THE POSTERIOR RADIOULNAR REGION 483 



The Extensor carpi ulnaris is the most superficial muscle on the ulnar side of 

 the forearm. It arises from the external condyle of the humerus by the common 

 Extensor tendon ; by an aponeurosis from the posterior border of the ulna in common 

 with the Flexor carpi ulnaris and the Flexor profundus digitorum; and from the 

 deep fascia of the forearm. This muscle terminates in a tendon which runs 

 through a groove behind the styloid process of the ulna, passes through a separate 

 compartment in the annular ligament, and is inserted into the prominent tubercle 

 on the ulnar side of the base of the metacarpal bone of the little finger. 



The Anconeus (m. anconaeus) is a small triangular muscle placed behind and 

 below the elbow-joint, and appears to be a continuation of the external portion of 

 the Triceps. It arises by a separate tendon from the back part of the outer 

 condyle of the humerus, and is inserted into the side of the olecranon and upper 

 fourth of the posterior surface of the shaft of the ulna; its fibres diverge from their 

 origin, the upper ones being directed transversely, the lower obliquely inward. 



The Deep Layer (Fig. 362). 



i^Supinator [brevis]. "" Extensor brevis pollicis. 



^Extensor ossis metacarpi pollicis. ^ Extensor longus pollicis. 



Extensor indicis. 

 ^ 



The Supinator [brevis] (m. supinator) (Figs. 361 and 362) is a broad muscle, 

 of hollow cylindrical form, curved around the upper third of the radius. It con- 

 sists of two distinct planes of muscle fibres, between which lies the posterior 

 interosseous nerve (Fig. 361). The two planes arise in common the superficial 

 one by tendinous, and the deeper by muscle, fibres from me external condyle 

 of the humerus, from the external lateral ligament of the elbow-joint and the orbic- 

 ular ligament of the radius; from the ridge on the ulna, which runs obliquely 

 downward from the posterior extremity of the lesser sigmoid cavity; from the 

 triangular depression in front of the cavity; and from a tendinous expansion 

 which covers the surface of the muscle. The superficial fibres surround the 

 upper part of the radius, and are inserted into the outer edge of the bicipital 

 tuberosity and into the oblique line of the radius, as low down as the insertion 

 of the Pronator teres. The upper fibres of the deeper plane form a sling-like 

 fasciculus, which encircles the neck of the radius above the tuberosity and is 

 attached to the back part of its inner surface; the greater part of this portion of 

 the muscle is inserted into the posterior and external surface of the shaft, midway 

 between the oblique line and the head of the bone. Between the insertion of 

 the two planes the posterior interosseous nerve lies on the shaft of the bone 

 (Fig. 361). 



Relations. By its superficial surface, with the superficial Extensors and the Braohioradialis 

 muscles, and the radial vessels and nerve; by its deep surface, with the elbow-joint, the inter- 

 osseous membrane, and the radius. 



Extensor ossis metacarpi pollicis (m. abductor pollicis longus) is the most 

 external and the largest of the deep Extensor muscles; it lies immediately below 

 the Supinator [brevis], with which it is sometimes united. It arises from the outer 

 part of the posterior surface of the shaft of the ulna below the insertion of the 

 Anconeus, from the interosseous membrane, and from the middle third of the 

 posterior surface of the shaft of the radius. Passing obliquely downward and 

 outward, it terminates in a tendon which runs through a groove on the outer side 

 of the styloid process of the radius, accompanied by the tendon of the Extensor 

 brevis pollicis, and is inserted into the base of the metacarpal bone of the thumb. 



