RELATION OF MUSCLES AND BONES. 21 



of the muscle, we know that the live muscle has the power 

 of shortening when stimulated, and in this respect is totally 

 unlike the rubber. 



The action of the bones of the forearm as a lever (we say 

 lever, not levers, because the two bones of the forearm move 

 as one in flexing and extending the forearm upon the arm) 

 may perhaps be better understood by the following considera- 

 tions : If the arm consisted merely of the biceps, suspended 

 from the shoulder, it is evident that its only action would be 

 a straight pull. Suppose the biceps, thus hanging alone from 

 the shoulder, had a hook at its lower end, it could, when it 

 shortened, lift a weight just as far as it shortened, and no 

 farther. It could not swing the weight outward, or push it 

 upward. But from the way in which the biceps is attached 

 to the bones of the forearm, when the muscle shortens an 

 inch it may move the hand a foot. Of course the hand moves 

 much faster, and we have a great gain in speed by reason of 

 this lever arrangement. Of course we must see that we can- 

 not lift so heavy a weight at this faster rate as we could at 

 the elbow. For instance, suppose one were to carry a heavy 

 basket with a bail handle by slipping the arm through the 

 bail up to the elbow. Now, it is evident that the biceps is 

 supporting the weight. If it is as heavy as can be held here, 

 we know that we could not hold the same weight in the hand 

 with the elbow bent at a right angle. 



Levers. The essentials of a lever are, a point about 

 which the lever turns, which in the body is some joint ; the 

 Power as it is called, which in the body is a muscle pulling, 

 and a weight to be moved (not always to be lifted). The 

 point about which a lever turns is called its Fulcrum. The 

 place where the power is applied is called the Power (where 

 the muscle is attached), and the part to be moved is the 

 Weight. In flexing the forearm, the weight is the hand, or the 



