174 THE MUSCLES. 



Insertion (Fig. 84, r). The tendon passes across the car- 

 pus and is inserted onto the dorsal surface of the base of the 

 second metacarpal on the radial side just distad of the groove 

 for the radial artery. 



Relations. Outer surface at the proximal end with the 

 caput laterale of the triceps (Fig. 75, //), farther distad with 

 the integument and the brachioradialis (k) ; the distal tendon 

 with the extensor pollicis brevis (Fig. 84, d}. Radial surface 

 with the brachialis (Fig. 75, z) and the integument. Ulnar 

 surface with the extensor carpi radialis brevis (Fig. 77, /), and 

 near the proximal end with the extensor communis digitorum 

 (Fig. 75, ;). 



Action. Extensor of the hand. 



M. extensor carpi radialis brevis (Fig. 77, /). A slender 

 muscle from the humerus to the third metacarpal. 



Origin (Fig. 81, /) from the distal part of the lateral supra- 

 condyloid ridge distad of the extensor longus. Near the 

 carpus the muscle ends in a slender tendon which passes 

 through the groove with the extensor longus tendon. The 

 tendon then diverges from the extensor longus and has its 



Insertion (Fig. 84, f) into the radial side of the dorsal sur- 

 face of the base of the third metacarpal. 



Relations. Outer surface with the extensor longus (Fig. 

 77, <?), the integument, and distad with the extensor pollicis 

 brevis (Fig. 84, d}. Radial surface with the extensor longus. 

 Ulnar surface with the extensor communis digitorum (Fig. 75, 

 i)i] and the extensor brevis pollicis. Inner surface with the 

 pronator teres (Fig. 77, </), supinator (Fig. 85, b], and extensor 

 brevis pollicis (Fig. 85, a). 



Action. Extensor of the hand. 



M. extensor digitorum communis (Fig. 75, m}. A slen- 

 der muscle from the lateral supracondyloid ridge of the humerus 

 to the phalanges of the second, third, fourth, and fifth digits. 



Origin (Fig. 81, ;//). The distal surface of the lateral 

 supracondyloid ridge dorsad of the origin of the extensor carpi 

 radialis brevis. 



At the junction of the middle and distal thirds of the radius the 

 muscle passes into a large, flat tendon (Fig. 75, m'} which passes 



