SUPPORT AND MOVEMENT 



385 



qos+rocnemius 



biceps brochi 



Fig. 15-7. Muscle action in the human arm and leg. In the upper left figure, the triceps brachii muscle contracts in 

 extending or straightening the arm while the biceps brachii relaxes. In the lower left figure, the opposite action 

 occurs, that is, the triceps brachii relaxes and the biceps brachii contracts. This flexes or bends the arm as in 

 lifting. To rise on the toes as in walking the large gastrocnemius contracts (right). 



tion is the most common in the animal 

 body. Antagonist muscles are not equally 

 matched as to strength. For example, the 

 muscle which raises the jaw is stronger than 

 the one that lowers it. Hence, when bodi 

 contract violently as they do in convulsions 

 the jaw is closed tightly (lockjaw). 



Muscles vary considerably in size and 

 shape, some being long and fusiform, 

 whereas others are thin and flat (Fig. 

 15-8). Most of them have a fleshy middle 

 or belly part and two tapering ends which 

 terminate in round or flat cords called 

 tendons. Tendons consist of tough, fibrous 

 tissue that attaches the muscle to the bone. 

 The two ends of the muscle are identified 

 by the amount of movement that takes 

 place in the bones to which the tendons are 

 attached. The end which moves the bone 

 the greater distance is called the insertion; 

 the end which moves the bone the shorter 

 distance is the origin. Thus the biceps 

 brachii muscle (Fig. 15-7) has its origin 

 on the point of the scapula and its insertion 



on the radius because the latter bone 

 moves the greater distance when contrac- 

 tion occurs. 



Tendons act like cables, attaching a mus- 

 cle to a bone sometimes at a considerable 

 distance from the muscle. This is a very 

 convenient arrangement because it makes 

 possible the location of muscles some dis- 

 tance from the point where action must 

 occur. For example, the muscles that sup- 

 port and operate the foot are located in 

 the lower leg. If one feels the calf of his 

 leg while standing, tlie tenseness of the 

 muscles in supporting the body weight is 

 clearly apparent. The large tendon at the 

 heel, the tendon of Achilles (Fig. 15-7), is 

 like a steel cable when one is standing, par- 

 ticularly if he is on his toes. If the large calf 

 muscles that are necessary in operating the 

 foot were located in the foot itself the latter 

 appendage would reach astounding propor- 

 tions. Furthermore, the foot would function 

 poorly as compared to the slim-ankled in- 

 strument that is man's, or better, woman's 



