THE METACARPUS. - 205 
has only one lateral articular facet on its base, namely, that on its radial side for 
the fourth metacarpal. The carpal articular surface is saddle-shaped, and there is 
Radial 
side 
r 
ave Metacarpal 
Unciform —_ 
7 > IIL. Metacarpal 
V. Metacarpal Scare 
—Os mag ‘ ee 
CIS ME Ie Fig. 153.—FirrH METACARPAL BONE. 
Fic. 152.—FourTH METACARPAL BONE. 
a tubercle on the ulnar 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 
palmar surfaces of the metacarpals, those of the four inner bones being directed upwards towards 
the base or carpal end, differing in this respect from that of the first metacarpal, which is directed 
downwards towards the head or phalangeal extremity. The opening of the latter canal usually 
hes to the wlnar side of the palmar aspect of the shaft. 
Architecture.—Similar in arrangement to that of long bones generally, though it may be 
noted that the compact walls of the shaft are thicker in proportion to the length of the bone 
than in the other long bones of the upper extremity. 
Variations.—As previously stated (ante, p. 202), the styloid apophysis of the third meta- 
carpal bone appears as a separate ossicle in about 1°8 per cent. of cases examined (“ Fourth Annual 
Report of the Committee of Collect. Invest. Anat. Soc. Gt. Brit. and Ireland,” Journ. Anat. and 
Physiol. vol. xxviii. p. 64). In place of being united to the third metacarpal, the styloid 
apophysis may be fused with either the os magnum or the trapezoid, under which conditions the 
base of the third metacarpal bone is without this characteristic process. 
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 inner metacarpals, for whilst the shaft and head of the first metacarpal 
are developed from the primary ossific centre, and its base from a secondary epiphysis, in 
the case of the second, third, fourth, and fifth metacarpals, the shafts and bases are de- 
veloped from the primary centres, the heads in these instances being derived from the 
secondary epiphyses. 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. 
The primary centres for the shafts and bases of the second, third, fourth, and fifth meta- 
carpals appear in that order during the ninth or tenth week of intrauterine life, some 
little time after the terminal phalanges have begun to ossify, that for the shaft and head 
of the metacarpal bone of the thumb a little later. At birth the shafts 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 
i i i, i i ee, ee eee 
