THE MUSCULAR SYSTEM 227 



to the shaft of the humerus. These do not homologize readily 

 with the muscles of the same region in urodeles, but the last 

 muscle, which appears in some mammals as two, compares well 

 with those of like name in Necturus. This leaves the humero- 

 antebrachialis to be compared with both biceps and brachialis, 

 and it may well be the ancestral form from which both have 

 originated. In origin it is like the latter; and shows no sim- 

 ilarity with the biceps, which arises, usually by a single head, 

 from the scapula and the coracoid process ; it is, however, pre- 

 cisely like the biceps in its mode of insertion, and must be at 

 least in part homologous with this latter muscle. 



The second region to be considered is the proximal portion 

 of the posterior limb and includes the muscles of the pelvic 

 girdle and thigh. As in the skeleton there is in the muscula- 

 ture little suggestion of serial homology between the two pairs 

 of appendages, although in Necturus the two limbs closely cor- 

 respond in the distal portion. A fundamental difference in the 

 muscles of the two girdles is that in the posterior limb they 

 are nearly all intrinsic, and arise from the appendicular skel- 

 eton, while in the anterior limb an extensive system of extrinsic 

 muscles controls in part both the girdle as a whole and the 

 proximal part of the free limb. This difference is undoubtedly 

 correlated with the definite attachment of the posterior girdle 

 to the axial skeleton through the formation of a sacrum, while 

 in the anterior girdle there is either no attachment to the verte- 

 bral column, or a freely movable one through clavicle, 

 sternum and ribs. 



In Necturus (Fig. 61), in which the pelvic girdle is in the 

 form of a flat pubo-ischiadic plate and a narrow ilium, the 

 muscles are naturally divided into those of the outer (ventral), 

 and those of the inner (dorsal) side of the plate, and, thirdly, 

 those which arise from the ilium. Of the first there are two, 

 forming as many layers on the outer side of the plate; the 

 pubo-ischio-tibialis, which runs down the inner side of the leg, 

 passes the femur and inserts into the proximal end of the tibia, 

 and the pubo-ischio-femoralis externus, which inserts into the 

 femur. Upon the inner side of the plate there is a single large 



