196 ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



Fig. 279. — The superficial layer of muscles of the extensor surface of the forearm. 



Fig. 280. — The deep layer of muscles of the extensor surface of the forearm. 

 The superficial layer of the extensors has been removed, the cavities of the dorsal carpal ligament have been opened 

 and the tendons of the superficial muscles removed. 



muscle is in immediate relation with the insertion of the tendon of the biceps and with the bicipito- 

 radial bursa. 



It arises from the external epicondyle of the humerus, from the radial lateral and annular 

 ligaments of the elbow-joint, and from the supinator ridge of the ulna. It is tendinous at its 

 origin and usually exhibits a superficial aponeurosis in the middle of its course. Some of the 

 fibers run almost horizontally and some of them obliquely forward and downward, passing to 

 the upper, outer, and lower portion of the tuberosity of the radius and to the volar surface and 

 border, the outer surface, the dorsal surface and border of the same bone. The fibers passing 

 to the volar border terminate immediately above the insertion of the pronator (radii) teres. 



The muscle usually consists of a superficial and of a deep portion which are not sharply 

 differentiated, and between the two portions the deep (posterior interosseous) branch of the radial 

 nerve passes. 



The supinator is supplied by the radial nerve. As its name indicates, it supinates the forearm. 



THE SUPERFICIAL LAYER OF THE EXTENSORS. 



This layer consists of three muscles, adherent to each other at their origins, which are partly 

 in common with the extensor carpi radialis brevis from the external epicondyle of the humerus 

 and the antibrachial fascia, and they terminate in tendons which are distinctly directed toward 

 the ulnar side and pass beneath the dorsal carpal ligament to the back of the hand. They are 

 situated in the dorsal region of the forearm, chiefly upon the dorsal surface of the ulna (and upon 

 the supinator above), and are in relation externally with the extensor carpi radialis brevis, inter- 

 nally with the anconeus and below with the flexor carpi ulnaris. 



The extensor digitorum communis (Figs. 278, 279, and 289) is a broad, flat, strong 

 muscle which arises from the external epicondyle of the humerus and the antibrachial 

 fascia; it is intimately adherent to the extensor digiti Vand partly to the extensor carpi radialis 

 brevis. Somewhat below the middle of the forearm it divides into three (or four) bellies 

 (Fig. 279) which terminate in round tendons; the ulnar one gives off tendons for both the ring 

 and little fingers when only three bellies are present. The tendons pass beneath the dorsal 

 carpal ligament to the back of the hand (Fig. 289), where those for the ulnar fingers are 

 connected by slender transverse or oblique tendinous fasciculi, the junctures tendinum. In the 

 fingers, these tendons form the main portion of the dorsal aponeurosis. 



The extensor digiti V proprius (Figs. 279 and 289) is a thin slender muscle which is so 

 closely connected with the extensor digitorum communis that it seems to be a part of it. Its 

 slender tendon passes through a separate compartment in the dorsal carpal ligament to the 

 dorsal aponeurosis of the little finger (Fig. 281) and is sometimes double, taking the place of the 

 tendon of the extensor communis to the little finger, which may be poorly developed or even 

 absent (Fig. 289). 



