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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 81

development. Let us take as a standard the lateral length of the
hemisphere, and equal to 100, and compare with this the length of the
Sylvian fossa measured along its longest axis. The following Table gives
the results of such an inquiry:—

Kislative Length of the Sylvian Fossa {i.e. Insula or Central Lobe).

Lateral Length of the Cerebrum = 100.

Period of Development.

Number of
Hemispheres
Measured.

Fronto-central
Index.

Central Index.

Occipito-
central Index.

3J months, ....

2



19-4



4£ to 5 months, ....

5

26-7

23-4

49-9

5 to 5£ months, ....

10

25-8

24-2

50

5£ to 6 months, ....

11

25-7

25-8

48-5

6 to 6£ months, ....

9

26-3

26-2

47-5

6£ to 7% months,

3

26-2

28

45-8

7^ to 8£ months,

12

26-5

28-7

44-8

JO.—In this Table the central index gives the relative length of the Sylvian fossa
as compared with the lateral length of the hemisphere, which is taken as 100. The
fronto-central index gives the relative length of the portion of hemisphere which lies in
front, and the occipito-central index the relative length of the portion of hemisphere which
lies "behindthe Sylvian fossa. It must he clearly understood, however, that the "lateral
length " of the hemisphere referred to in this case is not the distance between the fore and
hind points of the cerebrum measured in a straight line along the lateral surface of the
cerebrum, but is the sum of three measurements—(1) the distance between the fore-point
of the hemisphere and the foremost point of the fossa Sylvii; (2) the length of the
fossa Sylvii measured between its two most distant points ; and (3) the distance of the
hindmost point of the fossa Sylvii from the occipital pole.

This Table brings out very clearly the steady growth of the Sylvian
region from its first formation up to the end of intra-uterine life. In the
third month it forms only 19'4 per cent, of the lateral length of the hemisphere
; in the eighth month it is found to form 28*7 per cent. Its growth,

ROYAL IRISH ACADEMY.-CUNNINGHAM MEMOIRS, NO. VII. [11]