200 



CHORDATE ANATOMY 



myotome so that the surface of attachment of the muscles is increased, 

 it has not been proved that this increase is adaptive. See Fig. i86, C. 



A novelty first appearing in this group is the division of the lateral 

 trunk myotomes by a horizontal connective-tissue septum into epaxial 



D NECTURUS. 



OCPffESSOft MANOIBUl>e 

 SPHINCTER COLLI 

 TRAPEZIUS 



DOnSAUS SCAPULAE 



EPAXIN. MUSCLES 



HYRUUAL UUSXCS 



Fig. 190. — Superficial lateral trunk muscles in an amphibian, a reptile, and a mam- 

 mal. D, Necturus. E, Sphenodon. F, Felis. The metamerism of the lateral trunk 

 muscles which is such a striking feature of the lower vertebrates is retained in urodeles 

 and reptiles, but disappears in mammals. The factors in this change are chiefly the 

 increasing dominance of the appendicular muscles and the fusion of the primarily 

 metameric embryonic trunk muscles. The primitive metamerism, however, appears in 

 mammalian embryos 



and hypaxial groups of muscles. Five post-branchial myotomes send 

 buds anteriorly into the floor of the pharynx to form the hypobranchial 

 musculature innervated by the hypoglossal nerve. 



