Nor have we succeeded
in finding out why they are so liable to break down in some boys and
girls and not in others. Some day we shall discover this too.
We are now to look at another part of the human machine so that we may
study a lever of the third order. The lever formed by the forearm and
hand will suit our purpose very well. It is pivoted or jointed at the
elbow; the elbow is its fulcrum (Fig. 9 B). At the opposite end of the
lever, in the, upturned palm of the hand, we shall place a weight of 1
lb. to represent the load to be moved. The power which we are to yoke to
the lever is a strong muscular engine we have not mentioned before,
called the _brachialis anticus_, or front brachial muscle. It lies in
the upper arm, where it is fixed to the bone of that part--the humerus.
It is attached to one of the bones of the forearm--the ulna--just beyond
the elbow.
In the second order of lever, we have seen that the muscle worked on one
end, while the weight rested on the lever somewhere between the muscular
attachment and the fulcrum.
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