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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    pendicular,' running vertically on the inner aspect of each hemisphere, where the hemisphere is in contact with its fellow, and separating the parietal and occipital lobes. It may be stated here that, corresponding with this last, an external fissure is in some cases seen, but it6 presence would appear to be indicative of comparative lowness of cerebral organisation. Thus the Mangabey, one of the Catarrhini, has an external perpendicular fissure. It is well marked again in the Gibbon, in the Chimpanzee and in the Gorilla. In the Orang, how- ever, it is shorter and less obvious, and in Man it is but very poorly represented. Even on a single and not very impor- tant point like this, the reader will notice how the grada- tions go. But besides these fissures that we have seen to be present in Primates lower than the man-like apes, two new fissures appear. These are the calloso-marginal and the hippocampal. Both of them are only to be seen on the inner face of the hemi- sphere. The calloso-marginal is a fissure or furrow that lies just above the thick transverse band of nerve-tissue that joins the two hemispheres near their bases, and is known as the corpus callosum (hard body). Its position just above this body, and just on the margin of the hemisphere, accounts for its name. The fissure of the hippocampus is hard, by that nervous mass, the hippocampus minor, that lies in the pos- terior extension of the brain ventricle into the occipital lobe. It lies behind the middle of the inner face of the hemisphere, and is just by the junction of that inner face with the under surface. Both of these new fissures, then, are present in Man. But both of them appear first in his allies. The Orang, Chimpanzee and Gorilla have all of them a calloso-marginal and a hippocampal fissure on each side. The fissure of Sylvius and that of Rolando remain for con- sideration. As to the former, the most noticeable thing in the ascending series is the gradual movement of it towards the horizontal plane. As the Sylvian fissure lies between the parietal and temporal lobes, it follows that the more vertical is ite direction the smaller relatively is the anterior part of the brain. But as the line of the fissure passes from the nearly vertical position, parallel to that of Rolando, that w© see in the lower Primates, towards the almost horizontal posi- tion it has taken in the human brain, the frontal and parietal
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