Anatomy of the Cat

by Jacob Reighard; H. S. Jennings

(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.