[Illustration: FIG. 25.—Left humerus of Wolf, from the front and outer sides, the latter somewhat oblique. _h._, head. _int.t._, internal tuberosity. _ext.t._, external tuberosity. _bc._, bicipital groove. _dt._, deltoid ridge. _sh._, shaft. _s._, supinator ridge. _int. epi._, internal epicondyle. _s.f._, anconeal foramen. _tr._, trochlea. _tr′._, trochlea, posterior side. _ext. epi._, external epicondyle. _a.f._, anconeal fossa.] [Illustration: FIG. 26.—Left humerus of Horse, front side. _i.t._, internal tuberosity. _ex.t._, external tuberosity. _bc._, outer part of bicipital groove. _dt._, deltoid ridge. _s._, supinator ridge. _tr._, trochlea.] [Illustration: 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_. [Illustration: FIG. 28.—Left fore-arm bones of Wolf, front side. _R._, radius. _U._, ulna. _ol._, olecranon. _h._, head of radius.] [Illustration: 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.]