174 THE MUSCLES. 
Insertion (Fig. 84, ¢).—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, 7), farther distad with 
the integument and the brachioradialis (2); the distal tendon 
with the extensor pollicis brevis (Fig. 84, 2). Radial surface 
with the brachialis (Fig. 75, 2) and the integument. Ulnar 
surface.with the extensor carpi radialis brevis (Fig. 77, p), and 
near the proximal end with the extensor communis digitorum 
(Fig. 75, m7). 
Actton.—Extensor of the hand. 
M. extensor carpi radialis brevis (Fig. 77, £).—A slender 
muscle from the humerus to the third metacarpal. 
Origin (Fig. 81, 7) 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, 7) 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, 0), the integument, and distad with the extensor pollicis 
brevis (Fig. 84, 7). Radial surface with the extensor longus. 
Ulnar surface with the extensor communis digitorum (Fig. 75, 
m) and the extensor brevis pollicis. Inner surface with the 
pronator teres (Fig. 77, 7), supinator (Fig. 85, 4), and extensor 
brevis pollicis (Fig. 85, a). 
Action.—Extensor of the hand. 
M. extensor digitorum communis (Fig. 75, #z).—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. 
Atthe junction ofthe middle and distal thirds of the radius the 
muscle passes into a large, flat tendon (Fig. 75, 7’) which passes 
