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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 129

be borne in mind that the Sylvian Point in the female is placed relatively
further back than in the male.

For purposes of comparison I have also measured the Sylvian fissure in
a number of ape brains. In the lower apes the measurements were made
from the " Sylvian Point" to the extremity of the fissure where this ends
on the surface, or up to the submerged supramarginal convolution in cases
where the Sylvian fissure enters into superficial connexion with the parallel
sulcus. The lateral length of the hemisphere was reckoned as 100, and the
length-index of the fissure calculated accordingly :—

Length Index of the Fissure of Sylvius in the Apes.
Lateral Length of Hemisphere = 100.

Ape.

Number of Hemispheres
Examined.

General Index
for all.

Index for
Right Side.

Index for
Left Side.

Chimpanzee,

4 (2 right and 2 left), .

30-2

28-8

31-6

Orang,

2 (1 right and 1 left), .

35

35-7

344

Mangaby,

6 (3 right and 3 left), .

35-4

35-5

35-4

Cynocephalus, .

7 (4 right and 3 left), .

38-1

37-5

38-9

Macaque,

5 (3 right and 2 left), .

38'7

37-2

40-4

Haniadryas,

2 (1 right and 1 left), .

39-2





Cebus,

6 (3 right and 3 left), .

37-7

38-7

36-8

The greater relative length of the fissure in the ape is made very
evident by this Table, and it is interesting to observe, although the number
of hemispheres examined is too small to decide the point, that in the
greater number of the varieties examined the left fissure is relatively
longer than the right.

The greater relative length of the fissure in the ape is partly due to the
fact that in man we have not included in the measurements the ramus
posterior ascendens. But this is not the sole cause. The decrease in the
length of the Sylvian fissure in man is largely brought about by an increase
in the size of the arching convolutions which connect the supramarginal

EOYAX IRISH ACADEMY.—CUNNINGHAM MEMOIRS, NO. VII. [17]