396 THE MUSCULAE SYSTEM. 



The deep muscles are five in number : one, the supinator, extends between 

 the proximal parts of the ulna and radius ; the others are the special extensors 

 of the thumb and forefinger, viz., the abductor pollicis longus, extensor pollicis longus 

 and extensor pollicis brevis, and extensor indicis proprius. They cover the dorsal 

 surface of the bones of the forearm and the interosseous membrane, and are almost 

 wholly concealed by the superficial muscles. Only the abductor pollicis longus and 

 the extensor pollicis brevis become superficial in the distal part of the forearm, 

 where they emerge between the radial extensors of the carpus and the extensor 

 digitorum communis. 



Superficial Muscles. 



M. Brachioradialis. The brachioradialis arises, by fleshy fibres, from the 

 anterior aspect of the proximal two-thirds of the lateral epicondylic ridge of the 

 humerus, and from the anterior surface of the lateral intermuscular septum 

 (Fig. 340, p. 380). 



The muscle lies in the lateral side of the hollow of the elbow, passes distally 

 along the lateral border of the forearm, and ends about the middle of the forearm 

 in a narrow, flat tendon which is inserted, under cover of the tendons of the abductor 

 pollicis longus and extensor pollicis brevis, by a transverse linear attachment, into 

 the proximal limit of the groove for the above-named muscles on the lateral side 

 of the distal extremity of the radius. Some of its fibres gain an attachment 

 to the ridge on- the volar margin of the groove, and others spread over the surface 

 of the groove for a variable distance (Figs. 355, p. 397, and 348, p. 389). 



Nerve-Supply. The muscle is supplied by a branch of the radial nerve (C. 5. 6.) in the 

 hollow of the elbow. 



Actions. The muscle is primarily a flexor of the elbow-joint. It is also a semi -prona tor and 

 semi-supinator of the forearm, bringing the limb from the supine or prone position, into a 

 position in which the radius is uppermost. It thus assists both the pronator and supinator muscles. 



M. Extensor Carpi Radialis Longus. The extensor carpi radialis longus 



arises, by fleshy fibres, from the anterior aspect of the distal third of the lateral 

 epicondylic ridge of the humerus, from the anterior surface of the lateral inter- 

 muscular septum, and from the common tendon of origin of succeeding muscles, 

 attached to the lateral epicondyle (Figs. 356 and 357, p. 399). 



In the distal half of the forearm, it ends in a tendon which passes beneath the 

 dorsal carpal ligament, to be inserted into the dorsal surface of the base of the 

 second metacarpal bone on its radial side (Fig. 353, p. 394). 



The muscle is concealed in its proximal part by the brachioradialis, and its 

 tendon, in the distal half of the forearm, is crossed, obliquely, by the abductor pollicis 

 and by the extensor pollicis brevis. 



Nerve-Supply. The muscle is supplied by a branch of the radial nerve in the hollow of the 

 elbow (C. (5.) 6. 7. 8.). 



Actions. The muscle is an extensor of the wrist, and also an accessory flexor of the elbow- 

 joint. 



M. Extensor Carpi Radialis Brevis. The extensor carpi radialis brevis 

 arises from the common tendon, from the radial collateral ligament of the elbow, 

 from the fascia over it, and from intermuscular septa on either side. 



It passes distally, in the dorsal surface of the forearm and under the dorsal 

 carpal ligament, in close relation to the previous muscle, to be inserted, by a tendon, 

 into the bases of the second and third metacarpal bones (Fig. 353, p. 394). A 

 bursa is placed beneath the two radial extensor tendons close to their insertion. 



It is practically concealed, in the forearm, by the extensor carpi radialis longus, 

 and in the distal half is crossed obliquely by the abductor pollicis longus and the 

 extensor pollicis brevis. The tendons of the two muscles are crossed, on the dorsum j 

 of the wrist, by the tendon of the extensor pollicis longus. 



Nerve-Supply. The deep branch of the radial nerve (C. (5.) 6. 7. (8.)). 



Actions. Like the long extensor, this muscle extends the hand at the wrist; and is a 

 subsidiary flexor of the elbow- joint. 



