THE ANIMAL MACHINE 



If, while holding the weight, you will grasp the elbow 

 with the other hand, you will feel the point of attach- 

 ment of the biceps, and discover that it does not seem 

 to be, roughly speaking, more than about an inch from 

 the joint. Obviously, then, if you are lifting a pound 

 weight, the actual equivalent of energy expended by 

 the contracting biceps must be twelve pounds. But, 

 in the meantime, when the pound weight in your hand 

 moves through the space of one inch, the muscle has 

 contracted by one-twelfth of an inch; and you may 

 sweep the weight through a distance of two feet by utiliz- 

 ing the two-inch contraction, which represents about 

 the capacity of the muscle. 



A similar consideration of the muscles of the legs will 

 show how the muscular system which is susceptible of 

 but trifling variation in size, gives to the animal great 

 locomotive power. With the aid of a series of levers, 

 represented by the bones of our thighs, legs, and feet, 

 we are able to stride along, covering three or four feet 

 at each step, while no set of the muscles that effect this 

 propulsion varies in length by more than two or three 

 inches. It appears, then, that the muscular system 

 gives a marvelous illustration of capacity for storing 

 energy in a compact form and utilizing it for the de- 

 velopment of motion. 



THE TWO TYPES OF MUSCLES 



The muscles of animals and men alike are divided into 

 two systems, one called voluntary, the other involuntary. 

 The voluntary muscles, as their name implies, are sub- 



[45] 



