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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 133

the outer surface of the brain, in a direction corresponding to the
transverse line on the glass, and the joint being placed just below the
Sylvian Point. The short metal band was then moved into the line of
the fissure (fig. 32). The angle obtained in this way by the instrument,
was traced on paper, and measured in the ordinary way.

Sylvian Angle in Man.



Number of Hemispheres









Examined.



Right.

Left.

Period of Growth.





Average Angle.



Right.

Left.







Adults,

16

15

67°-8

66°-3

70°-3

11 to 15 years,

6

4

65°-8

62°-

7l°-7

4 to 5 years,

4

4

64°-l

61°-2

67°-

First year,

4

2

67°-2

660>5

69°-

Full-time foetuses,

5

4

62°-l

60°-5

64°-

7^ to 8£ months, .

6

6

61°-5

57°'3

64°-8

In the adult the average angle is 67°'8, and there appears to be no
difference in this respect between the male and the female. In the former
I obtained an average angle of 67°"9, and in the latter an average angle
of 67°-7. The angle, however, is subject to very great variations in individual
cases. Thus, in one female hemisphere the angle was 54°, whilst in
four hemispheres (three males and one female) it was 74°.

But a further examination of the above Table makes it evident that the
angle is not of equal magnitude at all stages of growth, and also that it
differs considerably in the right and left hemisphere. In the early condition
of the brain the angle is much more acute, but as growth advances
the posterior part of the fissure becomes more depressed, and the angle,
in consequence, opens out. This is apparently due to the fact that the
fronto-parietal operculum continues to grow downwards even after the
insula is closed in and its margin has met the temporal operculum.

The greater angle on the left side is evident at every stage of growth,
and constitutes a marked difference between the two hemispheres.