We have already seen why our muscles are so easily exhausted when we stand stock-still; they then get no rest at all.  Now, it sometimes happens in people who have to stand for long periods at a stretch that these muscular engines which maintain the arch are overtaxed; the arch of the foot gives way.  The foot becomes flat and flexible, and can no longer serve as a lever.  Many men and women thus become permanently crippled; they cannot step off their toes, but must shuffle along on the inner sides of their feet.  But if the case of the overworked muscles which maintain the arch is hard in grown-up people, it is even harder in boys and girls who have to stand quite still for a long time, or who have to carry such burdens as are beyond their strength.  When we are 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 considering, the brachial muscle is attached about half an inch beyond the fulcrum at the elbow, while the total length of the lever, measured from the elbow to the palm, is 12 inches.  Now, it is very evident that the muscle or power being attached so close to the elbow, works under a great disadvantage as regards