xii. 12 LIMB MUSCLES 323 



limbs is also shown by the fact that they are innervated by branches 

 of the spinal nerves of several segments (2 for the fore-limb, 4 for the 

 hind-limb in the frog). Presumably the original arrangement was such 

 as to move the limbs in association with the waves of contraction 

 passing down the body. In modern urodeles the limb is brought 

 forward and its joints flexed as the epaxial muscles at the level of its 

 front end contract, and then passes back and extends as the wave of 

 contraction moves past. This may have been the primitive movement, 

 making the limb more useful as a lever during the early attempts to 

 'swim on land' (Fig. 176). 



The muscles of the limbs of tetrapods are presumably derived from 

 those that raise and lower the fins of fishes, modified, as we have seen, 

 to brace the limbs and move them, allowing standing and walking. 

 The muscles that run from the girdles to the humerus and femur are 

 therefore able to draw the leg forward and backward, as well as to 

 raise and lower it in the transverse plane. The actions of the various 

 bundles are of course not confined to a single plane: all the muscles 

 running from the back to the humerus can raise (abduct) the upper 

 limb, but the more anterior members also protract, the more posterior 

 retract it. Similarly there is a ventral series whose anterior members 

 work with the anterior dorsal muscles as protractors, although they 

 antagonize the action of raising the whole limb. Moreover, many of the 

 muscles have a rotating action on the humerus and femur. It is, how- 

 ever, possible to consider the muscles of the arm and leg in two great 

 groups; first a more anterior and ventral ('ventro-lateral') set serving 

 to draw the limb mainly forward and towards the midline (protraction 

 and adduction) and to flex its more distal joints, second a more 

 posterior, dorsal ('dorso-medial') mass serving mainly to draw the 

 limb backwards and away from the body (retraction and abduction) 

 and to extend its joints. 



In the fore-limb the proximal members of the ventral group make a 

 sheet of fibres running transversely to the main body axis and attached 

 to the sternum and hypaxial muscles at one end and to the humerus at 

 the other (Fig. 190). Within this sheet can be recognized the del- 

 toideus, pectoralis, coraco-radialis, and coraco-brachialis muscles. In 

 the limb itself this group is continued, there being, roughly speaking, 

 a set of muscles in each segment that serves to flex it on the next. 

 Thus the brachio-radialis flexes the elbow joint and in the forearm 

 the flexor carpi radialis and flexor carpi ulnaris flex the wrist. The 

 flexor digitorum longus muscle arises from the medial epicondyle of 

 the humerus and is inserted by tendons to the carpus and terminal 



