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302 Cunningham Memoirs.

In the apes, the prgecentral sulcus is placed relatively much further
forwards on the surface of the cerebrum than in man. The fronto-
prsecentral indices in several different genera are given in the following
Table :—

fronto-pericentral indices in the apes.

Lateral Length of the Hemisphere = 100.



Number of
Hemispheres
Examined.

Average fronto-
prsecentral
Index.

Cebus, .....

8

27-4

Baboon, .....

11

28-7

Macaque, .....

6

30-8

Chimpanzee, . . .

6

31-6

Orang, ......

2

33-6

The position of the vertical stem of the inferior prsecentral sulcus with
reference to the coronal suture varies somewhat in different apes. Sometimes
it coincides more or less closely with the sutural line. In the various
ape-heads figured in Plate viii. (figs. 18 to 22) it is covered by the coronal
bar of bone; in other cases it is placed a short distance in front of the
suture (vide Plate iv., figs. 8 and 9).

XIII. Summary.—1. The sulcus prsecentralis inferior in the human
brain is composed of a vertical stem and a horizontal limb. The latter is
carried forwards into the middle frontal convolution.

2. This furrow is the earliest to appear on the outer surface of the
frontal lobe of the foetal brain. In some cases it is seen in the fifth month
cerebrum in the form of a long, deep, vertical sulcus, which subsequently
undergoes a retrograde development before its adult condition is reached.
In many cases, in its early condition, it presents a form in every respect
comparable with that observed in the cerebrum of a low ape (Cebus).
Frequently it is developed in several separate pieces.

3. The sulcus prsecentralis superior is closely connected with the basal