ABSTRACT
RADIUS-ULNA (FIGS. 35-37) The ulna is a long bone with a tapering shaft, bearing at its distal end in some animals a somewhat swollen articulation, called in humans the ulna head (Fig. 37). At the proxi mal end, the articulation for the trochlea of the humerus is carried in a deep notch, the trochlear notch (Fig. 36). To proximal of this extends a lever-like process for attachment of the triceps muscle, called the olecranon. In some animals, the distal end of the trochlear notch is extended into a prominent coronoid process. The radius runs alongside the ulna. In humans, the radius is able to rotate around the ulna, allowing the hand to be twisted from side-to-side. With the thumb out to lateral, the radius lies parallel to, and to lateral of, the ulna. With the thumb in to medial, the shaft of the radius crosses over to cranial of the ulna, so that the radius remains to lateral of the ulna at its proximal end, but to cranial and medial of the ulna at its distal end. In quadrupeds, the radius moves much less and is held in the crossed-over (thumb to medial) position. In most hoofed mammals (ungulates), the two bones are fused in this position and the ulna's shaft and distal end are very greatly reduced. The radius has a swollen proximal articular area which is called the radius head, whose form varies greatly. In animals with a separate radius and ulna, there are facets for articulation between the two bones on both the radius head and the trochlear notch area of the ulna. The distal end of the radius bears a complex surface for the joints with the carpal bones, which again varies widely in form. Animals with separated radius and ulna also have surfaces on the head of the ulna and the lateral surface of the distal radius, for articulation between them.
