We may include among the bones of the wrist the lower ends of the radius and ulna and the first row of bones of the carpus, - the scaphoid, lunate (semilunar), cuneiform, and pisiform.

Of the bones of the forearm - the radius and ulna - we have seen that at the elbow the ulna is the larger of the two. This is because the main function of the ulna is to act as a support to the parts beyond. The radius is intended mainly as a means of enabling the hand to perform the functions of pronation and supination. At the wrist we find the radius supporting the hand and consequently its lower end is large and well developed. The ulna, on the contrary, contributes but little to the support of the hand and does not even enter directly into the wrist-joint, as does the radius at the elbow-joint, but serves as a fixed point around which the radius rotates. The functional value of the ulna at the wrist is so much less than that of the radius as amply to account for its diminished size.

Fig. 340.   Posterior view of the lower end of the radius and ulna and the carpal bones.

Fig. 340. - Posterior view of the lower end of the radius and ulna and the carpal bones.

Lower End Of The Radius

The lower end of the radius is large and spongy. The compact tissue forms a quite thin superficial layer (Fig. 338). Its anterior surface is hollowed out to receive the pronator quadratus muscle, with a prominent articular edge to which is attached the anterior ligament (Fig. 339).

The posterior surface is convex and marked with a number of ridges with grooves between them which lodge the extensor tendons (Fig. 340). In its middle is a prominence, the dorsal radial tubercle, which marks the position of the extensor longus pollicis muscle. On its inner side is a concave articular facet, the ulnar notch (sigmoid cavity), for articulation with the ulna; it is plane from above downward, thus showing that it permits movement in one direction only, like a hinge.

Between the lower edge of the ulnar notch and the articular surface is a rough ridge that gives attachment to the triangular interarticular fibrocartilage.

The lower or radiocarpal articular surface slopes downward and outward to end in the styloid process, which is thereby placed lower than the styloid process of the ulna. The articular surface is divided into two facets: the outer is the smaller, is triangular in shape, and articulates with the navicular {scaphoid) bone; the inner or larger is quadrilateral and articulates with the lunate (semilunar) bone. The styloid process at its base or upper outer portion has inserted into it the tendon of the brachioradialis muscle. To its tip is attached the external lateral ligament.

The Ulna

The lower extremity of the ulna is rounded in shape, forming its head, with the styloid process projecting downward on its inner and posterior aspect To its tip is attached the internal lateral ligament. On its outer side is a rounded smooth surface for articulation with the ulnar notch of the radius. The inferior or articular surface is flat and rests on the flat interarticular fibrocartilage.

The navicular (scaphoid), lunate (semilunar), cuneiform, and pisiform bones form the first row of the carpal bones. The pisiform rests on the anterior surface of the cuneiform and does not enter into the articulations between the hand and bones of the forearm.

The navicular and lunate articulate directly with the lower end of the radius, but the cuneiform articulates with the under surface of the triangular interarticular fibrocartilage.