70 THE HUMAN BODY. 



seen and felt on the biceps in front of the burner us when 

 the elbow is bent, or in the ball of the thumb when that 

 digit is moved so as to touch the little finger ; when a muscle 

 contracts its belly may also be felt to grow harder. The 

 swelling and hardening of a contracted muscle are daily 

 illustrated when one schoolboy invites another to feel his 

 "biceps." 



The Origin and Insertion of Muscles. Almost invaria- 

 bly that part of the skeleton to which one end of a mus- 



FlG. 29. The biceps muscle and the arm-bones, to illustrate how, under ordi- 

 nary circumstances, the elbow joint is flexed when the muscle contracts. 



cle is fixed is more easily moved than that part on which 

 it pulls by its other tendon ; the less movable attachment 

 is the origin, the other the insertion of the muscle. Tak- 

 ing the biceps of the arm, we find that when the belly of 

 the muscle contracts and pulls on its upper and lower 

 tendons, the result is commonly that only the forearm is 

 moved, the elbow joint being bent as shown in Fig. 29. 



How may its thickening be recognized? What change besides 

 thickening and shortening occurs in the belly of a contracted muscle? 

 Give an example. 



What is meant by the origin of a muscle? What by the insertion? 

 Give an example. 



