Bones of the Upper Extremity. 109 



The five ossa metacarpalia (see also Figs. 146, 148 151) are short cylindrical 

 bones, each of which is divisible into a shaft, or corpus, a proximal extremity, basis, and a 

 distal extremity, capitulum. The whole bone is curved, so as to be slightly convex toward 

 the back of the hand. 



The corpus or shaft is triangular so that one side looks dorsalvvard, the other two 

 radial- and ulnarward ; the latter surfaces meet in an angle directed volarward. On the volar 

 surface is a large foramen nutricium, which leads into a canalis nutricius directed in the 

 thumb distalward, in the second to the fifth finger, proximalward. 



The basis is somewhat broadened and presents on its proximal surface an articular 

 'surface for connection with the carpus, between the second and the fifth finger also small 

 articular surfaces on the radial and ulnar surface for the neighboring metacarpal bones. The 

 basis ossis metacarpalis I, has a saddle-shaped joint surface (for the os multangulum majus) 

 but no lateral articular surfaces. On the basis ossis metacarpalis III, from the radial part of 

 the dorsal surface, a blunt process, the processus styloideus ossis metacarpalis III, (insertion 

 of the in. extensor carpi radialis brevis) projects toward the carpus. The basis ossis meta- 

 carpalis V presents proximalward a slightly saddle-shaped articular surface and a flat articular 

 surface on the radial side. 



Each capitulum possesses a spherical articular surface, covered with cartilage, which 

 extends upon the volar more than upon the dorsal surface. On the two sides there are 

 depressions and roughened areas for the ligaments of the joint. 



The four spaces lying between the ossa metacarpalia (see Figs. 148 and 149) are called 

 spatia interossea metacarpi. The first, between thumb and index finger is the broadest. 

 They are all filled out by the mm. interossei. 



The phalanges digitorum (see also Figs. 148 and 149) are three in number from the 

 second to the fifth finger ; while on the thumb there are two ; they are called phalanx prima, 

 secunda, lerlia. Each phalanx is a short cylindrical bone which is divisible into a shaft or 

 corpus, a proximal end, basis, and a distal end. 



The corpus is flat volarward and curved dorsalward so as to be convex in transverse 

 direction. A large foramen nutricium on its volar surface leads into a canalis nutricius 

 directed distalward. The basis is somewhat broadened and possesses a deep articular surface, 

 covered with cartilage, which on the first phalanx is simple, on the others is divided by a 

 ridge. The distal end of the first phalanx of the thumb and also of the first and second 

 phalanx of the remaining fingers has a trochlea, a small transverse roller surface covered with 

 cartilage, with a groove in the middle; on the sides lie small depressions for the attachment 

 of the ligaments of the joint. On the terminal phalanx the distal end is transformed to a 

 broad flat rough area, the tuberositas uncjuicularis. 



Ossa sesamoidea (sesamoid bones) (see Fig. 148) are small masses of bone which, 

 embedded in tendons or ligaments, lie upon the other bones. There are constantly five or 

 almost constantly five such on the volar surface, two at the metacarpophalangeal joint of the 

 thumb, one at the interphalangeal joint of the thumb and one each at the metacarpophalangeal 

 joint of the second and of the fifth finger. 



