112 THE BONES. 



two narrow, rousrhened surfaces, parallel and elongated vertically, commencing 

 near the superior extremity, to disappear a little below the middle of the bone ; 

 these surfaces are held in apposition to the rudimentary metacarpals, by an 

 interosseous ligament which is often ossified in old Horses. The borders — external 

 and internal — are very thick, round, and smooth, like the anterior face. 



Extremities. — The superior is flattened before and behind, and presents. 1. 

 Above, an undulating articular surface, formed by the union of several flat facets 

 more or less inclined towards each other ; they articulate with all the lower row 

 of carpal bones. 2. Anteriorly and inwardly, a tuberosity for muscular insertion. 

 3. Posteriorly — and directly above the roughened surfaces of the posterior face — 

 four small diarthrodial facets in pairs, and running into the larger articular 

 surface by their superior border ; they are adapted to similar facets on the 

 rudimentary metacarpals. The inferior extremity, elongated transversely, articulates 

 with the first phalanx and the sesamoid bones by an articular sm-face — convex 

 from before to behind — which is composed of tivo lateral condyles separated by a 

 median spine. The two condyles would be exactly alike, if the antero-posterior 

 diameter of the external condyle were not less extensive than that of the opposite 

 condyle. Both are hollowed on the sides by an excavation for the attachment 

 of ligamentous fasciculi. 



Structure and development. — The principal metacarpal is one of the most 

 compact bones in the body. It is developed from two centres of ossification, 

 one of which is for the inferior extremity. 



RuDiMENTAEY METACARPALS (ossa metacctrpi pavva). — The two rudimentary 

 (sm.aU) metacarpal (or splint) bones are elongated, and placed against the 

 posterior face of the principal bone, one without, the other within. Each is in 

 the form of an inverted pyramid, and exhibits a middle part and two extremities. 



Middle portion. — Prismatic and triangular, this ofl^ers : 1. Three faces — an 

 external, smooth and rounded from one border to the other ; an internal, plane, 

 and equally smooth ; an anterior, covered with asperities to give attachment to 

 the interosseous ligament uniting the lateral metacarpal bone to the median. 

 2. Three salient borders which markedly separate the faces from each other. 



Extremities. — The superior, the largest, is named the head, and shows : above, 

 a diarthrodial facet which articulates with one or two bones of the inferior row 

 of the carpus ; in front, other two small facets continuous with the preceding, 

 and in contact with similar facets on the large metacarpal bone ; on the other 

 points of its periphery are rugosities for the attachment of ligamentous and 

 tendinous fibres. The inferior extremity only reaches to about the lower fourth 

 of the large metacarpal bone, and terminates in a small button-shaped enlarge- 

 ment, which is never fused with the latter. 



The two small metacarpals, although very much alike, may yet be easily 

 distinguished from each other. For instance, the internal bone is always the 

 thickest, and often the longest ; besides, the superior articular surface of its 

 head results from the union of the two facets articulating with the two last 

 carpal bones of the lower tier. 



Structure and development. — Of a somewhat compact texture, like all the long 

 bones, these have no medullary canal, and are developed from only one ossifying 

 centre. Not unfrequently, however, the tubercle is formed from a special centre. 



The metacarpus of the Ass is recognized by : 1. The great inequality of the 

 small metacarpals. 2. The thickness of the large metacarpal (the width is to the 

 thickness as 1-35 : 1 in the A&s, and as 1*53 : 1 in the Horse). 3. The depth of 



