=Humerus= (Figs. 47 and 48).--The humerus forms the support of the upper arm and articulates by its proximal end with the scapula at the glenoid cavity, and by its distal end with the radius and ulna, the bones of the lower arm. It is a nearly cylindrical bone with enlarged ends, and is so curved that its dorsal and ventral borders are hooked at the opposite ends so that it has the form of an Italic _f_. The proximal end of the bone bears on its dorsomedial portion a thickening, the =head= of the humerus (_a_), which bears a smooth ovoid articular facet by which the bone articulates with the glenoid cavity of the scapula. The head is not separated from the body by a distinct anatomical =neck= as in the human humerus. [Illustration: FIG. 47.--HUMERUS, VENTRAL SIDE. FIG. 48.--HUMERUS, MEDIAL SIDE. _a_, head; _b_, greater tuberosity; _c_, lesser tuberosity; _d_, bicipital groove; _e_, pectoral ridge; _f_, deltoid ridge; _g_, rough area for insertion of latissimus dorsi and teres major; _h_, nutrient foramen; _i_, capitulum; _j_, trochlea; _l_, coronoid fossa; _m_, radial fossa; _n_, medial epicondyle; _o_, lateral epicondyle; _q_, supracondyloid foramen.] Along the lateral border of the proximal end of the shaft is a high rough ridge semicircular in side view, the =great tuberosity= (_b_). It gives attachment to muscles and is marked on its dorsal border by a deep depression for the tendon of the infraspinatus muscle. On the medial margin of the proximal end closely associated with the head is a smaller elevation, the =lesser tuberosity= (_c_), also for muscular attachment. Between the greater and lesser tuberosities on the ventral surface is seen a broad groove, the =sulcus intertubercularis= or =bicipital groove= (_d_), which passes distad onto the surface of the shaft. In the natural state it is converted into a canal by overlying tendons and lodges the tendon of the biceps muscle. The shaft is nearly cylindrical at its middle, but its dorsoventral diameter is slightly greater than its mediolateral diameter. Its proximal end is flattened mediolaterad, while its distal end is flattened dorsoventrad. From the ventral margin of the greater tuberosity a ridge, the =pectoral= ridge (_e_), is continued onto the surface of the shaft, and from the dorsal margin another ridge, the =deltoid= ridge (_f_), passes distad and ventrad so as to meet the pectoral ridge near the middle of the ventral surface of the bone. On the medial margin of the bone near the junction of the first and second fourths is a roughened area (_g_) for the attachment of the tendons of the latissimus dorsi and teres major muscles, and on the same surface near the junction of the second and last thirds is a nutrient foramen (_h_).