178 THE SUPERIOR EXTREMITY 



be followed to the thumb, that of the abductor pollicis longus 

 to the base of the metacarpal bone, that of the extensor pollicis 

 brevis to the base of the first phalanx, and the tendon of the 

 extensor pollicis longus to the base of the terminal phalanx. 



There are four tendons of the extensor digitorum communis, 

 one for each finger. Each of the four expands, on the dorsum 

 of the first phalanx of the finger to which it belongs, into an 

 extensor expansion which covers the whole of the dorsum of the 

 phalanx. 



Near the distal extremity of the first phalanx the extensor 

 expansion divides into three pieces, an intermediate piece and 

 two collateral pieces. The weaker intermediate piece is inserted 

 into the base of the second phalanx. The lateral pieces unite 

 with one another on the dorsum of the second phalanx before 

 they are inserted into the base of the terminal phalanx. Care- 

 fully define this arrangement on at least one finger. The 

 tendon of the extensor indicis passes along the ulnar side of the 

 tendon of the extensor digitorum communis to join the extensor 

 expansion of the index finger, and the two tendons of the extensor 

 digiti quinti join the extensor expansion of the little finger. 



Clean the margins of the extensor expansion of one or more 

 fingers, and pass the handle of a scalpel between the expansion 

 and the middle of the first phalanx, then note, as the expansion 

 is put on the stretch, that, just beyond the metacarpo-phalangeal 

 joint, the expansion is joined, on each side, by structures which 

 pass to it through the interosseous spaces. On the radial side 

 it is joined by the tendon of a lumbrical muscle and by a slip 

 from the tendon of an interosseous muscle, and, on the ulnar 

 sides, by a slip from a tendon of an interosseous muscle, except 

 in the case of the little finger where there is, of course, no inter- 

 osseous muscle on the ulnar side. The dissector will have no 

 difficulty in displaying the association ot the lumbrical muscles 

 with the extensor expansions at the present stage of the dis- 

 section. The connections of the interossei cannot be properly 

 demonstrated until a later stage. 



The Extensor Expansions on the Fingers. The dissector 

 who has carefully followed the above instructions will have 

 recognised that by means of the extensor expansions a 

 common extensor tendon, a lumbrical, and one or more inter- 

 ossei gain insertion into the dorsal aspects of the bases of 

 the second and terminal phalanges of each finger, and 

 through the same medium the second and terminal phalanges 

 of the index and little fingers are attached to the special 

 extensors of those digits. The dissector should now note 

 the movements he can make with his own fingers : (i) by use 

 of the flexor muscles alone he can flex all three joints of the 

 fingers, the metacarpo-phalangeal, and the proximal and distal 

 interphalangeal joints; (2) by use of the extensor muscles alone 

 he can extend all three joints ; (3) by use of the flexors and 

 extensors simultaneously he can flex the interphalangeal and 



