88 THE SKELETON OF THE HORSE 



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

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

 nearly straight in its length. The posterior or 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 upper two-thirds it is roughened for the 

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

 junction of the upper and middle thirds. The lower tliird is wider and flattened. 

 The borders are rounded. 



The proximal extremity (Basis) bears an undulating articular surface adapted 

 to the lower row of carpal bones. The greater part supports the third carpal l:)one; 

 the oblique outer part, separated from the preceding l3y a ridge, articulates with 

 the fourth, and a small facet for the second is usually found at the postero-internal 

 angle. On either side is a notch separating two small facets which articulate with 

 the proximal ends of the small metacarpal bones. Toward the inner side of the 

 anterior surface is the metacarpal tuberosity, into which the extensor carpi radialis 

 is inserted. The posterior surface is roughened for the attachment of the sus- 

 pensory ligament. 



The distal extremity (Trochlea 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 inner condyle is slightly the larger. 

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

 lateral ligaments of the fetlock joint. 



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

 is specially thick in front and internally. The medullary canal extends further toward the ends 

 than in most of the long bones of the horse and there is little spongy bone. 



The Small Metacarpal Bones 



These are situated on either side of the posterior 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 anterior surface is flattened 

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

 by an interosseous ligament, except near the distal end. The abaxial surface is 

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

 smooth and concave from edge to edge, except below, where it forms a rounded 

 edge. 



The proximal extremity or head (Basis) is relatively large. In the case of the 

 inner bone it usually bears two facets above which support the second and third 

 carpal bones, while the outer bone has here a single facet for articulation with the 

 fourth carpal bone. Each has also two facets for articulation with the large meta- 

 carpal, and is elsewhere roughened for the attachment of ligaments and nmscles. 

 The inner bone may present a small facet behind for the first carpal bone. 



The distal extremity (Capitulum) is usually a small nodule, 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 inner bone is the longer; in other subjects tlio outer 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 varialMe in size and may be a mere point. 



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

 proximal extremity unites with the shaft before birth, the distal extremity toward 

 the middle of the second year. The small metacarpal bones ossify from two cen- 

 ters, one of which is for the proximal extremity. Their distal ends are cartilaginous 



