THE POSTERIOR RADIO-ULNAR REGION. 399 



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 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, accom- 

 panied by the tendon of the Extensor brevis pollicis, and is inserted into the base 

 of the metacarpal bone of the thumb. It occasionally gives off two slips near its 

 insertion one to the Trapezium, and the other to blend with the origin of the 

 Abductor pollicis. 



Relations. By its superficial surface, with the Extensor communis digitorum, 

 Extensor minimi digiti, and fascia of the forearm, and with the branches of the 

 posterior interosseous artery and nerve which cross it ; by its deep surface, with 

 the ulna, interosseous membrane, radius, the tendons of the Extensor carpi radialis 

 longior and brevior, which it crosses obliquely, and, at the outer side of the wrist, 

 with the radial vessels ; by its upper border, with the Supinator brevis ; by its 

 luiri'f bordi-r, with the Extensor brevis pollicis. 



The Extensor brevis pollicis (Extensor primi internodii pollicis), the smallest 

 muscle of this group, lies on the inner side of the preceding. It arises from the pos- 

 terior surface of the shaft of the radius, below the Extensor ossis metacarpi pollicis, 

 and from the interosseous membrane. Its direction is similar to that of the 

 Extensor ossis metacarpi pollicis, its tendon passing through the same groove on 

 the outer side of the styloid process, to be inserted into the base of the first phalanx 

 of the thumb. 



Relations. The same as those of the Extensor ossis metacarpi pollicis. 



The Extensor longus pollicis (Extensor secundi internodii pollicis) is much larger 

 than the preceding muscle, the origin of which it partly covers in. It arises from 

 the outer part of the posterior surface of the shaft of the ulna, below the origin of 

 the Extensor ossis metacarpi pollicis, and from the interosseous membrane. It 

 terminates in a tendon which passes through a separate compartment in the annular 

 ligament, lying in a narrow, oblique groove at the back part of the lower end of the 

 radius. It then crosses obliquely the tendons of the Extensor carpi radialis lon- 

 gior and brevior, being separated from the other extensor tendons, of the thumb by 

 a triangular interval, in which the radial artery is found, and is finally inserted 

 into the base of the last phalanx of the thumb. 



Relations. By its superficial surface, with the same parts as the Extensor ossis 

 metacarpi pollicis ; by its deep surface, with the ulna, interosseous membrane, the 

 posterior interosseous nerve, radius, the wrist, the radial vessels, and metacarpal 

 bone of the thumb. 



The Extensor indicis is a narrow, elongated muscle placed on the inner side of, 

 and parallel with, the preceding. It arises from the posterior surface of the shaft 

 of the ulna, below the origin of the Extensor longus pollicis and from the inter- 

 osseous membrane. Its tendon passes with the Extensor communis digitorum 

 through the same canal in the annular ligament, and subsequently joins the tendon 

 of the Extensor communis which belongs to the index finger, opposite the lower 

 end of the corresponding metacarpal bone, lying to the ulnar side of the tendon 

 from the common Extensor. 



Relations. The relations are similar to those of the preceding muscles. 



Nerves. The Supinator longus is supplied by the sixth, the Extensor carpi 



