64 THE ENGINES OF THE HUMAN BODY 



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. 17B). 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 antlcus 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 fore- 

 arm — 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 



