the hemisphere, but, in others, it extends for some distance more or
less transversely outwards. I saw it in the right hemisphere of a
female brain pass more than two inches outwards; and on another
specimen, also the right hemisphere, it proceeded for four-tenths of an
inch outwards, and then extended downwards, as far as the lower margin
of the outer surface of the hemisphere. The imperfect definition of
this fissure in the majority of human brains, as compared with its
remarkable distinctness in the brain of most Quadrumana, is owing to
the presence, in the former, of certain superficial, well marked,
secondary convolutions which bridge it over and connect the parietal
with the occipital lobe. The closer the first of these bridging gyri
lies to the longitudinal fissure, the shorter is the external
parieto-occipital fissure (loc. cit. p. 12).
The obliteration of the external perpendicular fissure of Gratiolet,
therefore, is not a constant character of the human brain. On the other
hand, its full development is not a constant character of the higher
ape s brain. For, in the chimpanzee, the more or less extensive
obliteration of the external perpendicular sulcus by bridging
convolutions, on one side or the other, has been noted over and over
again by Prof. Rolleston, Mr. Marshall, M. Broca and Professor Turner.
At the conclusion of a special paper on this subject the latter writes:
(72. Notes more especially on the bridging convolutions in the Brain of
the Chimpanzee, Proceedings of the Royal Society of Edinburgh,
1865-6.)
The three specimens of the brain of a chimpanzee, just described,
prove, that the generalisation which Gratiolet has attempted to draw of
the complete absence of the first connecting convolution and the
concealment of the second, as essentially characteristic features in
the brain of this animal, is by no means universally applicable. In
only one specimen did the brain, in these particulars, follow the law
which Gratiolet has expressed. As regards the presence of the superior
bridging convolution, I am inclined to think that it has existed in one
hemisphere, at least, in a majority of the brains of this animal which
have, up to this time, been figured or described. The superficial
position of the second bridging convolution is evidently less frequent,
and has as yet, I believe, only been seen in the brain (A) recorded in
this communication. The asymmetrical arrangement in the convolutions of
the two hemispheres, which previous observers have referred to in their
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