The two bones of the fore-arm, the _radius_ and _ulna_, are, in most mammals, entirely separate from each other, but in certain of the more highly specialized hoofed animals are immovably coössified. Primitively, the two bones were of nearly equal size, but in most of the mammalian orders there is a more or less well-defined tendency for the radius to enlarge at the expense of the ulna. These bones are normally crossed, the radius being external at the upper end and passing in front of the ulna to the inner side of the arm. The radius varies considerably in form in accordance with the uses to which the hand is put; if the capacity of rotation is retained, the upper end, or head, of the radius is small, circular or disk-like, covering little of the humeral trochlea, but when the head of the radius is broadened to cover the whole width of the humerus, then all power of rotation is lost. (Cf. Figs. 28 and 29.) As a rule, the radius broadens downward and covers two-thirds or more of the breadth of the wrist-bones. [Illustration: FIG. 30.—Coössified bones of left fore-arm of Horse, front side. For most of its length, the ulna is concealed by the radius.] [Illustration: FIG. 31.—Left fore-arm bones of the Tapir (_Tapirus terrestris_). _R._, radius. _U._, ulna. _h._, head of radius. _h′._, sigmoid notch of ulna. _ol._, olecranon. N.B. This figure is on a much larger scale than Fig. 30.] The ulna is longer than the radius, its upper end being extended into a heavy process, the _olecranon_, or _anconeal process_, into which is inserted the tendon of the great triceps muscle, the contraction of which straightens the arm; this process is the bony projection at the back of the elbow-joint. Below the olecranon is a semicircular articular concavity, which embraces the humeral trochlea and its upper angle fits into the anconeal fossa of the humerus. The ulna contracts and grows more slender downwards and its lower end covers but one of the wrist-bones. While in the more primitive mammals, and in those which retain the power of rotating the hand, the ulna has nearly or quite the same thickness as the radius, it is often much more slender and in the more highly specialized of the hoofed animals, such as the horses, camels and true ruminants, the radius carries the entire weight and the ulna has become very slender, more or less of its middle portion is lost and the two ends are coössified with the radius, so that the fore-arm appears to have but a single bone. The reverse process of enlarging the ulna and reducing the radius is very rare and practically confined to the elephant tribe. [Illustration: FIG. 32.—Left manus of Wolf, front side. _SL._, scapho-lunar. _Py._, pyramidal. _Pis._, pisiform. _Tm._, trapezium. _Td._, trapezoid. _M._, magnum. _U._, unciform. _Mc. I-V_, first to fifth metacarpals. _Ph. 1_, first phalanx. _Ph. 2_, second phalanx. _Ung._, ungual phalanx. _I_, first digit, or pollex. _II-V_, second to fifth digits.]