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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 131

extremity of the insula. This produces an undermining of the cerebral
mantle by the hinder end of the island of Reil.

I have noted the condition of the transverse gyri of Heschl in 73
hemispheres, and have found that in 2*8 per cent, only one was present; in
20*5 per cent, two; in 56*1 per cent, three; in 19"2 per cent, four ; and in
1*4 per cent. five. Two other points of some importance were established:
(1) It is more usual for the hindmost gyrus to reach the surface on the
right side than on the left; and (2) in those hemispheres in which four
gyri were present, 28'6 per cent, only belonged to the right side, whilst
71 "4 per cent, belonged to the left side.

In the anthropoid ape there are two, and in many cases three
transverse temporal gyri of Heschl, but these are feeble and resemble the
corresponding gyri in the human brain in the early stages of their
development (PI. iv., figs. 9 and 11). Further, the anterior gyrus does
not stand out so prominently from the others as in the case of the
human brain.

In the lower apes it is usual to find only one transverse temporal gyrus
of Heschl, and with it the supramarginal convolution is, as a rule,
concealed within the Sylvian fissure. This is the condition present in
cebus. In the baboon the supramarginal convolution is more usually on
the surface. Thus in ten hemispheres it was superficial in seven, and
depressed within the fissure in three. In rare cases also a second transverse
gyrus of Heschl may be noted in the baboon. In the mangaby the
supramarginal convolution is usually deep and hidden from view. In two
out of seven hemispheres I found it superficial. In all cases only one gyrus
of Heschl was present.

XL The Angle of the Sylvian Fissure.—By this I mean the angle
which is formed by the posterior horizontal limb of the Sylvian fissure with
a line drawn at right angles to the longest antero-posterior diameter of the
hemisphere. For the purpose of measuring this angle I had a special
instrument constructed. Two straight, narrow, and flexible bands of brass,
measuring 12 inches and 18 inches respectively, were prepared. These
were jointed together by a pin which fixed the extremity of the short band

[17*]