THE WORK OF MUSCLES AND GLANDS 39 



ily the body excretes all the water which it receives plus 

 the water which arises within it by oxidation. 



A distinction must be borne in mind between the com- 

 pounds which for the most part make up the muscle and 

 those substances which it is generally found to use as 

 sources of energy. The muscle is mainly composed of 

 proteins. But, as just stated, it is most apt to destroy 

 carbohydrates when at work. One is reminded of the 

 fact that a steam-engine is composed chiefly of steel, but 

 burns coal as its fuel. The comparison is somewhat 

 faulty, however, for it suggests a more radical difference 

 between structure and fuel than we can safely infer 

 for the muscle. Under some conditions muscular work 

 may involve some destruction of protein material. 



All that has been said of contraction up to this time 

 applies equally to all three classes of muscle. Neverthe- 

 less each type is adapted to its particular work by peculiar 

 properties. Skeletal muscle is capable of quick shortening 

 and prompt relaxation. A contraction may occur and 

 the return to an extended condition be accomplished in 

 one-tenth of a second. The trained finger of a pianist may 

 strike a key ten times in a second. Such movements are 

 in strong contrast with those executed by the form of 

 muscle found in the viscera. The contractions of the 

 stomach develop very slowly, are maintained for some 

 time, and are correspondingly slow in fading out. Of 

 course, it is true that skeletal muscles may also make 

 prolonged contractions, as in keeping the body erect, carry- 

 ing a suit-case, and in countless other instances. Experi- 

 mental study has shown that such contractions as these are 

 really compounded of successive brief twitches occurring 

 too rapidly to permit relaxation. In view of this the 

 possibility of having prolonged contractions in skeletal 

 muscle does not invalidate the statement that it is essen- 

 tially a quick-acting tissue. 



Muscle and Nerve. The conception that muscular 

 activity is due to the nervous system is probably suffi- 

 ciently familiar. Every skeletal muscle has its own strand 



