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Gratiolet originated the statement that there is a fundamental difference in the development of the brains of apes and that of man consisting in this; that, in the apes, the sulci which first make their appearance are situated on the posterior region of the cerebral hemispheres, while, in the human foetus, the sulci first become visible on the frontal lobes.
In the lower and smaller forms of placental Mammals the surface of the cerebral hemispheres is either smooth or evenly rounded, or exhibits a very few grooves, which are technically termed 'sulci,'separating ridges or 'convolutions' of the substance of the brain; and the smaller species of all orders tend to a similar smoothness of brain.
In no case, however, are either the frontal or the temporal sulci the earliest. As a rule the occipito- parietal is the earlier of the two. At the latter part of this period, another sulcus, the "posterio- parietal," or "Fissure of Rolando" is developed, and it is followed, in the course of the sixth month, by the other principal sulci of the frontal, parietal, temporal and occipital lobes.
In like manner, the brain of a human foetus, at the fifth month, may correctly be said to be, not only the brain of an ape, but that of an Arctopithecine or marmoset-like ape; for its hemispheres, with their great posterior lobster, and with no sulci but the sylvian and the calcarine, present the characteristics found only in the group of the Arctopithecine Primates.
As to the convolutions, the brains of the apes exhibit every stage of progress, from the almost smooth brain of the Marmoset, to the Orang and the Chimpanzee, which fall but little below Man. And it is most remarkable that, as soon as all the principal sulci appear, the pattern according to which they are arranged is identical with that of the corresponding sulci of man.
Ecker, figure of the human embryo; on the development of the gyri and sulci of the brain; on the sexual differences in the pelvis in man; on the presence of a sagittal crest in Australians. Edentata, former wide range of, in America; absence of secondary sexual characters in. Edolius, racket-shaped feathers in. Edwards, Mr., on the proportion of the sexes in North American species of Papilio.
There remains, then, no dispute as to the resemblance in fundamental characters, between the ape's brain and man's: nor any as to the wonderfully close similarity between the chimpanzee, orang and man, in even the details of the arrangement of the gyri and sulci of the cerebral hemispheres.
It is admitted that the man's cerebral hemispheres are absolutely and relatively larger than those of the orang and chimpanzee; that his frontal lobes are less excavated by the upward protrusion of the roof of the orbits; that his gyri and sulci are, as a rule, less symmetrically disposed, and present a greater number of secondary plications.
The swollen limb presents immense lobulated masses, heaped up at different parts, separated from one another by deep sulci, which are especially marked at the flexures of the joints.
The brain weighed 1390 grams and showed little or no gross lesion, if we except a pigmentation of the right prefrontal region under an area of old pias hemorrhage. There was also a chronic leptomeningitis, with numerous streaks and flecks along the sulci, especially in the frontal region.
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