RADIAL ARTERY. 289 



ment, and ascends on or through the fibres of the supinator, but beneath 

 the anconeus, to supply both those muscles and the elbow joint, and to 

 anastomose with the superior prof'unda artery in the last-named muscle, as 

 well as with the recurrent radial. 



The posterior interosseous nerve ( 3 ) takes origin from the musculo-spiral 

 trunk (p. 259), and winds backwards through the fibres of the supinator 

 brevis. Escaped from the supinator, the nerve is placed between the 

 superficial and deep layers of muscles as far as the middle of the forearm. 

 Much reduced in size at that spot, it sinks beneath the extensor of the 

 second phalanx of the thumb, and runs on the interosseous membrane to 

 the back of the carpus. Finally the nerve enlarges beneath the tendons 

 of the extensor comnumis digitorum, and terminates in filaments to the 

 ligaments and the articulations of the carpus. 



Branches, It furnishes offsets to all the muscles of the deep layer ; 

 and to those of the superficial layer with the exception of the three follow- 

 ing, viz., anconeus, supinator longus, and extensor carpi radialis longior. 



RADIAL ARTERY AT THE WRIST (fig. 88). The radial artery (d), 

 with its venae comites, winds below the radius to the back of the carpus, 

 and enters the palm of the hand at the first interosseous space, between the 

 heads of the first dorsal interosseous muscle. At first the vessel lies 

 deeply on the external lateral ligament of the wrist joint, and beneath the 

 tendons of the extensors of the metacarpal bone, E, and first phalanx of 

 the thumb, F ; but afterwards it is more superficial, and is crossed by the 

 tendon of the extensor of the second phalanx of the thumb, G. 



Offsets of the external cutaneous nerve entwine around the artery, and 

 the radial nerve is superficial to it. Its branches are numerous but incon- 

 siderable in size: 



. The dorsal carpal branch (f) passes transversely beneath the ex- 

 tensor tendons, and forms an arch with a corresponding offset of the ulnar 

 artery ; with this arch the posterior interosseous artery joins. 



From the carpal arch branches (g) descend to the third and fourth 

 interosseous spaces, and constitute two of the three dorsal interosseous 

 arteries: at the cleft of the fingers each divides into two, which are con- 

 tinued along the dorsum of the digits. In front they communicate witli 

 the digital arteries ; and behind they are joined by the perforating 

 branches of the palmar arch. 



b. The metacarpal or first dorsal interosseous branch (fig. 87, 5), 

 reaches the space between the second and third metacarpal bones, and 

 anastomoses, like the corresponding arteries of the other spaces, with a 

 perforating branch of the deep palmar arch. Finally it is continued to 

 the cleft of the fingers, where it joins the digital artery of the superficial 

 palmar arch, and gives small dorsal branches to the index and second 

 fingers. 



c. Two small dorsal arteries of the thumb (c) arise opposite the meta- 

 carpal bone, along which they extend, one on each border, to be distributed 

 on its posterior aspect. 



d. The dorsal branch of the index finger is distributed on the radial 

 edge of that digit. 



The different compartments of the annular ligament may be seen more 

 completely by dividing the sheaths of the ligament over the several ten- 

 dons passing beneath. There are six different spaces, and each is lubri- 

 cated by a synovial membrane. The most external one lodges the first 

 two extensors of the thumb. The next is a large hollow for the two radial 

 19 



