9 o THE SUPERIOR EXTREMITY 



and an oblique head, which are both supplied by the deep branch of the ulnar 

 nerve (C. 8). It forms a triangular sheet of muscle, with its base attached to the 

 third metacarpal bone and its apex to the base of the first phalanx of the thumb. 

 In the interval between the two heads the radial artery enters the palm. 



The three volar {palmar) interossei, which arise from the metacarpal bones 

 of the fingers on which they act, adduct the little, ring, and index fingers to the 

 middle line of the hand. The four dorsal interossei abduct the ring, middle, and 

 index fingers from the same line. Both groups are supplied by the deep branch 

 of the ulnar nerve (C. 8 and T. i), and, in addition to the above actions, they 

 help to flex the fingers at the metacarpo-phalangeal joints and to extend them 

 at the inter-phalangeal joints, the latter by virtue of their partial insertion 

 into the dorsal extensor expansion. 



Fascial Spaces of the Palm. Behind the flexor tendons and the 

 lumbrical muscles, there exists a large fascial space, which is subdivided into 

 a Middle Palmar and a Thenar Space by a septum of fibrous tissue attached 

 to the third metacarpal bone (Kanavel). This septum is weakened at its 

 proximal end, but it is usually strong enough to prevent the spread of pus 

 from the one space to the other. 



The Middle Palmar Space lies to the radial side of the hypothenar 

 eminence, from which it is completely separated by a fascial septum (p. 84). 

 It is bounded behind by the strong fascia which covers the interosseous 

 muscles in the third and fourth spaces, and in front by the tendons of the 

 middle, ring, and little fingers with their lumbrical muscles. This fascial 

 space may be primarily infected by punctured wounds or by compound 

 fractures of the third, fourth, and fifth metacarpals. It can be involved 

 secondarily to superficial infections of the same fingers, the spread occurring 

 along the sheath of the lumbrical muscle ; or it may be involved by direct 

 spread from the digital synovial sheaths of the third and fourth fingers, 

 the pus rupturing through the closed proximal end of the sheath. If not 

 evacuated, pus in the middle palmar space may spread distally to point at 

 the web of the fingers ; or it may spread proximally behind the tendons 

 into the forearm ; or, in neglected cases, it may pass backwards between 

 the metacarpal bones and infect the dorsal subaponeurotic space (p. 91). 

 Incisions into the space are made over the distal half of the fourth metacarpal 

 bone (p. 86), and the tendons of the ring finger are exposed at a point where 

 they have no synovial sheath. In this way the middle palmar space can be 

 reached without opening into the common palmar sheath, which is not 

 necessarily infected. 



The Thenar space lies between the middle palmar space and the tendon 

 of the flexor pollicis longus. It is bounded behind by the adductor of the 

 thumb and in front by the tendons of the index finger and the first lumbrical 

 muscle. Primary infections of this space may occur through punctured 

 wounds, and, very rarely, through compound fracture of the second meta- 

 carpal, which is separated from it by the adductor muscle of the thumb. 

 Septic infections of the index finger may involve the space secondarily, 

 following the same routes as have been described above in connection with 

 the middle palmar space. Pus in the thenar space may spread backwards 

 between the two heads of the adductor muscle, or over the distal border of 

 the transverse head to point on the dorsal surface of the web of the thumb. 

 It may also pass proximally into the forearm or distally to the interval be- 

 tween the index and middle fingers. Incisions into the thenar space are made 

 over the distal half of the second metacarpal (Fig. 32), and " through and 

 through " drainage may be obtained in the space between the first and 

 second metacarpal bones. 



The Dorsal Carpal (Posterior Annular) Ligament is a 



