138 THE UPPER EXTREMITY 



pollicis longus, and accompanies it beneath the dorsal carpal 

 ligament. It may be traced on the dorsal aspect of the 

 metacarpal bone of the thumb to the base of the proximal 

 phalanx, into which it is inserted. This muscle is supplied 

 by the dorsal interosseous nerve. 



M. Extensor Pollicis Longus (O.T. Extensor Secundi Inter- 

 nodii Pollicis). The extensor pollicis longus takes origin 

 from the lateral part of the dorsal surface of the body of the 

 ulna in its middle third, and also from the interosseous 

 membrane. It, to some extent, overlaps the preceding 

 muscle, and it ends in a tendon which passes under cpver 

 of the dorsal carpal ligament, where it occupies a deep narrow 

 groove on the dorsum of the distal end of the radius. 

 On the carpus it takes an oblique course, and, crossing the 

 tendons of the two radial extensors and the radial artery, 

 reaches the thumb. It is inserted into the base of the distal 

 phalanx of that digit. The extensor pollicis longus is supplied 

 by the dorsal interosseous nerve. 



When the thumb is powerfully extended in the living person the 

 tendons of its three last - mentioned muscles become prominent on the 

 lateral aspect of the wrist. The oblique course of the tendon of the 

 extensor pollicis longus is rendered evident, and a distinct depression 

 between it and the other two tendons is seen. 



M. Extensor Indicis Proprius (O.T. Extensor Indicis). 



The extensor indicis proprius arises below the preceding 

 muscle from a limited area on the dorsal surface of the ulna 

 and from the interosseous membrane. Its tendon accom- 

 panies those of the extensor communis under cover of the 

 dorsal carpal ligament, and will afterwards be traced to its 

 insertion on the index finger. This muscle is supplied by 

 the dorsal interosseous nerve. 



Arteria Interossea Dorsalis (O.T. Posterior Interosseous 

 Artery). This vessel arises in the volar part of the 

 forearm, from the common interosseous branch of the 

 ulnar artery. It at once proceeds dorsally between the 

 two bones of the forearm, in the interval between the proxi- 

 mal border of the interosseous membrane and the oblique 

 cord. In the present dissection it makes its appearance 

 between the contiguous borders of the supinator and the 

 abductor pollicis longus, and then it extends distally between 

 the superficial and deep muscles on the back of the forearm. 

 It gives branches to these, and by the time it has reached the 



