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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 117

measure at least responsible for the difference in the shape of the human
and the anthropoid frontal lobes.

In the lower apes the long axis of the insula in comparison with the
lateral length of the cerebrum is relatively greater than in the anthropoid
apes. This does not necessarily mean that there is a more extensive insular
area in the lower apes, seeing that the measurements were only taken with
reference to the length of the island. As is well known, the island of Reil
in the lower apes is very narrow. The following Table gives the results of
these measurements expressed in indices which correspond with those in
the preceding Table :—

Relative Length of the Long Axis of the Insula in the Lower Apes.

Lateral Length of the Hemisphere = 100.

Ape.

Number of
Hemispheres
Examined.

Fronto-Central
Index.

Central Index.

Occipito-
Central Index.

Cebus, . . . .

6

31-5

26-7

41-8

Cercopithecus, ....

2

31-4

24-9

43-8

Macaque, .....

6

29-8

24-9

45-3

Mangaby,.....

6

31-3

25-3

43-4

Cynocephalus, ....

8

30

27-9

42-1

Hamadiyas, ....

2

32-8

23-9

43-3

If we compare these indices with those obtained for the anthropoid
apes, it will be seen that the greater length of the insula in the low apes
is chiefly due to a closer approach of its posterior end to the occipital
extremity of the cerebrum. This is particularly noticeable in the cebus
and the baboon, in which the posterior end of the insula is placed relatively
nearer the occipital pole than in man.

VIII. Relation of the Island of Reil to the Cranial Wall.—We have
studied the position of the insula with reference to the anterior and