MUSCLES OF THE THORACIC LIMBS. 177 
Origin (Fig. 86, 7) 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, c’), which runs nearly 
the whole length of the radial surface. 
Insertion (Fig. 84, c).—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 divisions going to the 
first three digits. 
Fic, 85.—DrEEr MUSCLES ON THE EXTENSOR SURFACE OF THE Rabius 
AND ULNA. 
a, M. extensor brevis pollicis (a’, its tendon); 4, M. supinator; ¢, M. extensor in- 
dicis (c’, its tendon); d@, tendon of M. extensor carpi radialis brevis; ¢, tendon of M. 
extensor carpi radialis longus. 1, humerus; 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, 7). Dorsal border with the flexor profundus digi- 
torum (Fig. 75, g). Inner surface with the extensor brevis 
pollicis (Fig. 85, a). 
Action as indicated by the name. 
M. supinator (Fig. 85, 4).—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 
