MUSCLES OF FOREARM AND HAND 383 



Nerve-supply. — -From the musculo-cutaneous nerve by a branch which enters the ventral 

 surface of the muscle near the junction of the upper and middle thirds of the medial border. 

 In addition the radial (musculo-spiral) nerve usually sends a small branch into the distal lateral 

 portion of the muscle. A branch from the median nerve frequently supplies the medial side of 

 the muscle near the elbow-joint (Frohse). 



Action. — To flex the forearm. 



Relations. — It lies behind the biceps, on each side of which it projects. The distal lateral 

 portion of the muscle is grooved by the brachio-radialis, which here is closely applied to it. 

 The radial (musculo-spu-al) nerve runs between these two muscles. On the medial side run the 

 brachial vessels and median nerve. 



Variations. — It may be divided into two distinct heads continuous with the projections on 

 each side of the deltoid tuberosity. A great number of supernumerary slips have been recorded. 

 These may be attached to the radius, ulna, fascia of the forearm, capsule of the joint, brachio- 

 radialis, and extensor carpi radialis muscles. It may be partially fused with neighbouring 

 muscles. It has also been reported absent. 



BURS^ 



B. m. coraco-brachialis. — Between the subscapularis muscle, the tendon of the coraco- 

 brachiahs, and the coracoid process. Frequent. 



B. bicipito-radialis. — Between the ventral half of the radial tuberosity and the tendon of 

 the biceps. Constant. 



B. cubitalis interossea. — Between the tendon of the biceps and the ulna and the neighbour- 

 ing muscles. Frequent. 



D. MUSCULATURE OF THE FOREARM AND HAND 



(Figs. 366-379) 



The muscles of the forearm arise in part from the humerus, in part from the 

 radius and ulna. Their bellies lie chiefly in the proximal half of the forearm. 

 They are divisible into two groups: — a radio-dorsal, composed of extensors of the 

 hand and fingers and supinators of the forearm; and an ulno-volar, composed of 

 flexors of the hand and fingers and pronators of the forearm. The brachio- 

 radialis, which belongs morphologically with the former group, is physiologically 

 a flexor of the forearm. 



The two groups are separated on the medial side of the back of the forearm 

 by the dorsal margin of the ulna (figs. 366, 369). Ventrally they are separated 

 by the insertions of the biceps and brachialis and by an intermuscular septum 

 (figs. 366, 370). 



In the hand, in addition to the tendons of the muscles of the forearm mentioned 

 above (fig. 376), there are several sets of intrinsic muscles. About the meta- 

 carpal of the thumb (figs. 375, 376, 377) is grouped a set of muscles which arise 

 from the carpus and metacarpus and are inserted into the metacarpal and first 

 phalanx of the thumb. A similar set of muscles is grouped about the metacarpal 

 of the little finger (figs. 375, 376). These sets of muscles give rise respectively 

 to the thenar and hypothenar eminences. Between the metacarpals lies a series 

 of dorsal and palmar interosseous muscles (figs. 377, 378, 379) which are inserted 

 into the first row of phalanges and into the extensor tendons. From the tendons 

 of the deep flexor of the fingers a series of lumbrical muscles passes to the radial 

 side of the extensor tendons (figs. 373, 375). These various muscles abduct, 

 adduct, flex, and extend the digits. In addition to these deeper skeletal muscles 

 of the hand there is a subcutaneous muscle over the hypothenar eminence (fig. 

 375). Of the muscles of the hand, all are supplied by the ulnar nerve except 

 most of those of the thumb and the two more radial lumbricals, which are 

 supplied by the median nerve. 



An arrangement of the muscles of the forearm in which the dorsal extensor-supinator mus- 

 culture extends proximally on the radial side of the arm to the distal extremity of the humerus, 

 and the volar flexor-pronator musculature similarly on the ulnar side, is characteristic of all 

 limbed vertebrates and is associated with the pronate position of the forehmb characteristic of 

 quadrupeds. In amphibia and reptiles the musculature terminates distally on the carpus and 

 in the aponeuroses of the hand. In the higher forms special tendons are differentiated for those 

 muscles of the forearm which act on the fingers. On the back of the hand in many vertebrates 

 short extensor muscles are found running from the carpus to the phalanges. On the volar 

 surface a complex musculature is found in all forms which have freely movable fingers. In 

 animals which walk on the ends of the fingers, especially in the hoofed animals, the intrinsic 

 musculature of the hand is greatly reduced. The phylogenetic development of the muscles of 



