THE HAND 93 



of the fingers are made in the middle line to avoid injuring the 

 digital vessels and nerves. 



Incisions on the dorsal aspect of the first phalanx should be 

 confined, if possible, to the ulnar side to avoid injuring the 

 attachment of the lumbrical muscle (p. 84). In the case of 

 the second phalanx, dorsal incisions may be made to either side 

 of the extensor tendon. Septic infection on the dorsal surface 

 of the terminal phalanx is usually of the nature of an onychia 

 and involves the nail bed. It may spread to the bone unless 

 full exit is given to the pus by removal of the nail. 



Ossification of the Metacarpals and Phalanges. The 

 shafts and distal extremities of the phalanges ossify from primary 

 centres ; the proximal extremities from separate epiphyses, 

 which ossify during the third year and unite with the shafts at 

 twenty. The metacarpal bone of the thumb ossifies in the same 

 way as a phalanx, but in the other metacarpal bones the bases 

 and shafts are derived from the same primary centres, while 

 the heads are formed by separate epiphyses, ossifying at three 

 and uniting with the diaphyses at twenty. 



Tuberculous Dactylitis usually attacks the centres of the 

 shafts of the metacarpals and phalanges instead of their ex- 

 tremities, probably because the nutrient artery on entering the 

 bone at once breaks up into small branches, whereas in the other 

 long bones it divides into large ascending and descending 

 branches. Owing to their mode of ossification, the diaphyses 

 of these bones cannot be completely resected subperiosteally 

 without opening one of the joints. On this account, after the 

 periosteum has been elevated, the bone is divided close to 

 the extremity which has no epiphysis. The diaphysis can 

 then be lifted and broken away from the epiphyseal cartilage 

 at the opposite extremity. 



The Metacarpo-Phalangeal Joints possess volar and collateral 

 ligaments, but the dorsal ligament is replaced by the extensor 

 expansion. The volar accessory (glenoid) ligament is a plate 

 of fibrocartilage, which is firmly attached to the proximal 

 phalanx but only feebly connected with the metacarpal bone. 



Dorsal Dislocation of the Thumb, at its metacarpo- 

 phalangeal joint (Hey's dislocation), is produced by extreme 

 dorsi-flexion of the thumb. The volar accessory ligament gives 

 way at its weak proximal attachment, and the phalanx passes 

 backwards, carrying the ligament with it. The distal phalanx 

 is flexed by the flexor pollicis longus. Hyper-extension at the 



