84 THE SUPERIOR EXTREMITY 



The Hypothenar Eminence is produced by the abductor, the opponens 

 and the flexor brevis digiti quinti. These muscles are all supplied by the deep 

 branch of the ulnar nerve (C. 8 and T. i), and their actions are sufficiently 

 described by their names. When pus forms in the hypothenar eminence, it 

 is localised to that area, as it is shut off from the central portion of the palm 

 by & fibrous septum, which passes backwards from the ulnar side of the central 

 portion of the palmar aponeurosis. In these cases the incision is made over 

 the ulnar side of the fifth metacarpal bone, dorsal to the pisiform. The 

 nerves of supply, which enter the muscles close to the pisiform, are not inter- 

 fered with by this incision, and no damage is done to the first digital branches 

 of the superficial volar arch and ulnar nerve, which run distally, in front of 

 the flexor digiti quinti brevis, to supply the ulnar side of the little finger. 



The Thenar Eminence is formed by the abductor pollicis, the flexor 

 pollicis brevis, and the opponens pollicis, which lies deep to the preceding 

 two muscles. These three muscles lie to the radial side of the tendon of the 

 flexor pollicis longus, and are all supplied by the median nerve (C. 8 and T. i). 

 They are shut off from the central space of the palm by a fibrous septum, 

 which passes backwards from the radial border of the central portion of the 

 palmar aponeurosis. When pus forms amongst these muscles, it is very 

 definitely localised, and shows no tendency to spread backwards or medially. 

 The incision lies over the distal part of the first metacarpal and on the radial 

 side of the eminence, thus avoiding the nerves of supply, which enter the 

 muscles at their carpal ends, and the digital nerve to the radial side of the 

 thumb (Fig. 30). 



Movements of the Thumb. Flexion and extension are the only move- 

 ments which occur at the metacarpo-phalangeal and inter-phalangeal joints 

 of the thumb. The additional movements of abduction, adduction, and 

 opposition all take place at the carpo-metacarpal joint, where the os mult- 

 angulum majus (trapezium) articulates with the first metacarpal bone. 

 This joint possesses a special synovial membrane, distinct from the synovial 

 membrane of the neighbouring joints. Abduction and adduction occur in 

 an antero-posterior plane, and the former movement must not be confused 

 with true extension, which occurs in a medio-lateral plane. 



The Central Portion of the Palm. The tendons of 

 the flexor digitorum sublimis and profundus lie behind the 

 central portion of the palmar aponeurosis before they enter 

 the fibrous digital sheaths. Opposite the first phalanx each 

 sublimis tendon splits, to allow the corresponding profundus 

 tendon to pass on to be inserted into the base of the distal 

 phalanx. Under cover of the profundus tendon the two slips 

 of the sublimis tendon receive attachment to the borders of the 

 second phalanx. 



The four Lumbrical Muscles arise from the tendons of the 

 flexor digitorum profundus in the palm, and are inserted into 

 the radial side of the dorsal extensor expansion (p. 92) and 

 the adjoining part of the first phalanx of the medial four digits. 

 They flex the fingers at the metacarpo-phalangeal joints, but 

 extend them at the inter-phalangeal joints, through the medium 

 of the extensor expansion. The lumbricals of the little and 

 ring fingers are supplied by the deep branch of the ulnar nerve 



