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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 135

From this Table we see that eight heads, with an average breadth-index
of 75, give an average Sylvian angle of 66°\5 ; whilst an equal number of
heads, with an average breadth-index of 79, furnish us with an average
Sylvian angle of 67°. The difference in the angle in the two groups is so
slight that we need not take it into account. The same will be seen if we
study the individual cases. In fact, no law can be formulated from the
results. The Sylvian fissure, therefore, must be placed on a different platform
from the other fissures of the cerebrum. It is altogether different in
its mode of formation, and, unlike them, it does not seem to be affected in
its inclination by the shape assumed by the cranium. The influence chiefly
concerned in determining its direction is probably the growth energy of
the two opercula between which it intervenes.

The following Table gives the Sylvian angle as it has been observed in
the apes:—

Sylvian Angle in the Apes.

Ape.

Number of
Hemispheres
Examined.

Average
Angle.

Eight.

Left.

Chimpanzee, ....

4

54°-5

52°-5

56°-5

Orang,.....

2

55°

54°

56°

Cebus, .....

6

54°-l

53°-3

55°-

Mangaby, .....

4

49°-2

48°

50°-5

Baboon, .....

7

48°-6

48°

49°-3

Macaque, .....

5

46°'5

47°-3

45°

Haniadryas, ....

2

44°

41°

47°

In the ape the angle is very much more acute than in man, and a considerable
amount of variation exists between the different forms. It is
interesting to note that with the exception of one form (macaque) all the
apes show a wider Sylvian angle on the left side than on the right side.
We have observed the same difference in man. I am quite unable to
explain this curious difference. Can it be due to a greater development on