They
are among the most distinctly Simian peculiarities which the human
organism exhibits.
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. The surface of the brain of a monkey
exhibits a sort of skeleton map of man's, and in the man-like apes the
details become more and more filled in, until it is only in minor
characters, such as the greater excavation of the anterior lobes, the
constant presence of fissures usually absent in man, and the different
disposition and proportions of some convolutions, that the Chimpanzee's
or the Orang's brain can be structurally distinguished from Man's.
So far as cerebral structure goes, therefore, it is clear that Man
differs less from the Chimpanzee or the Orang, than these do even from
the Monkeys, and that the difference between the brains of the
Chimpanzee and of Man is almost insignificant, when compared with that
between the Chimpanzee brain and that of a Lemur.
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