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300

Cunningham Memoirs.

the surface of the cerebrum of the early sulcus prsecentralis inferior. These
growth changes are exceedingly difficult to follow, and they tend to obscure
the homologies which exist between the parts in this region of the simian
and human brains.

XII. Topography of the Inferior Prsecentral Sulcus.—The sulcus
prsecentralis inferior when it is first formed lies relatively further back on
the cerebral surface than in the adult. About the seventh month, however,
it attains a position which it maintains unaltered in all the subsequent
stages of growth. To appreciate these relations it is necessary to construct
an index which may be termed the fronto-prsecentral index. The cerebrum
is measured with the tape from the frontal point to the middle of the
vertical limb of the prsecentral sulcus, and from this again to the occipital
point. The sum of these two measurements is taken as equal to 100, and
the index calculated accordingly.

Fronto-prsecentral Indices in Man.
Lateral Length of the Cerebrum = 100.

Period of Growth.

Number of
Hemispheres
Examined.

Fronto-
prsecentral
Index.

Intrauterine
Life.

/

Fifth month, ......

Sixth month, ......

Seventh month, .....

Eighth month, .....

Full-time foetuses, .....

V

5
5
9
13
12

39-7
38-8
37-7

37- 0

38- 0

Extrauterine
Life.

3 months to 5 years, ....

10

46

37-6
37-8

There is no sexual distinction to be detected in connexion with the
position of the inferior prascentral sulcus. Twenty-seven male cerebral