THE SMALL METACARPAL BONES 99 



The shaft (Corpus) is semicylindrical, and presents two surfaces and two 

 borders. The dorsal surface is smooth, convex from side to side, and nearly 

 straight m its length. The volar surface is somewhat convex from side to side and, 

 with the small bones, forms a wide groove which lodges the suspensory ligament! 

 On either side of its proximal two-thirds it is roughened for the attachment of the 

 small metacarpal bones. The nutrient foramen occurs at the junction of the 

 proximal and middle thirds. The distal part is wider and flattened. The borders 

 are rounded. 



The proximal extremity (Extremitas proximalis s. basis) bears an undulating 

 articular surface adapted to the distal row of carpal bones. The greater part sup- 

 ports the third carpal bone; the oblique lateral part, separated from the preceding 

 by a ridge, articulates with the fourth, and a small facet for the second is usually 

 found at the medio-volar angle. On either side is a notch separating two small 

 facets which articulate with the proximal ends of the small metacarpal bones. 

 Toward the medial side of the dorsal surface is the metacarpal tuberosity, into 

 which the extensor carpi radialis is inserted. The volar surface is roughened for 

 the attachment of the suspensory ligament. 



The distal extremity (Extremitas distalis s. capitulum) presents an articular 

 surface for the first phalanx and the proximal sesamoid bones, which is composed of 

 two condyles, separated by a sagittal ridge; the medial condyle is slightly the larger. 

 On either side is a small fossa, surmounted by a tubercle, for the attachment of the 

 collateral ligaments of the fetlock joint. 



The large metacarpal is one of the strongest bones in the skeleton. The compact substance 

 IS specially thick m front and medially. The medullary cavity extends further toward the ends 

 than in most of the long bones of the horse and there is Httle spongy substance. 



The Small Metacarpal Bones 



These, numerically the second and fourth metacarpal bones (Ossa metacarpalia 

 secundrun et quartum),are situated on either side of the volar surface of the large 

 metacarpal bone, and form the sides of the metacarpal groove. Each consists of a 

 shaft and two extremities. 



The shaft (Corpus) is three-sided and tapers to the distal end. It is variably 

 curved, convex toward the middle line of the limb. The attached surface is flat- 

 tened and is rough, except in its lower part; it is attached to the large metacarpal 

 bone by an interosseous hgament, except near the distal end. The dorsal or ab- 

 axial surface is smooth and rounded from side to side above, grooved below. The 

 volar or axial surface is smooth and concave from edge to edge, except below, 

 where it forms a rounded edge. 



The proximal extremity (Extremitas proximalis s. basis) is relatively large. In 

 the case of the medial bone it usually bears two facets above which support the sec- 

 ond and third carpal bones, while the lateral bone has here a single facet for articu- 

 lation with the fourth carpal bone. Each has also two facets for articulation with 

 the large metacarpal, and is elsewhere roughened for the attachment of ligaments 

 and muscles. The medial bone may present a small facet behind for the first carpal 

 bone. 



The distal extremity (Extremitas distalis s. capitulum) is usually a small nod- 

 ule, which projects to a variable extent in different subjects, and is easily felt in the 

 living animal. It is situated two-thirds to three-fourths of the way down the region. 



The small metacarpal bones vary much in length, thickness, and curvature. In the ma- 

 jority of cases the medial bone is the longer; in other subjects the lateral one is the longer or there 

 is no material difference. Sometimes the curvature is very pronounced, so that the distal end 

 causes a decided projection. The distal end is very variable in size and may be a mere point; in 

 other oases, especially in large draft horses, it may present a prolongation which is regarded as the 

 vestige of the digital skeleton. 



Development. — The large metacarpal bone ossifies from three centers. The 



