110 THE BONES. 



articular facets : the superior, concave, corresponds to the radius ; the inferior, 

 convex, is in contact with the second bone of the upper row. 



The other three bones of this row increase in volume from without to within. 



The second bone, os pyramidalis, or cuneiform, articulates with the radius, 

 the first bone of the lower row, the third of the upper, and the supercarpal 

 bone ; it has, in all, five articular facets. 



The third, or os lunctre, has six facets, and is united below to the first and 

 second bones of the second row. 



The fourth, or os scajjhoides — the most voluminous of the row— has only four 

 facets, and articulates by its inferior face with the os magnum and trctpezoides. 



Collectively, the second, third, and fourth bones of the upper row form two 

 articular surfaces. 



The superior, or radial articular surface, is very irregular ; but in examining 

 it from without to within, there may be observed : 1. A glenoid cavity on the 

 cuneiform bone. 2. In front, a transversely elongated condyle on the lunar and 

 scaphoid bones. 3. A groove placed behind the preceding condyle. 



The inferior articulating surface, which corresponds to the second row, is 

 constituted by several undulated facets ; it is convex outwardly and in front, 

 concave posteriorly and inwardly. 



Bones of the Infekior or Metacarpal Row (Figs. 68, 69). — The thick- 

 ness of these bones decreases from without to within. 



The first, unciform, or hookbone {os hainatum), has four diarthrodial facets, 

 and responds, above, to the two first bones of the superior row ; below, to the 

 first and second metacarpals. 



The second, os magnum, or os capitatum — ^the largest — has seven articular 

 facets, three of which are on the interno-lateral face. It articulates, above, with 

 the lunar and scaphoides ; below, with the principal metacarpal and the internal 

 rudimentary metacarpal. 



The third, or trapezoides — the smallest — is provided with five facets, and is 

 in contact with the scaphoides above, and the middle and internal metacarpals 

 below. 



Collectively, these bones of the lower row form two large diarthrodial 

 surfaces. The upper surface responds to the bones of the upper row, and is 

 constituted in front, and from without to within, by a small condyle and two 

 glenoid cavities ; behind, by two isolated condyles, formed by the os magnum 

 and the trapezoides. The inferior articular surface is only formed by more or 

 less long and plane facets, which incline towards each other. It articulates with 

 the three portions of the metacarpus. 



Structure and deveJopm£nt. — Each carpal bone is formed by a nucleus of 

 close spongy substance, enveloped in a very thick layer of compact tissue. Each 

 is developed from a single centre of ossification. 



The carpal bones of the Ass much resemble those of the Horse, but the 

 upper face of the Junar is more concave ; the facet of the cuneiform, for the 

 pisiform, is less concave, but larger and triangular, and is always separated from 

 the superior articular surface by a rough, wide, and deep groove. The pisifot m 

 is more circular than that of the Horse, and is modified in shape to correspond 

 with the cuneiform ; while the inferior surface of the os magnum is almost plane 

 in the Ass, but in the Horse it is cut into a condyle in front and a glenoid cavity 

 behind. Lesbre states that the trapezium is more frequent in the carpus of the 

 Ass than the Horse. 



