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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 175

To express these relations clearly, it was necessary to construct four
indices, which may be respectively termed the mesial fronto-Rolandic, the
mesial corono-Rolandic, the lateral fronto-Rolandic, and the lateral corono-
Rolandic. In calculating the mesial indices the length of the upper margin
of the hemisphere, measured by the tape from its anterior to its posterior
end, is taken as the standard, and equal to 100; and in theicase of the
lateral indices the length of the cerebrum, measured over its lateral
surface between the same points, is taken as the standard, and equal
to 100.

The determination of the point which should be considered the upper
limit of the fissure of Rolando on the upper border of the hemisphere was a
matter of some difficulty. As already noted, the fissure usually cuts the
upper border of the hemisphere, and I have always measured to the point
at which it reaches this margin. But there are cases in which it falls short
of the upper border of the hemisphere, and then it usually bends suddenly
backwards for a short distance. In these cases I have measured to the
angle of bending, and my reason for selecting that point is simply this—
that, even when the fissure reaches the mesial surface of the hemisphere,
as a general rule the same backward inflexion of its extremity is seen, and
the angle of bending corresponds with the point where it cuts the upper
border of the hemisphere.

In measuring the early foetal brains, where in many cases the lower
portion of the fissure of Rolando was alone present, the point on the mesial
border of the cerebrum, from which the measurement was taken, was
obtained by drawing a line upwards to the margin of the hemisphere in
the direction of the furrow. In the early stages of its development (from
its first appearance up to the middle of the seventh month of intrauterine
life) the fissure traverses the surface of the brain in a more transverse
direction than at later stages of growth. Thus we shall point out, in a
later part of this chapter, that whilst in the adult the fissure of Rolando
intersects the mesial plane at an angle of 71° 7, in the sixth-month foetus
the corresponding angle is 73° 6. This must be borne in mind in comparing
the different indices which are given in the following Table, and
which brings out the average results I have obtained in my endeavours to