2i6 A MANUAL OF ANATOMY 



The Sesamoid Bones. — These are usually two in number, 

 and are placed on the palmar aspect of the head of the first 

 metacarpal bone. They are originally nodules of cartilage, one of 

 which is developed in the tendon of insertion of the superficial head 

 of the flexor brevis pollicis, and the other in that of the adductor 

 obliquus pollicis. Similar ossicles are sometimes met with on the 

 palmar aspects of the heads of the second and fifth metacarpal 

 bones; 



Ossification of the Metacarpal Bones and Phalanges. — Each of these bones 

 ossifies in cartilage from one primary, and one secondary, centre. The primary 

 centre appears about the ninth week of intra-uterine life at the middle of the 

 shaft, except in the case of the ungual phalanges, in which it appears at the 

 distal end. In the inner four metacarpal bones the epiphysis is placed at the 

 distal end or head, but in the first metacarpal bone (that of the thumb), and 

 in all the phalanges, it is placed at the proximal end or base. The secondary 

 centre in each case appears from the third to the fifth year, and the epiphyses 

 are united to their respective shafts about the twentieth year. The first 

 metacarpal bone has sometimes a thin distal epiphysis, as well as a proximal, 

 which begins to ossify about the eighth year, and joins soon thereafter ; and 

 the second metacarpal bone has sometimes a proximal epiphysis, as well as a 

 distal. The first metacarpal bone, being a phalanx as regards its epiphysial 

 ossification, is regarded by some as the proximal phalanx of the thumb, 

 according to which view the bone wanting in the thumb is the metacarpal 

 bone. The stjdoid process of the third metacarpal bone has sometimes a 

 special centre, and, instead of joining the rest of the bone, it may remain 

 persistent as a supernumerary bone of the carpus, or it may unite with the 

 postero-internal angle of the trapezoid. 



The Hand as a Whole. 



The hand presents two surfaces — dorsal and palmar — -two borders 

 ■ — outer and inner — and two extremities — proximal and distal. 



The dorsal surface is convex, both longitudinally and trans- 

 versely. 



The palmar or volar surface is concave, both longitudinally and 

 transversely. 



The outer or radial border is in line with the thumb or pollex, 

 and is constructed by the scaphoid or navicular, trapezium, and the 

 phalanges of the thumb. 



The inner or ulnar border is in line with the little finger, and is 

 constructed by the cuneiform or pyramidal, unciform, and the 

 phalanges of the Httle finger. This border presents the tubercle on 

 the inner side of the proximal end or base of the fifth metacarpal 

 bone for the tendon of the extensor carpi ulnaris. 



The proximal extremity is formed by the scaphoid or navicular, 

 semilunar, and cuneiform or pyramidal bones. 



The distal extremity is formed by the third or ungual phalanges. 



It is to be noted (i) that the thumb is the shortest of all the 

 digits ; (2) that it stands off at an angle from its fellows ; and (3) that 

 it occupies a more anterior plane than its fellows, in which respects 

 it presents a striking contrast to the great toe. 



The palmar aspect of the carpus is rendered concave by the 



