86 THE SUPERIOR EXTREMITY 



transverse head of the adductor pollicis, and the medial two 

 lumbricals. 



The Median Nerve enters the palm behind the transverse 

 carpal (anterior annular) ligament. At the lower border of 

 the ligament it breaks up into lateral and medial divisions. 

 The lateral division at once supplies the thenar muscles 

 (p. 84) and gives off digital branches to supply both sides of 

 the thumb and the radial side of the index ringer. The medial 

 division gives off two branches, which run to the second and 

 third clefts to supply the adjacent sides of the index, middle, and 

 ring fingers, and twigs to their dorsal apects (p. 72). The 

 digital branches of the radial side of the index finger and the 

 second cleft supply the first and second lumbricals respectively. 



The Superficial Volar (Palmar) Arch is the continuation of 

 the ulnar artery into the palm. It runs obliquely, distally, and 

 laterally across the hook of the hamate to reach the mid-point 

 between the base of the middle finger and the distal skin crease 

 at the wrist. The mode of termination of the arch is very 

 variable, but it is always joined by a branch from the radial 

 artery (superficialis volse, princeps pollicis or radialis indicis), 

 and the anastomosis is so free that in wounds of the arch both 

 the cut ends require to be tied. The artery lies immediately 

 under cover of the central part of the palmar aponeurosis, and 

 it crosses in front of the flexor tendons and their sheaths and 

 the digital branches of the median nerve. 



It gives off four Digital Branches. The first supplies the 

 ulnar side of the little finger (p. 84) ; the second passes to the 

 fourth cleft, and lies to the ulnar side of the fourth metacarpal 

 bone ; the third passes to the third cleft and lies to the radial 

 side of the same bone ; the fourth crosses the third metacarpal 

 bone to gain the second cleft. An incision can therefore be made 

 over the distal part of the fourth metacarpal bone, without injuring 

 the superficial volar arch or any of its digital branches (Fig. 30). 



Synovial Sheaths of the Flexor Tendons. As the flexor 

 tendons pass into the palm, they are invested with synovial 

 sheaths, which are arranged in a somewhat complicated manner. 



The tendon of the flexor pollicis longus has a separate sheath, 

 which extends distally as far as its insertion and proximally for 

 an inch or more beyond the transverse carpal ligament. It 

 consists of a " visceral " layer, closely applied to the tendon, 

 and a parietal layer, which lines the walls of the space occupied 

 by the tendon. At both extremities of the sheath the parietal 



