Fig. 27.—Left humerus of Man, front side. Letters as in [Fig. 25].
The superior segment of the fore limb has a single bone, the humerus, the upper end of which is the rounded, convex head, which fits into the glenoid cavity of the shoulder-blade, forming the joint of the shoulder; in front of the head are two prominent and sometimes very large projections for muscular attachment, the external and internal tuberosities, separated by a groove, in which play the two tendons of the biceps muscle and is therefore called the bicipital groove. In a few mammals, such as the Horse, Camel and Giraffe, the groove is divided into two by a median tubercle or ridge. From the external tuberosity there generally passes down the front face of the shaft a rough and sometimes very prominent ridge, the deltoid crest, to which is attached the powerful deltoid muscle. At the lower end of the humerus is the trochlea, an irregular half-cylinder, for articulation with the two bones of the fore-arm and varying in form according to the relative sizes of those bones. On each side of the trochlea is frequently a rough prominence, the epicondyle, and above the inner one is, in many mammals, a perforation, the epicondylar foramen, for the passage of a nerve. Extending up the shaft from the outer epicondyle is a rough crest, the supinator ridge, to which is attached one of the muscles that rotate the hand and is conspicuously developed in those mammals which have the power of more or less free rotation and especially in burrowers. On the posterior face of the humerus, just above the trochlea, is a large, deep pit, the anconeal fossa.
Fig. 28.—Left fore-arm bones of Wolf, front side. R., radius. U., ulna. ol., olecranon. h., head of radius.
Fig. 29.—Left fore-arm bones of Man, front side. Letters as in [Fig. 28]. The small object at the right of each figure is the head of the radius, seen from above.
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.
Fig. 30.—Coössified bones of left fore-arm of Horse, front side. For most of its length, the ulna is concealed by the radius.
