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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 57

the index being no less than 28*5. In the course of a month the proportion
is reduced, to 24*7; and the cause of this change is the rapid
increase of the mesial part of the frontal lobe ; the occipital lobe remains
stationary in so far as its relative length is concerned. In the succeeding
month, by an increase in the occipital lobe, the parietal mesial length is still
further reduced, and now the index reaches its lowest point. In the newborn
child we find that its normal proportion is attained, and this it maintains
up to adult life. The fall in the index at the fourth and fifth year,
and the sudden rise from the eleventh to the fifteenth year, are somewhat
perplexing. The number of brains examined at the latter stage is hardly
sufficient to enable us to draw decisive conclusions from the curious rise in
the parietal index.

Let us now compare with the indices which are obtained for the human
brain, those which are characteristic of the different forms of ape.

The Ape Brain.

Occipital and Parietal Indices—Mesial length of the Cerebral Hemisphere along its

upper border = 100.

Number of
Hemispheres
examined.

Variety of Ape.

Occipital
Index.

Parietal
Index.

4

Orang, .....

23-2

21-3

4

Chimpanzee, ....

24-2

199

2

Hauiadryas, ....

29-5

20-5

5

Cynocephalus,

29-7

22-6

8

Mangaby, ....

30-5

24-1

5

Macaque, . ,

31

19

8

Cercopithecus,

32-9

19

7

Cebus, .....

Q l> . 1

66 L

20-6

The low parietal index, and the high occipital index, are the leading
peculiarities of the brain of the ape when examined from this point of view.

ROYAL IRISH ACAD EST Y.—CUNNINGHAM MEMOIRS, NO. VH. [8]