1 76 THE SUPERIOR EXTREMITY 



injecting them, by means of a large hypodermic syringe, with a 

 thin solution of coloured starch. If the sheaths have been 

 injured, and it is not possible to distend them, then open each and 

 examine its extent with the aid of a blunt probe. 



The Mucous Sheaths of the Extensor Tendons. Seven 

 mucous sheaths surround the tendons which pass under cover 

 of the dorsal carpal ligament. Two lie along the distal part 

 of the radial border of the forearm ; they are the sheaths of 

 the abductor pollicis longus and the extensor pollicis brevis. 

 They may communicate with one another. From the radial 

 to the ulnar border the remaining five sheaths are arranged 

 in the following order: the sheath of the extensor carpi 

 radialis longus, the sheath of the extensor carpi radialis 

 brevis, a sheath common to the extensor digitorum com- 

 munis and the extensor indicis, the sheath of the extensor 

 digiti quinti, and the sheath of the extensor carpi ulnaris. 



The proximal limit of the sheaths lies at, or slightly proxi- 

 mal to, the dorsal carpal ligament. The sheaths of the 

 abductor pollicis longus and the radial and ulnar extensors 

 of the carpus are prolonged distally to the insertions of the 

 tendons of these muscles. As regards the sheaths of the 

 extensors of the fingers and thumb, it may be said, speaking 

 generally, that they terminate distally about the level of the 

 mid-length of the hand (Figs. 75, 76). 



Dissection. The brachio -radialis has already been dissected ; 

 the other superficial muscles must now be cleaned and, as far 

 as possible, isolated from one another. The isolation of the distal 

 parts is not difficult, but the proximal parts spring not only from 

 the humerus and the super jacent deep fascia, but also from 

 strong intermuscular septa which intervene between adjacent 

 muscles. To isolate the proximal parts of the muscles, therefore, 

 the dissector must split the intermuscular septa with the scalpel 

 and so isolate each muscle as far as its bony origin. 



M. Brachio-radialis (O.T. Supinator Longus). The brachio- 

 radialis muscle lies more on the volar than on the dorsal 

 surface of the forearm. It takes origin, in the arm, from 

 the proximal two- thirds of the lateral epicondylar ridge of the 

 humerus and from the lateral intermuscular septum. Near 

 the middle of the forearm a flat tendon emerges from its 

 fleshy belly, and proceeds distally to gain insertion into the 

 lateral aspect of the expanded distal extremity of the radius, 

 at the base of the styloid process. The nerve of supply is 

 a branch of the radial nerve (O.T. mttsculo- spiral), which 



