BRANCHES OF THE RADIAL ARTERY. 91 



joined by the interosseous arteries, especially by the posterior 

 terminal branch of the anterior interosseous. 



From the dorsal carpal arch branches (g) descend to the third dorsal inter- 

 and fourth interosseous spaces, and constitute two of the three osseous " 

 dor*ul interosseous arteries : at the cleft of the fingers each divides 

 into two, which are continued along the dorsum of the digits. 

 Below, they communicate with the digital arteries ; and above, 

 they are joined by the perforating branches of the deep palmar 

 arch. 



2. The metacarpal or first dorsal interosseous branch of the radial metacarpal; 

 (fig. 35, 6) gains the space between the second and third metacarpal 



bones, and receives, like the corresponding arteries of the other 

 spaces, a perforating branch from 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 middle fingers. 



3. Two small dorsal arteries of the thumb arise opposite the dorsal 

 metacarpal bone, along which they extend, one on each border, to 



be distributed on its posterior aspect. 



4. The dorsal branch of the index finger is distributed on the radial and fore- 

 edge of that digit. 



The different divisions of the annular ligament may now be seen Sheaths of 

 more completely by cutting the sheaths of the ligament over the 

 several tendons passing beneath. There are six separate compart- 

 ments, and each is lubricated by a synovial membrane. The most out inwards, 

 external one lodges the first two extensors of the thumb. The next 

 is a large hollow for the two radial extensors of the wrist ; and a 

 small space for the long extensor of the thumb follows on the ulnar 

 side. Farther to the inner side is the common sheath for the 

 extensor of the fingers, and that of the forefinger ; and then comes 

 a slender compartment for the extensor of the little finger. Internal Bones 

 to all is the space for the extensor carpi ulnaris. The last muscle the tendons, 

 grooves the ulna ; but the others lie in hollows in the radius in the 

 order mentioned above, with the exception of the extensor minimi 

 digiti which is situate between the bones. 



Dissection. If the supinator brevis be divided by a vertical To see 

 incision, and reflected from the radius, its attachment to that bone 

 will be better understood. 



The posterior interosseous nerve, and the offsets from its gangli- interosseous 

 form enlargement, may be traced more completely after the tendons 

 of the extensor of the fingers and indicator muscle have been cut 

 at the wrist. 



The posterior surface of the dorsal interosseous muscles of the and inter- 

 hand may next be cleaned, so that their double origin, and their muscles 

 insertion into the side, and on the dorsum of the phalanges, may be 

 fully observed. Between the heads of origin of these muscles the 

 posterior perforating arteries appear. 



Lastly, the outer head of the first dorsal interosseous muscle is to Passage 

 be divided, and carefully separated from the first metacarpal bone, artery into 

 so as to display the passage of the radial artery into the palm. P alm - 



