386 A MANUAL OF ANATOMY 



as in inserting that finger into the ear canal. It also acts as a 

 feeble auxiliary extensor of the wrist- joint. 



3. Extensor Carpi Ulnaris — Origin. — (i) The external epicondyle 

 of the humerus by the common tendon ; (2) the deep fascia ; 

 (3) the intermuscular septum on either side ; and (4) the upper 

 two-thirds of the posterior border of the ulna by an aponeurosis 

 which is common to it, the flexor carpi ulnaris, and the flexor 

 profundus digitorum. 



Insertion. — ^The tubercle on the inner side of the base of the 

 fifth metacarpal bone. 



'Nerve-supply. — ^The posterior interosseous nerve. 



Action. — (i) To extend the wrist-joint, and (2) to assist in 

 internal lateral flexion (adduction) of that joint. 



The muscle rests upon the inner half of the posterior surface of 

 the shaft of the ulna, and its tendon, in passing beneath the posterior 

 annular ligament, occupies the groove on the back of the ulna. 



4. Anconeus — Origin. — ^The back of the external epicondyle of 

 the humerus at its lower part. 



Insertion. — The outer surface of the olecranon process, and the 

 upper third of the posterior surface of the shaft, of the ulna. 



The muscle, which is continuous with the internal head of the 

 triceps, is triangular, the upper fibres being short and transverse, 

 whilst the others pass obliquely downwards and inwards. 



Nerve-supply. — ^The musculo-spiral nerve, by means of a long 

 branch which descends in the internal head of the triceps, and 

 enters the muscle on its deep surface near the upper border. 



Action. — ^To assist the inner head of the triceps in extending the 

 elbow. 



The posterior interosseous recurrent artery ascends beneath the 

 muscle to the back of the external epicondyle. 



The posterior interosseous nerve will be found described on 

 p. 358, and the corresponding artery on p. 361. 



Deep Layer. — ^There are five muscles in this layer, namely, the 

 supinator radii brevis, extensor ossis metacarpi pollicis, extensor 

 brevis pollicis, extensor longus pollicis, and extensor indicis. 



I. Supinator Radii Brevis — Origin. — (i) The external lateral 

 ligament of the elbow-joint ; (2) the orbicular ligament of the 

 radius ; (3) the back part of the bicipital hollow, and the supinator 

 ridge, of the ulna ; and (4) the aponeurosis covering the upper 

 part of the muscle. 



Insertion. — ^The shaft of the radius on its anterior, outer, and 

 posterior aspects over about the upper third. 



Anteriorly the muscle descends as low as the anterior oblique line, 

 externally as low as the insertion of the pronator radii teres, and 

 posteriorly as low as the posterior oblique line. It closely invests 

 the radius over about its upper third, except on its inner aspect. 



Nerve-supply. — The posterior interosseous nerve. 



Action. — To supinate the radius upon the ulna. 



The muscle is pierced by the posterior interosseous nerve, and, 



