Part 16

I, radius; II, ulna. a, head of radius; b, neck; c, bicipital tuberosity; d, articular circumference; e, facet for radial fossa of humerus; f, styloid process of radius; g, facet for articulation of scapholunar bone; h, semilunar (or great sigmoid) notch of ulna; i, coronoid process; j, olecranon; k, area for insertion of brachialis and clavobrachial muscles; l, rough area for attachment of interosseous membrane; m, styloid process of ulna.]

The dorsal surface of the distal end is marked by longitudinal grooves for tendons, and its lateral or ulnar surface bears a concave facet for articulation with the ulna.

=Ulna= (II, Figs. 49 and 50).--The ulna is a long slender bone, flattened mediolaterad. It is enlarged at its proximal end and becomes gradually smaller toward the distal end.

The proximal end is marked ventrally by a deep excavation, the =semilunar notch=, or =great sigmoid cavity= (h). By the saddle-shaped articular surface of the semilunar notch it articulates with the trochlea. This articular surface is divided into two parts by a transverse non-articular area. The distal boundary of the semilunar notch is a blunt process, the =coronoid= process (i), which bears on its lateral surface a concave facet, the =radial notch=, for the head of the radius.

The portion of the bone proximad of the semilunar notch is called the =olecranon= (j). It fits into the olecranon fossa of the humerus when the arm is straightened, and is rough at its end for the insertion of tendons.

The body of the ulna becomes triangular distad. The distal end is slightly larger than the shaft just proximad of it, and bears on its radioventral side a hemispherical head for articulation with the radius. Distad of the head the bone continues as the flattened =styloid= process (m), which projects distad from its dorsolateral side and is smooth on the medial side of its apex, for articulation with the cuneiform bone of the wrist.

=Carpus= (Fig. 51).--The carpus (wrist) consists of seven bones arranged in two rows, three in the proximal row and four in the distal row. Beginning on the medial side of the hand (thumb or radial side), the first bone in the proximal row is the =scapholunar= (a) (equal to the scaphoid or navicular and lunar of the human hand). It articulates with the radius. The next is the =cuneiform= (b), articulating with the styloid process of the ulna, and the next, which is attached to the cuneiform and projects freely ventrad, is the =pisiform= (c).

In the distal row the bone on the radial side is the =trapezium= (d); the next is the trapezoid (e), the next the os magnum (f), and the last the unciform (g). The distal row articulates with the metacarpals or bones of the palm of the hand (1-5).

In the kitten the scapholunar is represented by three bones, the =scaphoid= or navicular, on the radial side, the =lunare=, between the scaphoid and the cuneiform, and a =centrale=, which lies distad of the other two.

Scapholunar Bone. Os scapholunaris (Fig. 51, a).--The scapholunar is a quadrangular bone with the ventroradial angle produced into a blunt process. Its proximal surface is smooth and articulates with the distal end of the radius. The distal end is marked by oblique ridges and articulates with the unciform, os magnum, trapezoid, and trapezium. The ulnar surface articulates with the cuneiform, and the dorsal surface of the ventroradial process with the radial sesamoid.

Cuneiform Bone. (Os triquetrum BNA) (Fig. 51, b).--The cuneiform bone has the form of a flattened pyramid. Its base articulates with the unciform, its proximoulnar surface with the pisiform except at its dorsal margin, where it articulates with the styloid process of the ulna. On its proximoradial surface is a smooth facet for articulation with the scapholunar.

Pisiform Bone. Os pisiforme (Fig. 51, c).--The pisiform bone is about twice as long as broad, with enlarged ends. Its dorsal end articulates with the cuneiform, and on its proximal surface, separated from the above by a smooth ridge, is a smooth facet for articulation with the styloid process of the ulna.

a, scapholunar bone; b, cuneiform; c, pisiform; d, trapezium; e, trapezoid; f, os magnum; g, unciform; h, radial sesamoid; i, proximal phalanges; j, second phalanges; k, distal phalanges; 1, 2, 3, 4, 5, metacarpals in order from the radial side.]

Unciform Bone. (Os hamatum BNA) (Fig. 51, g).--The unciform is a wedge-shaped bone with the apex of the wedge directed proximad, and smooth for articulation with the scapholunar. By a part of its ulnar surface it articulates with the cuneiform, and by its radial surface with the os magnum. Its distal end articulates with the fourth and fifth metacarpals.

