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Dk. Cunningham—Surface Anatomy of the Primate Cerebrum. 107

VI. Antero-posterior length of the Island of Reil.—We have seen
that, from its first appearance to the end of foetal life, there is a steady
and rapid increase of the area of the Sylvian fossa, and further, that this
growth is relatively much greater than that of the cerebral hemisphere, as
a whole, during the same period. It is now necessary to establish the
ratio which the same area (viz. the insula) presents to the cerebral hemisphere
from the time of birth up to adult life. If we again assume the
lateral length of the hemisphere to be equal to 100, the following Table will
show the percentage of this which is formed by the insula, and also of the
portion of cerebrum in front of it, and the portion of cerebrum behind it,
at different periods of extra-uterine life.

It must be clearly understood, however, that by the u lateral length " of
the hemisphere in this instance we mean the sum of the following measurements
: (a) from anterior end of cerebrum to the anterior end of the
insula ; (b) the length of the insula ; and (c) the distance between the
posterior end of the insula and the occipital pole :—

Relative Length of the Island of Reil.
Lateral length of the Cerebral Hemisphere = 100.

Period of Development.

Number of
Hemispheres
Examined.

Fronto-Central
Index.

Central
or

Insular Index.

Occipito-
Central Index.

7£ to 8 months, ....

12

26-5

28-7

44-8

Full-time foetuses,

11

24-3

32

43-6

12 mouths, ....

8

25-3

30

44-7

4 to 5 years, ,

7

27-1

28-8

44-1

11 to 15, ....

6

26-8

29-1

44-1

Irish adults, ....

28

26-3

29-6

44-1

Adult Negroes, ....

5

28-7

28-3

43

From this Table it will be seen that the posterior end of the insula
maintains after birth a wonderfully constant position with reference to
the posterior end of the cerebrum. Slight variations, it is true, are

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