CLASS AMPHIBIA 



499 



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 back toward the median line is an adductor: one 

 that pulls a part forward toward the median line is an abductor; 

 one that lowers a part is a depressor : one that raises a part is a 

 levator ; and one that rotates onopart on another is ajrotator^. 

 The movements of an organ depend on the origin and insertion 

 of the muscles and the nature of the articulations of its bones 

 with each other and with other parts of the body. 



The muscles of the hind limb are usually selected for study 

 to illustrate the methods of action of muscles in general. Table 

 XVI gives the name, origin, insertion, and action of the principal 

 muscles of the hind limb, and Figure 421 shows most of them as 

 seen from the ventral side. 



TABLE XVI 



THE NAME, ORIGIN, INSERTION, AND ACTION OF THE PRINCIPAL 

 MUSCLES OF THE HIND LIMB OF THE FROG 



