On the Brain | Page 4

from the point of view of organisation alone, no one probably
would ever have disputed the view of Linnaeus, that man should be
placed, merely as a peculiar species, at the head of the mammalia and
of those apes. Both shew, in all their organs, so close an affinity, that
the most exact anatomical investigation is needed in order to
demonstrate those differences which really exist. So it is with the brains.
The brains of man, the orang, the chimpanzee, the gorilla, in spite of all
the important differences which they present, come very close to one
another" (loc. cit. p. 101).

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. Nor, turning to the differences between the
brains of the highest apes and that of man, is there any serious question
as to the nature and extent of these differences. 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. And it is admitted that, as a
rule, in man, the temporo-occipital or "external perpendicular" fissure,
which is usually so strongly marked a feature of the ape's brain is but
faintly marked. But it is also clear, that none of these differences
constitutes a sharp demarcation between the man's and the ape's brain.
In respect to the external perpendicular fissure of Gratiolet, in the
human brain for instance, Professor Turner remarks: (71. 'Convolutions
of the Human Cerebrum Topographically Considered,' 1866, p. 12.)
"In some brains it appears simply as an indentation of the margin of 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 descriptions, is also well illustrated in these specimens" (pp. 8, 9).
Even were the presence of the temporo-occipital, or external
perpendicular, sulcus, a mark of distinction between the higher apes
and man, the value of such a distinctive character would be rendered
very doubtful by the structure of the brain in the Platyrrhine apes. In
fact, while the temporo-occipital is