Darwin made easy / by Edward B. Aveling.

  • Edward Aveling
Date:
1893
Catalogue details

Licence: Public Domain Mark

Credit: Darwin made easy / by Edward B. Aveling. Source: Wellcome Collection.

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    lobes, in which are probably resident the higher mental funo- tions, increase in relative size. When we examine this fissure in the anthropoid brains, we find it least horizontal in the Gibbon, then in the Orang, then, in the Chimpanzee and Gorilla. In these last its direction is but very slightly different from the direction of the fissure in Man. As to the fissure of Rolando, the most important point there is its position rather than its direction. The higher the animal the farther back is this brain-cleft; the larger is the proportion of brain-substance before it as compared with that posterior to it; the larger, in a word, is the frontal lobe as compared with the rest of the brain. Now, in the Chimpanzee and in the Gorilla, this fissure lies well in front of the middle of the brain. Not more than ■§■ of the brain-substance lies in front of it. In Man, on the other hand, the fissure of Rolando lies either at about the middle of the encephalon or behind the middle. Not less than | of the brain-substance lies in front of it. But in the brain of the Orang the position of the fissure of Rolando is, by measurement, almost exactly mid- way between that held by it in the brain of the Gorilla and in Man. (d) Convolutions.—A word or two as to the folds in the brain of the Gibbon alone first. In this lowest of the Anthro- pomorpha the occipital lobe is nearly destitute of convolutions, and the ascending frontal and parietal folds are quite rudi- mentary. It will be remembered that these lie respectively before and behind the fissure of Rolando. And here it should be stated that these two convolutions are quite well marked in some monkeys below the Gibbon. Thus the Mangabeyr already mentioned, has them both very distinctly shown. In the Gibbon appear the first traces of the annectent or bridging- over convolutions (p. 37). It is upon these and the supra-marginal lobule that our last words may be said. And first, as to th® annectent. In Man there are generally two of these on each side. They run across the perpendicular fissure, and therefore connect th© occipital and parietal lobes of each side. One of them lies lower in a vertical line than the other. In the Chimpanzee, the first, or upper of the two annec- tent convolutions of Man is wanting, and the second, or lower
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