that all the great masses in the human brain have their anatomical representatives, or homologues, in the Chimpanzee. The frontal lobe (figs. 4 and 5) F, together with the parietal lobe P, marked off by the first ascending convolution 4,4′, which is included in the latter, lie above the Sylvian fissure, and in front of the vertical or perpendicular fissure; the temporo-sphenoidal lobe, T, lies below the Sylvian fissure; the central lobe is the island of Reil; and the occipital lobe, O, is the part behind the external vertical fissure. Though this latter fissure is broken across by convolutions, its place can