FOREARM AND HAND 169 



upon the medial side of the tendon, and are yet supplied 

 by the median nerve. 



Dissection. Clean the adductor of the thumb and then 

 examine the short muscles of the thumb and revise the short 

 muscles of the little finger. 



The Short Muscles of the Thumb. Three of the short 

 muscles of the thumb have already been dissected. All four 

 are now displayed, with the exception of the deep head of 

 the short flexor, which cannot be seen until the adductor 

 has been reflected. Their relations and attachments 

 should now be studied. Three of the four, the abductor 

 pollicis brevis, the superficial head of the flexor pollicis 

 brevis and the opponens pollicis lie on the lateral side of 

 the tendon of the flexor pollicis longus ; the fourth muscle, 

 the adductor pollicis, is on the medial side of that tendon. 



The abductor pollicis brevis forms the most prominent and 

 lateral part of the ball of the thumb. The superficial head 

 of the flexor pollicis brevis is immediately medial to the 

 abductor; the opponens is deep to both of them and is 

 exposed when they are pulled apart, or when they are 

 reflected. The fan-shaped adductor pollicis lies deeply in 

 the palm. It is imperfectly separated into a proximal or 

 oblique portion and a distal or transverse portion. The 

 separation is produced by the radial artery as it enters the 

 palm through the first interosseous space. The deep head 

 of the flexor pollicis brevis is concealed by the adductor, 

 and cannot be seen at this stage of the dissection. 



The branch of the median nerve which supplies the 

 three muscles which lie lateral to the tendon of the 

 flexor pollicis longus has already been found (p. 155), 

 and the twigs to the adductor which are derived from the 

 deep branch of the ulnar nerve have also been traced (p. 166). 



M. Abductor Pollicis Brevis. The short abductor of the 

 thumb arises from the volar aspect of the transverse carpal 

 ligament and from the ridge on the os multangulum 

 majus. It is inserted into the lateral sesamoid bone of 

 the thumb, into the lateral side of the base of the first 

 phalanx, and to a slight extent into the extensor tendon 

 on the dorsum of the first phalanx. It is supplied by the 

 median nerve. 



M. Flexor Pollicis Brevis (Caput Superficialis). The 



