young, the bony levers and muscular engines of our feet have not only
their daily work to do, but they have continually to effect those
wonderful alterations which we call growth. Hence, the muscular engines
of young people need special care; they must be given plenty of work to
do, but that kind of active action which gives them alternate strokes of
work and rest. Even the engine of a motor cycle has three strokes of
play for one of work. Our engines, too, must have a liberal supply of
the right kind of fuel. But even with all those precautions, we have to
confess that the muscular engines of the foot do sometimes break down,
and the leverage of the foot becomes threatened. 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. In levers of the third order, the load is
placed at the end of the lever, and the muscle is attached somewhere
between the load and the fulcrum (Fig. 9 A). In the example we are