226 OSTEOLOGY. 



fourth metacarpal. The extensor carpi radialis brevis muscle is inserted into the 

 dorsum of the base. 



The fourth metacarpal bone may be recognised by a method of exclusion. It 

 is unlike 'either the first, second, or third, and differs from the fifth, which it 

 resembles in size, by having articular surfaces on both sides of its base. Proximally 

 there is a quadrilateral surface on its base for articulation with the os harnatuui. 

 On its lateral side there are usually two small oval facets for the third metacarpal. 

 Of these facets the dorsal one not infrequently has a narrow surface for articula- 

 tion with the capitate. On the medial side there is a narrow articular strip for the 

 base of the fifth metacarpal. 



The fifth metacarpal bone can be recognised by its size and the fact that it 

 has an articular facet only on one side of its base, namely, that on its lateral side 

 for the fourth metacarpal. The carpal articular surface is saddle-shaped, and there 

 is a tubercle on the medial side of the base for the insertion of the extensor carpi 

 ulnaris muscle. 



As has been already pointed out, the openings of the arterial canals are usually seen on the 

 volar surfaces of the metacarpals, those of the medial four bones being directed proximally towards 

 the base or carpal end, differing in this respect from that of the first metacarpal, which is directed 

 distally towards the head or phalangeal extremity. The opening of the latter canal usually 

 lies to the medial side of the volar aspect of the body. 



Ossification. The metacarpal bones are developed from primary and secondary 

 centres but there is a remarkable difference between the mode of growth of the first and 

 the remaining four metacarpals, for whilst the body and head of the first metacarpal 

 are developed from the primary ossific centre, and its base from a secondary centre, in 

 the case of the second, third, fourth, and fifth metacarpals the bodies and bases are de- 

 veloped from the primary centres, the heads in these instances being derived from the 

 secondary centres. In this respect, therefore, as will be seen hereafter, the metacarpal 

 bone of the thumb resembles the phalanges in the manner of its growth, a circumstance 

 which has given rise to considerable discussion as to whether the thumb is to be regarded as 

 possessing three phalanges and no metacarpal, or one metacarpal and two phalanges. Broom 

 (Anat. Anz. vol. 28), by a reference to reptilian forms, offers an explanation in regard to the 

 difference in the mode of ossification of the first metacarpal on the ground that the most 

 movable joint is that between the first metacarpal and the carpus, whereas on the other 

 digits the most movable joints are those between the metacarpals and phalanges. In 

 consequence those ends of the bones which enter into the formation of the joints where 

 movement is most free are the ends where the epiphyses will appear. This is in accordance 

 with the law to be suggested in connexion with the fibula. The primary centres for the bodies 

 and bases of the second, third, fourth, and fifth metacarpals appear in that order during the 

 ninth or tenth week of intra-uterine life, some little time after the terminal phalanges 

 have begun to ossify ; that for the body and head of the metacarpal bone of the thumb a 

 little later. At birth the bodies of the bones are well formed. The secondary centres from 

 which the heads of the second, third, fourth, and fifth metacarpals and the base of the 

 first are developed, appear about the third year, and usually completely fuse with the 

 shafts about the age of twenty. There may be an independent centre for the styloid 

 process of the third metacarpal, and there is usually a scale-like epiphysis on the head of 

 the first metacarpal which makes its appearance about eight or ten, and rapidly unites 

 with the head. The occurrence of a basal epiphysis in the second metacarpal bone has 

 been noticed. 



Phalanges Digltorum Manns. 



The phalanges or finger bones are fourteen in number three for each finger, 

 and two for the thumb ; and they are named numerically in order from the 

 proximal toward the distal ends of the fingers. 



Phalanx Prima. The first phalanx, the longest and stoutest of the three, has a 

 semi-cylindrical body which is curved slightly forwards. The volar surface is flat, 

 and bounded on either side by two sharp borders to which the fibrous sheath of the 

 flexor tendons is attached. The dorsal surface, convex from side to side, is overlain 

 by the extensor tendons. The proximal end, considerably enlarged, has a simple oval 

 concave surface, which rests on the head of its corresponding metacarpal bone. On 

 either side of this the bone displays a tubercular form, and affords attachment to 



