THE BRACHIAL PLEXUS 151 



The Flexor Digitorum Profundns arises from the volar aspects of the 

 ulna and interosseous membrane. Near the wrist it breaks up into four 

 tendons which are inserted into the distal phalanges of the medial four 

 fingers. That part of the muscle which is destined for the ring and little 

 fingers is supplied by the ulnar nerve, while the remainder is supplied by 

 the volar interosseous. 



In the distal part of the forearm, the median nerve gives off 

 a volar cutaneous branch, which supplies the skin over the 

 central part of the palm of the hand. Near the wrist, the 

 nerve becomes more superficial and lies behind the palmaris 

 longus tendon. Neuromata or neuro-fibromata are frequently 

 found on the median nerve in this part of its course. 



As it enters the palm of the hand, the median nerve 

 divides into a lateral and a medial division. The former 

 supplies the skin on the volar aspect of the thumb and the 

 radial half of the volar aspect of the index finger, by means 

 of digital branches. These nerves are not restricted to the 

 volar aspects of the index finger and thumb, and they also 

 supply the distal halves of the dorsal aspects of both. 



In addition to its digital branches, the lateral division of 

 the median nerve gives off branches to supply the muscles of 

 the thenar eminence, which is formed by the abductor pollicis 

 brevis, the opponens pollicis and the flexor pollicis brevis. 



The Abductor Pollicis Brevis forms the lateral part of the thenar eminence. 

 It arises from the carpus and is inserted into the radial side of the base of 

 the proximal phalanx of the thumb. In cases where injury of the median 

 nerve near the wrist is suspected, the action of this muscle must be care- 

 fully tested. Abduction of the thumb carries it forwards from the palm in 

 an antero-posterior plane (p. 147), and it must be distinguished from 

 extension, which carries the thumb away from the hand in a lateral 

 direction. Pure abduction of the thumb is impossible when the abductor 

 pollicis brevis is paralysed, because the abductor longus (extensor ossis 

 metacarpi pollicis) is really an extensor muscle. 



The Flexor Pollicis Brevis forms the medial part of the thenar eminence, 

 while the Opponens Pol/ids lies under cover of both the flexor and the 

 abductor. The latter muscle arises from the radial side of the volar aspect 

 of the carpus and is inserted into the radial border of the first metacarpal 

 bone. When it contracts, it produces a slight amount of medial rotation 



