486 THE MUSCLES AND FASCIA. 



compartment in the annular ligament behind the inferior radio-ulnar joint, then 

 divides into two as it crosses the hand, one slip being united to the common 

 extensor by a cross-piece at the metacarpo-phalangeal articulation. Both finally 

 spread into a broad aponeurosis which blends with the common extensor to the 

 finger, and is inserted into the second and third phalanges. The tendon is 

 situated on the ulnar side of, and somewhat more superficial than, the common 

 extensor. 



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 

 tendon ; from the middle third of the posterior surface of the ulna, below the 

 Anconeus, and 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 compart- 

 ment 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. 



Relations. By its superficial surface, with the deep fascia of the forearm ; by 

 its deep surface, with the ulna and the muscles of the deep layer. 



The Anconeus 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. 



Relations. By its superficial surface, with a strong fascia derived from the 

 Triceps ; by its deep surface, with the elbow-joint, the orbicular ligament, the 

 ulna, and a small portion of the Supinator brevis. 



Deep Layer (Fig. 310). 



Supinator brevis. Extensor brevis pollicis. 



Extensor ossis metacarpi pollicis. Extensor longus pollicis. 



Extensor indicis. 



The Supinator brevis is a broad muscle, of a hollow cylindrical form, curved 

 round the upper third of the radius. It consists of two distinct planes of muscular 

 fibres, between which lies the posterior interosseous nerve. The two planes arise 

 in common : the superficial one by tendinous, and the deeper by muscular fibres 

 from the external condyle of the humerus ; from the external lateral ligament of the 

 elbow-joint and the orbicular 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 this ridge ; 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 to the oblique line of the radius, as low down as the insertion of the 

 Pronator radii 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. 



Relations. By its superficial surface, with the superficial Extensor and 

 Supinator muscles, and the radial vessels and nerve ; by its deep surface, with the 

 elbow-joint, the interosseous membrane, and the radius. 



The Extensor ossis metacarpi pollicis 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 posterior surface of the shaft of 

 the ulna below the insertion of the Anconeus, from the interosseous membrane, 



