in having a forearm which is considerably shorter than the upper arm;
whereas in anthropoid apes the forearm is much the longer. That fact
surprises us at first, especially when we remember that anthropoids
spend most of their lives amongst trees and use their arms much more
than their legs in swinging the weight of their heavy bodies from branch
to branch and from tree to tree. A long forearm and hand give them a
long and quick reach, so that they can seize distant branches and swing
themselves along safely and at a good pace. Our first thought is to
suppose that a long forearm, being a weak lever, will be ill adapted
for climbing. But when you look at Fig. 10, the explanation becomes
plain. When a branch is seized by the hand, and the whole weight of the
body is supported from it, the entire machinery of the arm changes its
action. The forearm is no longer the lever which the brachial muscle
moves (Fig. 10), but now becomes the base from which it acts. The part
which was its piston cord now serves as its base of fixation, and what
was its base of fixation to the humerus becomes its piston cord. The
humerus has become a lever of the third order; its fulcrum is at the
elbow; the weight of the body is attached to it at the shoulder and
represents the load which has to be lifted. We also notice that the
brachial muscle is attached a long way up the humerus, thus increasing
its power very greatly, although the rate at which it helps in lifting
the body is diminished. We can see, then, why the humerus is short and
the forearm long in anthropoid apes; shortening the humerus makes it
more powerful as a lever for lifting the body. That is why anthropoids
are strong and agile tree-climbers. But then watch them use those long
hands and forearms for the varied and precise movements we have to
perform in our daily lives, and you will see how clumsy they are.

[Illustration: Fig. 10.--Showing the action of the brachialis anticus in
the arm of an anthropoid ape.]