MUSCULAR SYSTEM 



137 



In those vertebrates which are sufficiently known the first traces of the de- 

 velopment of the musculature of the appendages are the appearance of two buds 

 (fig. 146) from the ventral border of a varying number of myotomes in the region 

 of the developing limb. These buds soon divide into inner and outer halves and 

 each proliferates a cord of cells, the inner and outer series going respectively to 

 the dorsal and ventral side of the appendage. The cells in the cords soon lose 

 their distinctness and form a blastema from which the intrinsic muscles arise, 

 the definitive muscles being innervated by as many spinal nerves as there are con- 

 tributing myotomes. The extrinsic muscles arise directly from the myotomes. 



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 

 differentiated and developed. Details cannot be given here as there 

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



jx 



\\ >v- 



Fig. 147. — Superficial muscles of anterior part of Salamandra tnaculata, after 

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

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

 oblique; Id, latissimus dorsi; m, petro-tympano-maxillaris (masseter); mh, mylohyoid; 

 mhp, mylohyoid posterior; /»c, pectoralis; ph, procoraco-humeralis; ra, rectus abdominis; 

 spc, supracoracoid; sth, sternohyoid. 



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

 rotators"which turn I t on its axfs. These undergo the most modifica- 

 tion in the peripheral regions, the muscles of the upper arm and thigh 

 being more constant in character and position. Even more constant 

 are the extrinsic muscles, which may be grouped as in fishes. Most 

 prominent of the levators of the fore Hmb are the trapezius and 

 levator scapulae muscles, while the pectoralis and serratus anterior 

 ict as depressors; the sternocleidomastoid and the levator 

 scapulae anterior act as protractors, the pectoralis minor and the 

 latissimus dorsi are their antagonists. In the pelvic region the ex- 

 trinsic muscles are less differentiated in function. The pectineus 

 ind adductors act as protractors, 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. 



