202 



CHORDATE ANATOMY 



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

 Such definitive muscles as the pectoralis 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 amphibians 



-M. PIRIFORMIS 



'M. ABDUCTOR CAODAE VENT 



M. EXTENSOR CAUOAE 

 LATERALIS 



M ABDUCTOR CAUDAE 

 DORSAL IS 



DORSAL 



Fig. 192. — Human caudal muscles viewed from A. ventral and B. dorsal side. 

 These rudimentary muscles are the last remnants of the powerful caudal muscles of the 

 lower vertebrates. The presence of such useless rudiments receives its best interpreta- 

 tion in the evolution theory. (Redrawn from Wilder's " History of the Human Body," 

 Henry Holt & Co.; after Lartschneider.) 



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

 longitudinal bundles connected with the head. 



A further novelty in amphibians 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 differentiated sternohyoid and geniohyoid 

 muscles, which connect sternum and lower jaw respectively with the hyoid. 



No very striking developments affect the muscles of reptiles. The 

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

 costalis, and ilio-costalis, appear. The fusion of the lateral trunk 



