BACK OF THE FOREARM AND HAND. 149 



terosseous membrane, and descends obliquely to the thumb ; 

 its tendon, crossing the radial artery and the extensor mus- 

 cles of the wrist, passes through a special sheath in the 

 annular ligament, and in a groove in the radius, to be in- 

 serted into the base of the second phalanx of the thumb. 

 The little triangular interval left between the tendon of this 

 muscle and the parallel tendons of the two preceding mus- 

 cles, has been called the "anatomist's snuff-box;" forced 

 abduction of the thumb will reveal the depression to which 

 this name is given. 



The EXTENSOR INDICTS is the lowest muscle toward the 

 wrist ; it arises from the shaft of the ulna, usually below 

 the middle ; its tendon passes through the annular ligament 

 with the common extensor of the fingers, and uniting with 

 the tendon of that muscle going to the forefinger, is inserted 

 with it into the second and third phalanges. 



The POSTERIOR ANNULAR LIGAMENT is formed from the 

 deep fascia of the arm, and, if divided over the tendons 

 passing through it, will be found to have six separate 

 canals, lined with synovial membrane ; with the exception 

 ^of that for the extensor minimi digiti, which lies in the 

 interval between the two bones, each of these canals has 

 a corresponding groove in the radius or ulna ; between each 

 of the canals, the ligament is firmly attached to the bone 

 beneath. 



The radial artery will be found passing round the lower 

 end of the radius, beneath the extensor tendons of the 

 thumb, to the space between the thumb and forefinger, 

 where it passes between the two heads of the first dorsal 

 interosseous muscle, and penetrates the palm of the hand. 

 Before disappearing, it gives off the dorsal carpal branch, 

 which crosses transversely beneath the extensor tendons, 

 to join a similar branch from the ulnar arteiy. From the 

 arch thus formed, dorsal interosseous arteries are given off 

 to the third and fourth interosseous spaces ; the metacarpal, 

 or first dorsal interosseous, larger than the others, passes 

 upward, in the space between the first and second metacarpal 

 bones, anastomosing, as do the others, with the perforating 

 branch of the deep palmar arch ; at the cleft of the index and 

 middle fingers it ends by joining with the digital branch of 

 the superficial palmar arch. The radial artery also gives off 

 two small dorsal branches to the thumb, and a dorsal 

 branch to the index finger. 



In addition to its posterior carpal branch, the ulnar 



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