xiv THE MUSCULAR SYSTEM 247 



a dense fibrous band. The end of a muscle which is fastened 

 to a relatively immovable part is called the origin ; the more 

 movable end is the insertion. The contraction of a muscle 

 has the effect of drawing the origin and insertion nearer to- 

 gether. The gastrocne mius muscle, which may be taken as 

 an illustration, has its origin by two heads, one from a ten- 

 dinous band extending from the lower end of the femur to 

 the tibio-fibula, the other by a narrow tendon which joins 

 the tendon of the triceps on the anterior side of the thigh. 

 At its lower extremity the muscle is inserted by means of a 

 strong tendon which passes over the ankle joint and spreads 

 out over the lower or plantar surface of the foot. When 

 the muscle contracts, it may produce two movements. It 

 may straighten out, or extend the foot, and hence is spoken 

 of as an extensor of that member. It may also bend the 

 leg upon the thigh at the knee, a movement which is called 

 flexion. The gastrocnemius is designated, therefore, as an 

 extensor of the foot and a flexor of the leg. A muscle 

 which draws the limb posteriorly toward the long axis of 

 the body is called an adductor, one which pulls it in the 

 opposite direction an abductor. Then there are rotators, 

 which cause a limb to rotate about its long axis ; levators, 

 which raise a part, such as the muscles which raise the lower 

 jaw ; and depressors, which produce the opposite movement, 

 r.uch as the depressor mandibulce, which lowers the jaw. The 

 kind of movements a limb may make is dependent on the 

 nature of its joints and the number and attachments of its 

 muscles ; and the actions which the frog as a whole is cap- 

 able of performing are dependent in a similar manner upon 

 the organization of its skeletal and muscular systems, 

 although the order and combinations of the movements are 

 directed by impulses from the central nervous system. 

 The action of muscles may be well illustrated by a study 



