i7o THE SUPERIOR EXTREMITY 



superficial head of the short flexor of the thumb takes origin 

 from the transverse carpal ligament. It is inserted into the 

 lateral sesamoid bone of the thumb, and into the lateral side 

 of the proximal phalanx, and it is supplied by the median 

 nerve. (For the deep head see below.) 



M. Opponens Pollicis. The opponens muscle of the 

 thumb springs from the transverse carpal ligament and the 

 ridge on the volar surface of the os multangulum majus. Its 

 fibres spread out as they approach their insertion into the 

 whole length of the radial border and the adjacent part of the 

 volar surface of the metacarpal bone of the thumb. It is 

 supplied by the median nerve. 



M. Adductor Pollicis. The adductor of the thumb 

 consists of an oblique and a transverse part. The oblique 

 part arises from the os capitatum, and the bases of the 

 second and third metacarpal bones (Fig. 73). In some 

 cases it has a slip of origin from the os multangulum minus. 

 The transverse part springs from the distal two-thirds of the 

 volar border of the third metacarpal bone. The two parts 

 converge as they pass to their insertion into the medial 

 sesamoid bone of the thumb and the medial side of its 

 proximal phalanx. In many cases a slip of muscle fibres leaves 

 the lateral side of the oblique part of the adductor, passes 

 dorsal to the tendon of the flexor pollicis longus, and fuses 

 with the superficial head of the flexor pollicis brevis. Both 

 parts of the muscle are supplied by twigs from the deep 

 division of the ulnar nerve. 



At this stage of the dissection the short muscles of the 

 little finger, the abductor, the short flexor and the 

 opponens, should be revised (see p. 152). 



Dissection. To display the branches of the palmar parts of 

 the radial artery and the deep head of the flexor pollicis brevis, 

 cut through the two parts of the adductor pollicis midway 

 between their origins and insertions, and turn the separated 

 portions aside. The deep head of the flexor pollicis brevis lies 

 along the ulnar border of the metacarpal bone of the thumb, 

 and the radial artery will be found entering the palm between 

 the two heads of the first dorsal interosseous muscle at the 

 proximal end of the first interosseous space. Its last two branches, 

 the princeps pollicis and the volar radial artery of the index 

 finger, spring from it as soon as it enters the palm, and whilst 

 it still lies behind the oblique part of the adductor pollicis. 



The Deep Head of the Flexor Pollicis Brevis. This weak slip of 

 muscle lies along the medial side of the metacarpal bone of the thumb 



