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