THE MUSCULAR SYSTEM 



271 



In the urodeles, the metamerism of the lateral trunk musculature 

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

 ages, however, become widely extended anterior and posterior to the legs. 

 Such definitive muscles as the pectoraUs and the latissimus dorsi now 

 appear, and the intrinsic muscles subdivide into those of the arm and 

 thigh, the forearm and shank, and the feet. By further sphtting of the 

 original muscle mass within the limb, many new muscles arise, some of 

 which may be homologized with those of man. On the sides of the body, 

 the lateral trunk muscles become delaminated into layers, some amphibia 

 having as many as four. The*epaxial muscles of the trunk divide into 

 longitudinal bundles connected with the head. 



Fig. 225. — Superficial muscles of dog. b, brachialis. be, brachiocephalic; bf, biceps 

 femoris; cc. cleidocervical; d, digastric; di, deltoid; g. gluteus; i, intercostals; Id, 

 latissimus dorsi; m, masseter; mh, mylohyoid; oe, external oblique abdominal; ol, 

 omotransversarius; p, pectoralis; pg, parotid gland; ra, rectus abdominis; 5, sartorius; 

 sc, sternocephalicus; sg, submaxillary gland; sh, sternohyoid; sm, semimembranosus; 

 St, semitendinosus; t, trapezius; lb, triceps brachialis. (From Kingsley's "Comparative 

 Anatomy of Vertebrates," after Ellenberger and Baum.) 



A further novelty in amphibia is a movable tongue. Its intrinsic 

 muscles are those which, as we have seen, grow from occipital myotomes 

 into the floor of the throat and are innervated by the hypoglossal nerve. 

 In this group also we find sternohyoid and geniohyoid muscles, which 

 connect sternum and lower jaw respectively with the hyoid, differentiated. 



No very striking developments affect the muscles of reptiles. The 

 three sets of epaxial muscles of the trunk, — transverso-spinalis, lumbo- 

 costaHs, and iUo-costaUs, appear. The fusion of the lateral trunk 

 myotomes and the consequent loss of metamerism leads towards the con- 

 ditions in mammals. An extreme degree of delamination affects the 

 lateral trunk muscles, some reptiles having as many as eight layers in the 

 body- wall. 



With the great enlargement of the appendages of mammals, there 

 appears a corresponding increase in the appendicular musculature and 



