A REPRESENTATIVE VERTEBRATE. FROG 



349 



part upon another, as the leg upon the 

 thigh, is a flexor; one that straightens out 

 a part, as the extending of the foot, is an 

 extensor; one that draws a part toward the 

 midhne of the body is an adductor; one that 

 moves a part away from the midhne of the 

 body is an abductor; one that lowers a part 

 is a depressor; one that raises a part is a 

 levator; and one that rotates one part on 



another is a rotator. The movements of an 

 organ depend on the origin and insertion of 

 the muscles and the nature of the articula- 

 tions of its bones with each other and with 

 other parts of the body (Fig. 232). 



The muscles of the hindlimb of the frog 

 are usually selected for study to illustrate the 

 methods of action of muscles in general. 

 The accompanying table gives the name, 



contraction 



of biceps muscle 



Extension of arm 



by contraction of 



triceps muscle 



Figure 232. Movements produced by muscles. Contraction is the only action produced by a 

 muscle. Note the two opposing muscles in the arm and bones to which they are attached. 



origin, insertion, and action of some of the 

 muscles of the hindlimb. Figure 230 shows 

 that the triceps femoris arises from three 

 distinct heads; namely, vastus internus, rec- 

 tus anticus femoris, and vastus externus. 



The following are a few of the muscles of 

 the other parts of the body: the rectus ab- 

 dominis extends longitudinally along the 

 ventral side of the trunk; the external 



oblique covers most of the sides of the 

 trunk; the transverse lies beneath the ex- 

 ternal oblique and serves to contract the 

 body cavity; the pectoralis major moves the 

 forelimbs; and the mylohyoid raises the 

 floor of the mouth cavity during respiration. 

 The muscles of animals in general and 

 of man in particular are described in Chap- 

 ter 31. 



