126 THE UPPER EXTREMITY 



Tendon of the Flexor Pollicis Longus. This tendon pro- 

 ceeds distally, in the interval between two of the muscles 

 of the thumb (viz. the superficial head of the flexor pollicis 

 brevis, and the oblique part of the adductor pollicis), and also 

 in the interval between the two sesamoid bones which play 

 upon the head of the metacarpal bone. Reaching the proximal 

 phalanx, it enters a fibrous sheath constructed upon a similar 

 plan to those of the fingers. When this is opened the 

 tendon will be observed to be inserted into the volar aspect 

 of the base of the terminal phalanx of the thumb. The 

 mucous sheath which surrounds the tendon during its passage 

 through the carpal tunnel is continuous with the sheath which 

 invests the tendon in front of the phalanges. 



Dissection. Throw distally the superficial volar arch. Divide it on 

 the medial side distal to the origin of the profunda artery of the palm, and 

 on the lateral side at the point where it is joined by the superficial volar 

 artery. The median nerve may also be severed and its branches turned 

 aside, but care should be taken to preserve the two branches which it gives 

 to the lumbrical muscles, and also the stout branch which enters the muscles 

 of the thenar eminence. Lastly, cut through the fleshy belly of the 

 flexor digitorum sublimis in the forearm, and, raising its tendons from the 

 carpal hollow, throw them as far distally as possible. The tendons of the 

 flexor digitorum profundus and the attached lumbrical muscles are now 

 fully displayed. 



Mm. Lumbricales. These are four slender fleshy bellies 

 which arise from the tendons of the flexor digitorum pro- 

 fundus as they traverse the palm. The first lumbrical arises 

 from the lateral side of the tendon for the index finger ; the 

 second lumbrical springs from the lateral border of the 

 tendon for the middle finger ; whilst the third and fourth 

 lumbricals take origin from the adjacent sides of the tendons 

 between which they lie (viz. the tendons for the middle, 

 ring, and little fingers). The little muscles pass distally and 

 end in delicate tendons on the lateral sides of the fingers. 

 Each is inserted into the lateral margin of the dorsal expansion 

 of the extensor tendon, which lies upon the dorsal aspect 

 of the proximal phalanx. 



Dissection. The flexor digitorum profundus may be divided in the fore- 

 arm and thrown distally. Great care must be taken in raising the tendons 

 and lumbrical muscles from the palm, because slender twigs from the 

 deep branch of the ulnar nerve enter the two medial lumbrical muscles 

 on their dorsal aspects. These can easily be secured if ordinary caution be 

 observed. The deep volar arch and the deep branch of the palmar part 

 of the ulnar nerve are now exposed, and a favourable opportunity is given 

 for studying the short muscles of the thumb and little finger. 



