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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 55

so that we can the more readily study the relative proportions of the different
sections of the mesial border in brains of different sizes and at different stages
of growth. This is readily done by supposing in every case the " mesial
length " of the hemisphere to be equal to 100.

I have determined the point at which the parieto-occipital fissure reaches
the upper border of the mantle—(1) in its relation to the occipital pole, and
(2) in its relation to the upper extremity of the fissure of Rolando. By this
means we obtain an occipital index which expresses the relative distance,
along the upper border of the hemisphere, of the fissure from the occipital
pole or, in other words, the mesial length of the occipital lobe; and a
parietal index which expresses the distance at which, on the upper cerebral
margin, the parieto-occipital fissure is placed behind the fissure of Rolando
or, in other words, the mesial length of the parietal lobe. Being provided
with these two indices a frontal index can be easily calculated ; but the consideration
of this we shall defer until we come to study the topography of
the fissure of Rolando.

In the following table are placed the occipital and parietal indices which
I have obtained for the human brain at its different periods of growth:—

The Human Bratn.

Occipital and Parietal Indices—Mesial length of the Cerebrum along its Upper Border = 100.

Number of Hemispheres Examined.

Stage of Growth.

Occipital
Index.

Parietal
Index.

CD

a

'12 ....

5£ to 6^- months,* .

18-8

28-5

• r-t
f-l

CD

+j

12 ....
20 ....

6£ to 7£ months, .
7% to 8j months, .

18-6
20-7

24-7
24-1

M

24 ....

Full-time foetuses,

20-8

25-7

CD
Pi

'12 ....

Eirst 12 months, .

22-3

25'6

tra-uteri
life.

14 ....
6 ....

4 to 5 years,
11 to 15 years,

23'2
20-8

24-2
27-4

Ex

82 ....

Adults,

21-2

25-5

* Of course, iu these hemispheres the parieto-occipital fissure had not reached the mesial
border, hut by a line drawn upwards in the direction of the fissure the proper point on the
margin of the hemisphere was obtained.