MUSCLE FORM AND DUTY 



131 



that one muscle of the pair bends the joint and the oppos- 

 ing muscle straightens it again. In the case of the elbow, 

 the muscle that bends the joint is the biceps and the 

 opposing muscle that straightens it is the triceps. The 

 hip joint which is capable of movement in several direc- 

 tions has more than one such pair of opposing muscles. 



ter, showing limbs, each with se\'ci:il joint.-. 



Muscle Form in Relation to Muscle Duty. — The 

 force which can be exerted by any muscle is simply the 

 combined pull of its individual fibers. Since any single 

 fiber is of microscopic size and of considerable fragility, 

 it is only by there being great numbers of them together 

 that muscles of such power as our bodies possess can be 

 obtained. The arrangement of fibers in a muscle depends 

 on whether the muscle is one in which strength is the 

 chief need or one in which amplitude of motion is re- 

 quired. Strong muscles must have many fibers side by 

 side, and so are thick. Long muscles have many fibers 

 placed end to end and so are capable of great decrease in 

 length, thus producing large motions of the parts to which 

 they are attached. 



The Strength of Many of the Muscles is remark- 

 able, particularly when we consider the mechanical dis- 

 advantage under which they work. The strongest 



