Part 22

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.

FIG. 54.—CARPAL BONES OF HORSE, WITH ADJACENT ENDS OF RADIUS AND METACARPUS; ANTERIOR VIEW. THE ACCESSORY AND FIRST CARPAL BONES ARE NOT SHOWN. ]

FIG. 55.—CARPAL ARTICULAR SURFACE OF RADIUS AND PROXIMAL ARTICULAR SURFACES OF CARPAL AND METACARPAL BONES, LEFT SIDE. THE ACCESSORY AND FIRST CARPAL BONES ARE NOT SHOWN.

Cr, Radial carpal; Ci, intermediate carpal; Cu, ulnar carpal; C2, C3, C4, second, third, and fourth carpals; Mc.II, second or inner small metacarpal bone; Mc.III, third or large metacarpal bone; Mc.IV, fourth or outer small metacarpal bone; 1, 2, grooves for tendons of anterior extensor and extensor carpi radialis; 3, metacarpal tuberosity. Arrows indicate relations of facets. Short arrow points to facet on ulnar carpal for articulation with accessory carpal. (After Schmaltz, Atlas. d. Anat. d. Pferdes.) ]

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.

THE METACARPUS

Three =metacarpal bones= (Ossa metacarpalia) are present in the horse. Of these, only one, the third or large metacarpal bone, is fully developed and carries a digit; the other two, the second and fourth, are much reduced, and are commonly called the internal and external small metacarpal or “splint” bones.

FIG. 56.—DISTAL ROW OF CARPAL BONES, METACARPUS, FIRST AND SECOND PHALANGES, AND PROXIMAL SESAMOID BONES OF HORSE; POSTERIOR VIEW, LEFT SIDE.

The fourth (external) metacarpal bone is wrongly numbered as I. (After Schmaltz, Atlas. d. Anat. d. Pferdes.) ]

FIG. 57.—LEFT METACARPAL AND DIGITAL BONES OF HORSE, INTERNAL VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

THE LARGE METACARPAL BONE

This (Os metacarpale tertium) is a very strong long bone, placed vertically between the carpus above and the first phalanx below. It consists of a shaft and two extremities.

The =shaft= (Corpus) is semicylindrical, 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 third 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 bone; the oblique outer part, separated from the preceding by 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 suspensory 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.