LAWS OF MUSCLE ACTION. 17 



us to consider the muscles of high importance. Add to this 

 the facts above noted, that the muscles are so largely con- 

 cerned in the nutrition of the body, the chief agents for its 

 protection, essential for the reception of ideas, and absolutely 

 indispensable for the expression of ideas, and we can see the 

 reason for beginning the study of physiology with the exami- 

 nation of the muscles and their action. 



Laws of Muscle Action. The chief characteristic of 

 muscle is its ability to shorten; incidentally, it at the same 

 time thickens and hardens. But it does its work by shorten- 

 ing, pulling on the bones by means of the strong, inelastic 

 tendons, thus producing motion. The muscle is ordinarily 

 said to " contract ; " but as it occupies almost exactly the same 

 amount of space at all times, the word " contract " is inappro- 

 priate. A muscle may be made to shorten one-third of its 

 length, but probably never shortens that much in the living 

 body. 



A muscle cannot be kept shortened for any great length 

 of time. If one holds his arm out horizontally as long as 

 possible he soon feels fatigue, later pain, and he may feel 

 soreness in the muscle for several days. The law of muscle 

 action is to alternate periods of rest with periods of action. 

 In many exercises, as in walking, the limbs act alternately, 

 one resting or recovering position while the other works. 



If we consider the biceps and triceps of the arm, we see 

 that they are compelled to act alternately if they would do 

 effective work. They might both shorten at the same time, 

 and are made to do so in such an attempt as that of holding 

 the arm rigidly bent at a right angle ; as, for instance, in wres- 

 tling " square hold," in which case one wishes to prevent his 

 opponent from either pushing or pulling him. But while the 

 two muscles act no motion is produced. When the flexor 

 shortens the extensor lengthens, and vice versa. 



