A STUDY OF THE MUSCLES 195 



end has two heads or tendons (Latin U = two -f caput = 

 head). The central portion or the part that contracts is 

 called the belly of the muscle. 



Since the biceps muscle is joined above to the shoulder 

 blade and below to the radius, it therefore passes across 

 two joints. When we lift a book with our forearm, the 

 upper tendons remain practically unmoved ; this end of the 

 muscle is then called its origin. The tendon attached to 

 the radius, however, is made to move considerably, and to 

 this end is given the name insertion of the muscle. If, on 

 the other hand, we climb a rope hand over hand, the elbow 

 joint is held firm, and the motion takes place at the shoul- 

 der. Under these conditions the radius end is the origin, 

 and the scapular end the insertion. By origin of a muscle 

 is meant the end that moves least; by insertion, the end that 

 moves most. In the majority of muscles one end is always 

 origin, the other insertion. 



The Triceps Muscle. If we straighten or extend the fore- 

 arm as far as possible, the belly of a muscle behind the 

 humerus is found to swell. This is the tri'ceps muscle 

 (Latin tri = three -f- caput = head), so called because it has 

 three tendons at its upper end. These tendons form the 

 origin of the triceps, and are attached to the shoulder blade 

 and to the humerus. The insertion of the muscle is on the 

 projecting head of the ulna (commonly known as the " funny 

 bone") (see Fig. 89). 



Arrangement of Muscles in the Body. If the biceps muscle 

 is made to contract, the forearm is brought upward or 

 flexed. When the triceps exerts its force, the biceps relaxes 

 and the forearm is straightened or extended. This illus- 

 trates the paired arrangement of muscles throughout the 

 body ; for a flexor muscle on one side of a joint is bal- 

 anced by an extensor on the other side, which acts as its 

 antagonist. 



Along the palm side of the forearm are the bellies of the 

 flexor muscles that bend the fingers, while on the back of 



