MUSCLES OF THE THORACIC LIMBS. 177 



Origin (Fig. 86, f) by short, fleshy fibres from the lateral 

 surface of the ulna between the semilunar notch and the junc- 

 tion of the third and fourth (distal) fourths. 



The muscle-fibres pass obliquely distad and toward the 

 radial side and join the tendon (Fig. 85, f'), which runs nearly 

 the whole length of the radial surface. 



Insertion (Fig. 84, <:). The tendon passes through the 

 groove on the ulnar side of the dorsal surface of the base of the 

 radius and divides into two. Both of these usually go to the 

 base of the second phalanx of the second digit, but one may 

 pass to the pollex, or there may be three division's going to the 

 first three digits. 



FIG. 85. DEEP MUSCLES ON THE EXTENSOR SURFACE OF THE RADIUS 



AND ULNA. 



a, M. extensor brevis pollicis (a', its tendon); ft, M. supinator; c, M. extensor in- 

 dicis (f 7 , its tendoni; d, tendon of M. extensor carpi radialis brevis; e, tendon of M. 

 extensor carpi radialis longus. I. humerns; 2, olecranon; 3, styloid process of 

 ulna; 4, distal end of radius; 5, head of radius. 



Relations. Outer surface with the integument, the extensor 

 carpi ulnaris (Fig. 75, 0), and the extensor digitorum lateralis 

 (Fig. 75, ;/). Dorsal border with the flexor profundus digi- 

 torum (Fig. 75, q]. Inner surface with the extensor brevis 

 pollicis 'Fig. 85, a). 



Action as indicated by the name. 



M. supinator (Fig. 85, b}. A flat muscle wrapped spirally 

 about the proximal end of the radius. 



Origin by a short, strong tendon from the lateral side of the 

 annular ligament of the radius and by tendinous fibres from the 

 radial collateral ligament passing from the humerus to the 

 radius. From the tendon of origin the fibres diverge, passing 

 toward the radial side and distad. 



Insertion (Fig. 86, g). The dorsal and part of the ventral 

 surface of the proximal two-fifths of the radius to within 



