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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 145

There is, apparently, no difference in this growth-change between the
male and the female, and, therefore, I have not introduced the indices which
I have obtained for the two sexes into this Table. If we take the adults
as an example, the average male indices were 69 and 31, whilst in the
female they were 69T and 309; and a similar correspondence was
found for each period of growth.

The figures in the above Table render it evident that the Sylvian fissure
from the last month of fostal life, and throughout the early years of childhood
, steadily descends until the parietal depth has gained 8 per cent, of
the entire breadth of the outer surface of the hemisphere, whilst the
temporal depth is diminished to a corresponding extent.

Throughout the whole of this growth-change the left hemisphere lags
slightly behind the right hemisphere. This is exactly the reverse of what
might be expected, seeing that on the left side the Sylvian angle is more
open than on the right side. It is difficult to explain this apparent discrepancy
; but it must be borne in mind that the measurements were made
along only one line, and that the Sylvian angle is greatly influenced by the
inclination of the terminal part of the fissure.

That this relation of the fissure of Sylvius to the two borders of the
hemisphere exercises a great influence in modifying its position with
reference to the squamous suture, is made very manifest by calculating the
parietal and temporal depth indices in special cases, and comparing the
results with the Sylvio-squamous indices. Thus, in the five adults in
which the fissure was immediately subjacent to the fore-part of the squamous
suture the average parietal depth index was 68-9 ; in the six adults in which
the fissure was placed above the suture the average parietal depth index
was 66-6 ; whilst in the six cases in which the fissure was below the
anterior part of the suture the average parietal depth index was 69'6.
The same is to be noted when we study exceptional cases. I have already
referred to the very low position of the fissure with reference to the suture
in the head represented in PI. vn., fig. 15. In this the parietal depth index
was 71-3. Again, in the head of a girl of four years old, where the fissure
lias already very nearly gained the adult position (PI. vi., fig. 6), the parietal
depth index is 67*3, whereas the average for this period of life is 65-7.

KOYAL IRISH ACAD KM Y.-CUNNINGHAM MKMOIR8, NO. TH. j~ 19"]