Ill 



THE ANIMAL MACHINE 



fTpMIE muscular system is not only the oldest 

 machine in existence, but also the most 



-* complex. Moreover, it is otherwise entitled to 

 precedence, for even to-day, in this so-called age of 

 steam and electricity, the muscular system remains by 

 far the most important of all machines. In the United 

 States alone there are some twenty million horses doing 

 work for man; and of course no machine of any sort is 

 ever put in motion or continues indefinitely in operation 

 without aid supplied by human muscles. All in all, then, 

 it is impossible to overestimate the importance of this 

 muscular machine which is at once the oldest and the 

 most lasting of all systems of utilizing energy. 



The physical laws that govern the animal machine 

 are precisely similar to those that are applied to other 

 mechanisms. All the laws that have been called to 

 our attention must therefore be understood as applying 

 fully to the muscular mechanism. But in addition 

 to these the muscular system has certain laws or methods 

 of action of its own, some of which are not very clearly 

 understood. 



The prime mystery concerning the muscle is its 

 wonderful property of contracting. For practical pur- 

 poses we may say that it has no other property; the 



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