The Carpus

The carpus of the horse consists of seven or eight bones (Ossa carpi) arranged in two rows, proximal or antibrachial, and distal or metacarpal. The (abbreviated) names and relative positions of the bones of the left carpus as seen from in front are indicated below.

Proximal Row:

Radial Intermediate Ulnar Accessory

Distal Row:

First Second Third Fourth

The Radial Carpal Bone

The radial carpal bone (Os carpi radiale, scaphoid) is the largest bone of the upper row; it is somewhat compressed laterally, and is clearly six-sided. The superior or proximal surface is convex in front, concave behind, and articulates with the inner facet on the distal end of the radius. The inferior or distal surface is also convex in front and concave behind; it articulates with the second and third carpal bones. The external surface bears upper and lower facets on its anterior part for articulation with the intermediate; between and behind these it is excavated and rough. The anterior or dorsal surface is rough and slightly convex. The internal surface and the posterior or volar surface are rough and tuberculate.

The Intermediate Carpal Bone

The intermediate carpal bone (Os carpi intermedium, semilunar, lunar) is somewhat wedge-shaped, wider in front than behind. The superior or proximal surface is saddle-shaped, and articulates with the middle facet on the distal end of the radius. The inferior or distal surface is smaller, convex in front, concave behind, and articulates with the third and fourth carpal bones. The internal surface has upper and lower facets for articulation with the radial carpal, and between these it is excavated and rough. The external surface is similar to the preceding and articulates with the ulnar carpal. The anterior or dorsal surface is rough and slightly convex. The posterior or volar surface bears a tuberosity on its lower part.

The Ulnar Carpal Bone

The ulnar carpal bone (Os carpi ulnare, cuneiform) is the smallest and most irregular bone of the upper row. The superior or proximal surface is concave, and fits the lower part of the outer facet on the distal end of the radius. The inferior or distal surface is oblique and undulating for articulation with the fourth carpal bone. The internal surface has upper and lower facets for articulation with the intermediate. The anterior or dorsal and external surfaces are continuous, convex, and rough. The posterior or volar surface is oblique, and bears a concave facet for articulation with the accessory carpal bone; below this is a tubercle.

The Accessory Carpal Bone

The accessory carpal bone (Os carpi accessorium, pisiform) is situated behind the ulnar carpal bone and the outer part of the distal end of the ulna. It is discoid and presents for description two surfaces and a circumference. The internal surface is concave and forms the outer wall of the carpal groove. The external surface is convex and rough; a smooth groove for the outer tendon of the flexor carpi externus crosses its anterior part obliquely downward and slightly forward. The anterior border bears two facets; the upper one is concave and articulates with the back of the outer facet on the distal end of the radius; the lower one is convex and articulates with the ulnar carpal bone. The remainder of the circumference is rounded and rough.

The accessory does not directly bear weight, and may be regarded as a sesamoid bone interposed in the course of the tendons of the middle and external flexors of the carpus, which it enables to act at a mechanical advantage. The posterior border furnishes attachment to the transverse carpal ligament, which completes the carpal canal for the flexors of the digit.

The First Carpal Bone

The first carpal bone (Os carpale primum, trapezium) is a small inconstant bone, commonly about the size and shape of a pea, which is situated in the lower part of the internal lateral ligament behind the second carpal bone.

This bone appears to be absent on both sides in about half of the cases; in a good many subjects it is present on one side only. In size it varies from a minute nodule to a discoid mass 10 to 12 mm. in length. In exceptional cases it articulates with both the second carpal and the second (inner) metacarpal bone, in other cases with the former only, but in the majority of specimens no articular facet is present.

The Second Carpal Bone

The second carpal bone (Os carpale secundum, trapezoid) is the smallest constant bone of the lower row, and is irregularly hemispherical in shape. The superior or proximal surface is a convex facet which is continued upon the posterior or volar surface and articulates with the posterior part of the radial carpal. The external surface faces obliquely outward and forward, and bears three facets for articulation with the third carpal bone. The anterior or dorsal and the internal surface bear a tuberosity to which the lateral ligament is attached. The inferior or distal surface is articular and consists of a large flattened facet for the inner (second) metacarpal bone, and a small one for the large (third) metacarpal bone. Some specimens have a small facet on the lower part of the posterior surface which articulates with the first carpal bone.

The Third Carpal Bone

The third carpal bone (Os carpale tertium, os magnum) is much the largest bone of the lower row, forming more than two-thirds of the width of the latter. It is flattened from above downward, and is twice as wide in front as behind. The superior or proximal surface consists of two facets separated by an antero-posterior ridge; the inner facet is concave and articulates with the radial carpal; the outer facet—for the intermediate carpal—is concave in front and convex behind, where it encroaches on the posterior surface. The inferior or distal surface is slightly undulating, and articulates almost entirely with the large (third) metacarpal bone, but it usually bears a small oblique facet at its inner side for the inner (second) metacarpal, and there is commonly a non-articular depression externally. The internal surface faces backward and inward, and bears three facets for articulation with the second carpal, between which it is excavated and rough. The external surface has two facets for articulation with the fourth carpal, and is depressed and rough in its middle. The anterior or dorsal surface is convex and is crossed by a rough transverse ridge. The posterior or volar surface is relatively small, and is rounded; its upper part is encroached upon by the superior articular surface, below which it is rough.

The Fourth Carpal Bone

The fourth carpal bone (Os carpale quartum, unciform) is somewhat wedge-shaped, and is readily distinguished from the second by its greater size and its posterior tubercle. The superior or proximal surface articulates with the intermediate and ulnar; it is convex and curves outward, backward, and downward, encroaching on the external and posterior surfaces. The inferior or distal surface bears two inner facets for the large (third) metacarpal and an outer one for the external (fourth) metacarpal bone. The internal surface has two or three facets for articulation with the third carpal, between which it is excavated and rough. The anterior or dorsal surface is convex and rough. The external surface is small, being encroached upon by the superior articular surface. The posterior or volar surface bears a tubercle on its lower part.

The Carpus as a Whole

The bones of the carpus, exclusive of the accessory, form an irregular quadrangular mass, the width of which is about twice the height or the antero-posterior diameter. The anterior or dorsal surface is convex from side to side, depressed along the line of junction of the two rows, and prominent below. The posterior or volar surface is in general slightly convex, but very irregular. It forms with the accessory the carpal groove (Sulcus carpi), which in the recent state is rendered smooth by the posterior ligament; it is converted into the carpal canal (Canalis carpi) for the flexor tendons by the transverse carpal ligament, which stretches across from the accessory bone to the inner side. The proximal surface is widest internally and is elevated in front, concave behind; it is entirely articular and adapted to the carpal articular surface of the radius. The distal surface is also articular and is irregularly faceted in adaptation to the surfaces of the metacarpal bones; each of the lower bones usually articulates with two metacarpal bones, but sometimes the third does not bear on the inner metacarpal bone. The lateral surfaces are both irregular and rough, the internal one being the wider. With the exception of the accessory, ulnar, and second, each bone articulates with two bones of the other row.

Development.—Each ossifies from a single center.