160 THE SUPERIOR EXTREMITY 



The superficial branch passes distally, under cover of the 

 palmaris brevis, to which it gives a twig of supply. Whilst 

 under cover of the palmaris brevis it divides into two digital 

 branches. The medial of the two branches passes to the 

 medial side of the little finger. The lateral branch pierces 

 the septum which passes dorsally from the medial border of 

 the intermediate part of the palmar aponeurosis, and enters 

 the intermediate compartment of the palm. There it is joined 

 by a communicating branch from the first medial digital 

 branch of the median nerve, and then it divides into two 

 branches which supply the adjacent sides of the ring and 

 little fingers. 



Dissection. After the branches of the median and ulnar 

 nerves have been examined, remove the lateral part of the palmar 

 aponeurosis from the muscles of the thenar eminence, but 

 preserve their nerve of supply, which has already been found. 

 As soon as the aponeurosis is removed two muscles are exposed ; 

 they are the abductor pollicis brevis and the superficial head of 

 the flexor pollicis brevis. The abductor is the lateral muscle ; 

 pass the handle of the scapel behind its lateral border and lift 

 the muscle from the subjacent opponens pollicis, then divide 

 the abductor about the middle of its length ; turn the proximal 

 part towards its origin, and the distal part towards its insertion. 

 When that has been done the opponens will be exposed, and 

 must be cleaned. Next divide the short flexor at its middle and 

 reflect it towards its extremities. The reflection of the short 

 flexor will bring into view parts of the adductor of the thumb, 

 emerging from behind the flexor tendons of the fingers, and 

 along the medial border of the opponens pollicis the tendon of 

 the flexor pollicis longus will be seen ; it is enclosed in its mucous 

 sheath, which should not be injured. 



At this stage the mucous sheaths of the flexor tendons may 

 be re-examined (see p. 134) by inflation or with the aid of a 

 blunt probe. 



Note that the common sheath which envelops the tendons of 

 the flexor digitorum sublimis and the flexor digitorum profundus 

 extends, from the distal part of the forearm, behind the transverse 

 carpal ligament, to the middle of the palm. Its proximal limit 

 is about 25 mm. above the transverse ligament. At its distal 

 limit, which is at the middle of the palm, practically at the same 

 level as the most distal part of the superficial volar arch, it 

 terminates, opposite the index, middle and ring fingers, in blunt 

 protusions on the tendons of the corresponding fingers, but on 

 its ulnar side it is prolonged into and is continuous with the 

 flexor digital sheath of the little finger, which is prolonged to the 

 base of the terminal phalanx of the little finger (Fig. 58). The 

 continuity is easily demonstrated, if inflation fails, by making 

 a small incision into the flexor sheath of the little finger and 

 passing a probe through the incision and along the sheath. 



The synovial sheath of the flexor pollicis longus reaches the 

 same proximal level as the common flexor sheath, and it is pro- 