Os magnum. (Os capitatum BNA) (Fig. 51, f).--The os magnum may be described as an oblong plate bearing on its proximal surface a semicircular ridge which crosses it diagonally. The proximal end of the bone articulates with the scapholunar. Its distal end articulates with the third metacarpal except near its ventroulnar angle, where it articulates with the fourth metacarpal. Its ulnar surface articulates with the unciform. Its radial border articulates with the trapezoid, the third metacarpal, and, by two facets, with the second metacarpal.

Trapezoid. (Os multangulum minus BNA.) (Fig. 51, e).--The trapezoid is somewhat wedge-shaped, with the apex of the wedge pointing ventrad. Its proximal side articulates with the scapholunar, its distal side with the second metacarpal, its ulnar side with the os magnum, and its radial side with the trapezium.

Trapezium. (Os multangulum majus BNA.) (Fig. 51, d).--The trapezium has the form of a triangular prism curved into a semicircle. The convex face looks proximad and articulates by its ventral half with the scapholunar. The ulnar surface articulates with the second metacarpal dorsally, and ventrally with the trapezoid. Its radial surface articulates with the first metacarpal.

=Bones of the Hand or Manus= (Fig. 51, 1-5).--The Metacarpals. Metacarpus.--The metacarpals are the five bones of the palm of the hand; they are numbered from one to five, beginning with the thumb. They are cylindrical elongated bones with enlarged ends. The distal end is called the head, and the proximal end the base. Each head bears a hemispherical articular facet which is marked over its ventral half by a prominent smooth ridge. The surface dorsad of the ridge articulates with the proximal head of a phalanx. The ridge and the surface at its sides are for a pair of sesamoid bones.

The first metacarpal (1) is the shortest. Its head is oblique, and it articulates by the ulnar half of its proximal surface with the trapezium (d); by the radial half with the radial sesamoid (h).

The second metacarpal (2) is marked on the proximal part of its dorsal surface by an oblique groove passing from the radial side distad to the ulnar side. The base articulates with the trapezoid (e). The ulnar surface of the proximal end articulates with the os magnum (f) and third metacarpal, while the radial surface articulates with the trapezium (d).

The third metacarpal (3) is the longest, and its base is rhomboidal with a projecting dorsoradial angle separated by a groove from the rhomboid surface. The proximal end articulates with the os magnum (f) and second metacarpal; the radial surface of the proximal end with the second, and the ulnar surface with the fourth, metacarpal.

The fourth metacarpal (4) has a base similar in form to that of the third, and when placed in position with the fifth the two form a hemispherical facet which articulates with the unciform (g) and os magnum (f). The fourth metacarpal articulates by its radial side with the third, and by its ulnar side with the fifth.

The fifth metacarpal (5) articulates by its proximal end with the unciform (g), and by the radial side of its proximal end with the fourth metacarpal.

Digits (Fig. 51).--The first digit of the hand is called the =pollex= (thumb), the second the =index=, the third the =medius=, the fourth the =annularis=, the fifth the =minimus=.

The first digit has two phalanges, each of the others three phalanges.

Phalanges (Fig. 51, i, j, k).--The phalanges of the proximal row (i) are elongated, flattened dorsoventrally and curved so as to be longitudinally convex dorsad. All have thickened ends. The proximal end is notched, and its proximal surface looks dorsad and is concave for the head of the metacarpal. The distal end is pulley-shaped, and the pulley surface extends farther on to the ventral than on to the dorsal surface, and serves for articulation with the middle phalanx.

The phalanges of the middle row (j) are like those of the proximal row, but shorter. The proximal surface is triangular and marked by a median facetted ridge. The whole surface is smooth and adapted to the distal end of the phalanx of the first row. The distal end is transversely elongated, so as to be cylindrical, and projects more toward the ulnar than toward the radial side. The distal phalanx (k) articulates with this cylinder so that when it is fully extended it lies on the ulnar side of the middle phalanx.

The distal phalanx (k) has the form of a quadrangular prism. It is excavated on its proximal surface for articulation with the middle phalanx. Its distal surface presents dorsad a deep excavation from the bottom of which arises a compressed plate of bone having the form of a bird’s beak. The depression receives the base of a claw, and the beak-like projection supports the claw.