132 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



With the development of the paired appendages into organs for the 

 support of the body (tetrapoda) the skeleton of the leg is converted into 

 a series of levers, and the intrinsic muscles are correspondingly dif- 

 ferentiated and developed. Details cannot be given here as there are 

 so many modifications, but they may be grouped as flexors, which 

 bend the limb or its parts; extensors which straighten it, and rotators 

 which turn it on its axis. These undergo the most modification in the 

 peripheral regions, the muscles of the upper arm and thigh being more 

 constant in character and position. Even more constant are the ex- 

 trinsic muscles, which may be grouped as in fishes. Most prominent 



3th 



FIG. 140. Superficial muscles of anterior part of Salamandra maculata, after Fiir- 

 bringer. a, anconeus; bi, humero-branchialis inferior (biceps); bs, levator scapulae; CMC, 

 cucularis; dtr, dorso-trachealis; dg, digastric; ds, dorsalis scapulae; eo, external oblique; 

 Id, latissimus dorsi; m, petro-tympano-maxiUaris (masseter) ; mh, mylohyoid; pc, pectoralis; 

 ph, procoraco-humeralis; ra, rectus abdominis; spc, supracoracoid. 



of the levators of the fore limb are the trapezius and levator scapulae 

 muscles, while the pectoralis and serratus anterior act as depressors; 

 the sternocleidomastoid and the levator scapulas anterior act as 

 protractors, the pectoralis minor and the latissimus dorsi are their 

 antagonists. In the pelvic region the extrinsic muscles are less dif- 

 ferentiated in function. The pectineus and adductors act as pro- 

 tractors, the pyriformis counteracts them; the limb is drawn toward 

 the middle line by a pubofemoralis, while the gluteus muscle acts as 

 a retractor and elevator. 



THE VISCERAL MUSCLES. 



In the gill-bearing vertebrates a special system of muscles is devel- 

 oped in connection with the visceral arches, which have to open and 

 close the visceral clefts, including the mouth. With the loss of the gills 

 some of these muscles are lost while others become changed in function, 

 several retaining their connection with the hyoid. These visceral 

 muscles may be divided into two sets according as they are derived 



