THE MUSCULAR SYSTEM 



20I 



,. The most important advance, however, made by the elasmobranchs 

 is the first appearance in vertebrates of the muscles of pectoral and pelvic 

 fins. As the myotomes extend ventrally in the body- wall, hollow epithe- 

 lial buds branch off laterally into the fin anlagen. See Fig. i88. The 

 appendicular muscles are thus seen to be derivatives of lateral trunk 

 muscles. Differentiation of the muscles thus formed takes place in two 

 directions in elasmobranchs and higher animals. First, the appendicular 

 muscles are subdivided into intrinsic muscles which lie within the fin and 

 extrinsic muscles which are connected with the fin but He within the 

 body-wall. Both groups are subdivided into levators and depressors. On 

 the anterior side of the fin, a muscle is formed which pulls the fin forward 



M. TRANSVERSOSPI NALIS, 

 TRA^4SVERSE PFOCESS- 



--M. LONGISSIMUS DORSl. 



M. RHOMBOIDEUS. 



/M. SERRATUS POSTERIOR. 

 SCAPULA. 



M. L4TISSIMUS DORSl 

 GLENOID CAVITY. 



^^ 'ACROMION. 



M EXTERNAL'yO^ ^S^f ''''^'?''' ~"^^i^( ^^"^ " '| ' ' " ^,e^^J<^-^^ tORACDID. 



M. RECTUS ABDOMINIS^ ^ / ^ M BRACHIALIS 



MANUBRIUM STEBNl! ^~~^^ ' '^^ PECTDRALIS MAJOR. INFERIOR. 



CL^VVICLE' ^M. TRAhJSVERSUS THORACIS. 



Fig. 191. — Thoracic and lumbar muscles of man as seen in cross section. Thoracic 

 muscles on the right, lumbar on the left. The muscle arrangement is fundamentally 

 like that of any mammal. (Redrawn after Braus.) 



towards the head. No special antagonistic muscle is differentiated in 

 elasmobranchs, the adduction of the fin being effected by the combined 

 action of the posterior part of the levator and depressor groups acting 

 together. The extension of the extrinsic muscles of the fins in fan-like 

 form over the lateral trunk muscles tends to obscure the metamerism of 

 these in the region of the appendages. The trapezius muscle, which 

 extends from the scapula anteriorly above the gills, makes its first appear- 

 ance in this group. 



In the head region, the visceral muscles become specialized in relation to 

 the jaws. The levators of the first two visceral arches form the jaw muscles, 

 including the masseter, temporalis, and pterygoids, while the depres- 

 sors of these arches become the intermandibularis muscles. The muscles 

 of the remaining visceral arches remain relatively unmodified. (Fig. 189) 



In the urodeles, the metamerism of the lateral trunk musculature 

 persists as a striking characteristic. The extrinsic muscles of the append- 



