Freiburger historische Bestände - digital

Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 299

tively very much shorter. The length index of the insula in each case is:
chimpanzee 18*2 ; orang 21 -5 ; and man 29'6. If we regard the insula of
the anthropoid apes as extending forwards to the sulcus fronto-orbitalis,
and measure accordingly, we find that the discrepancy is not nearly so
great. In the chimpanzee the length-index of the insula becomes 28;
in the orang it becomes 31.

Of course this manner of regarding the homologies of the parts in the
lower portion of the frontal lobe is in complete variance with the ideas
almost universally held in regard to the inferior frontal convolution in the
apes, but it renders the difference between this part of the simian brain
and the corresponding part of the human brain still more striking.

If we remove in the adult human brain the opercular part of the inferior
frontal convolution we find little, if any, of the gyrus left behind. In other
words, the inferior frontal furrow on the surface very nearly corresponds in
its position to the deep submerged upper limiting sulcus of the island of Reil.
In some cases, however, the inferior frontal furrow lies at a slightly higher
level, and in these cases after removal of its opercular part a narrow strip of
the inferior frontal gyrus still remains. This non-opercular part of the
gyrus corresponds to the greater part of the inferior frontal convolution in
the apes. The basal part of the inferior frontal convolution in the anthropoid
ape is truly opercular in its nature. It is composed of that part of the
fronto-parietal operculum which lies in front of the inferior prsecentral
sulcus, and corresponds to the similarly situated basal part of the gyrus in
the human brain. In short it is the upper part of the gyrus as described
by Bischoff, &c, after we abstract from it the lower portion which as we
have seen belongs to the insula.

But the non-opercular part of the inferior frontal gyrus in the human
brain is much more extensive in the foetus than in the adult. The inferior
frontal sulcus when it is first developed lies considerably above the level of
the upper limiting furrow of the island of Reil. In subsequent growth the
descent of the opercula appears to be accompanied by a downward movement
of the inferior frontal sulcus, so that when the fully-developed condition
is reached it is placed in near relation to the upper border of the
insula, We have already referred to an apparent downward movement on

[38*]