A Book of Exposition | Page 9

A long forearm and hand are ill
adapted for lifting heavy burdens; strength is sacrificed if they are too
long. Hence, we find that the laboring peoples of the world--Europeans
and Mongolians--have usually short forearms and hands, while the
peoples who live on such bounties as Nature may provide for them
have relatively long forearms and hands.
[Illustration: Fig. 9B.--The forearm and hand as a lever of the third
order.]
Now, man differs from anthropoid apes, which are distant cousins of
his, 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.]
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. What should
we think of a metal engine which could reverse its action so that it
could act through its piston-rod at one time and through its cylinder at
another? Yet that is what a great number of the muscular engines of the

human machine do every day.
There is another little point, but an important one, which I must
mention before this chapter is finished. I have spoken of the forearm
and hand as if they formed a single solid lever. Of course that is not so;
there are joints at the wrist where the hand can be moved on the
forearm. But when a weight is placed in the hand, these joints became
fixed by the action of muscles. The fixing muscles are placed in the
forearm, both in front and behind, and are set in action the moment the
hand is loaded. The wrist joint is fixed just in the same way as the
joints of the foot are made rigid by muscles when it has to serve as a
lever. Even when we take a pen in our hand and write, these engines
which balance and fix the wrist have to be in action all the time. The
steadiness of our writing depends on how delicately they are balanced.
Like the muscles of the foot, the fixers of the wrist may become
overworked and exhausted, as occasionally happens in men and women
who do not hold their pens correctly and write for long