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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 301

hemispheres yielded an average fronto-prsecentral index of 37*9 ; whilst
nineteen female hemispheres afforded an average index of 37*8.

The relative position of the sulcus with reference to the cranial wall is
not so fixed. In those instances in which the sulcus appears very early (in
the fifth month) it is placed in front of the coronal suture, as we have
already mentioned (PL ii., fig. 9; also PI. i., fig. 14). In other cases
the vertical stem may appear immediately subjacent to the coronal line
(PI. ii., fig. 15; PI. i., fig. 24; and PL v., fig. 2), or immediately in front
of it (PL ii., figs. 18 and 20). Between the eighth and ninth months of
intrauterine life it is usually found just behind the sutural line or perhaps
coinciding with it.

After birth the forward movement of the sutural line becomes more
marked as may be seen in the following Table :—•

CoRONO-PRiECENTRAL INDICES.

Lateral Length of the Cerebrum = 100.

Period of Growth.

Number of
Hemispheres
Examined.

Average
Corono-prae-
central Indices.

New-born foetuses,

5

2-0

First year of extrauterine life,

4

3-7

4 to 5 years, .....

3

4-3

11 to 12£ years, ....

2

4-6

Adult,.....

8

6-4

N.B.—The lateral length of the hemisphere was obtained in this case by one
measurement along the lateral surface of the cerebrum from frontal point to occipital
point. The corono-prascentral distance was measured from the sutural line to the
mid-point of the vertical stem of the fissure.

The steady advance forwards of the suture in relation to the sulcus can
be appreciated by an examination of the above indices. It is largely due
to the forward growth of the squamous overlapping process of the lower
part of the anterior border of the parietal bone (figs. 28 to 31, pp. 119-122).