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.]
In the human machine the levers of the arm have been fashioned, not for
climbing, but for work of another kind--the kind which brings us a
livelihood. We must have perfect control over our hands; the longer the
lever of the forearm is made, the more difficult does control of the
hand become. Hence, in the human machine the forearm is made relatively
short and the upper arm long.
We have just seen that the brachial muscle could at one time move the
forearm and hand, but that when they are fixed it could then use the
humerus as a lever and thereby lift the weight of the body.
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