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Muscles
of the upper limb
The
major changes in the musculature of the upper limb involve
the secondary effects that result from freeing the forelimb
from a major supportive or propulsive role. The hand of
a chimpanzee is dexterous, but the proportions of the digits
and the rearrangement and supplementation of muscles are
the major reasons for the greater manipulative ability of
modern humans. Most of these changes are concentrated on
the thumb. For example, modern humans are the only living
great apes to have a separate long thumb flexor, and the
short muscle that swings the thumb over toward the palm
is particularly well developed in humans. This contributes
to the movement of opposition that is crucial for the so-called
precision grip; i.e., the bringing together of the tips
of the thumb and forefinger.
Pronation
and supination of the forearm, which allows the palm of
the hand to rotate 180 degrees, is not peculiar to humans.
This movement depends upon the possession of both a small
disk in the wrist joint and on an arrangement of the muscles
such that they can rotate theradius to and fro. Both the
disk and the muscle arrangement are present in other great
apes.
