MOTION-. 13 



same time thickens and hardens. But it does its work by 

 shortening, pulling on the bones by means of the strong, 

 inelastic tendons, thus producing motion. The action of 

 the muscle as a whole is the result of the characteristics 

 of the cells of which it is composed. The individual cells 

 and fibers shorten, and their combined action is seen in 

 the muscular movement. 



Extent of Muscle Shortening. A muscle may be 

 made to shorten one third of its length, but probably 

 never shortens that much in the living body. 



Duration of Muscle Shortening. 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. 



Alternate Action of Flexors and Extensors. 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 wrestling " 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 length- 

 ens, and vice versa. 



Normal Condition of Muscle. The muscles are always 

 slightly stretched, as shown by the fact that when a cut 

 is made into a muscle the wound gapes open ; the tension 



