THOEACIC LIMBS 595 



The distal surface is transversely elongated, and in the main 

 cave from the dorsal to the palmar side, and convex from side to side. 

 The radial half is depressed below the ulnar half. The distal surface 

 is smooth, for articulation with the five metacarpal bones. The dorsal 

 and palmar surfaces meet at the obscurely defined radial and ulnar 

 borders. 



The seven carpal bones are arranged in two transverse rows, with 

 three in the proximal row and four in the distal row. Counting from 

 the radial side, the proximal row is composed of the scapho-lunar, the 

 cuneiform, and the pisiform, and the distal row is composed of the 

 trapezium, the trapezoid, the os magnum, and the unciform. 



These carpal bones are modified from a cubical type, and hence 

 present six surfaces, proximal, distal, dorsal, palmar, radial, and ulnar. 



Of these surfaces the dorsal and palmar are free from articular 

 facets, as they do not join other bones, but form the back and front 

 of the wrist. 



The proximal and distal surfaces articulate with the lower ends of 

 the bones of the forearm or the upper ends of the metacarpal bones, or 

 form the articulation between the two rows. 



The radial and ulnar surfaces are articular surfaces on some of 

 the carpal bones ; on others, depending on the position of the in- 

 dividual bone, they assist in producing the radial and ulnar edges of 

 the wrist. 



The dorsal surface of the carpus is convex because the dorsal sur- 

 faces of the constituent bones are larger than their palmar surfaces, 

 and because the bones themselves are arranged as the arc of a circle 

 whose centre would lie without the general palmar surface of the 

 carpus. It will also be noticed that the proximal ends of the carpal 

 bones are larger than their distal ends, and that the bones appear to 

 radiate from a centre in the proximal border of the wrist. The small 

 hemispherical ossicle on the radial side of the proximal row is not con- 

 sidered an element of the wrist, but is classed among the sesamoid 

 bones. 



Ossification. Each carpal bone develops from a single centre, 

 except the scapho-lunar, which is a composite of three bones, two of 

 which, the scaphoid and the lunar, are found as distinct bones in man, 

 and the third, the os centrale, is present as a separate element in a 

 number of other mammals. 



