FOREARM AND HAND 155 



nerve; (2) a medial compartment enclosing the short 

 muscles of the little finger, and the superficial division of 

 the ulnar nerve ; (3) a lateral compartment enclosing the 

 muscles of the thumb. Fluid or pus which collects in 

 the intermediate compartment can spread into the forearm 

 behind the transverse carpal ligament, and distally to the 

 clefts between the fingers, but it is excluded from the lateral 

 and medial compartments by the septa which pass into 

 the palm from the margins of the intermediate part of the 

 palmar aponeurosis. 



Dissection. Clean the superficial volar arch from the ulnar 

 to the radial side, where it is completed by the anastomosis of 

 the ulnar artery either with the superficial volar branch of the 

 radial artery,- with the radialis indicis, or with a branch of the 

 princeps pollicis artery. Clean also the four digital branches 

 which the arch gives to the fingers. Then clean the palmar 

 part of the median nerve and its branches. The median nerve 

 enters the palm at the distal border of the transverse carpal 

 ligament. There it is flattened from before backwards and 

 expanded from side to side. It divides, almost immediately, into 

 two branches, a smaller lateral and a larger medial. Follow 

 the lateral branch first. Soon after its origin it gives off, from its 

 lateral side, a stout branch to supply the short muscles of the 

 thumb. Secure that branch at once, and follow it across the 

 superficial surface of the flexor pollicis brevis to the point where 

 it disappears under cover of the abductor pollicis brevis. More 

 distally the lateral branch of the median nerve divides into three 

 digital branches, two for the thumb and one for the radial side 

 of the index finger. No especial care is needed as the branches 

 to the thumb are cleaned, but the branch to the index finger 

 gives a fine twig to the first lumbrical which will be destroyed 

 unless care is exercised. The larger medial division of the 

 median nerve divides into two branches, one for the adjacent 

 sides of the index and middle fingers, and one for the adjacent 

 sides of the middle and ring fingers. Care must be exercised as 

 each is cleaned, for the first gives a fine twig to the second 

 lumbrical muscle which should not be missed, and the second is 

 connected with the lateral branch of the superficial division of 

 the ulnar nerve by a communicating twig which should be secured 

 now if it was not found when the ulnar nerve was cleaned. 



Arcus Volaris Superficial (O.T. Superficial Palmar 

 Arch). The term superficial volar arch is applied to an arterial 

 arcade which lies immediately subjacent to the intermediate 

 part of the palmar aponeurosis, its most distal point being 

 situated at the level of the distal border of the fully abducted 

 thumb. It is formed by the ulnar artery. That artery 

 crosses the volar surface of the transverse carpal ligament, 

 immediately lateral to the pisiform bone, then it passes across 



