The Humerus is a long bone with a shaft and two extremities; it is the bone of the upper arm. It has a rounded head forming about one-third of a sphere, which is bounded by the anatomical neck, a shallow depression all round the head, much less marked inferiorly. On the outer side of the head is the great tuberosity, which becomes continuous with the shaft, and has facets for the attachment of muscles. On the anterior surface of the upper end is the lesser tuberosity, which also becomes continuous with the shaft. Between the two tuberosities lies the bicipital groove, which gives attachment to the adductor muscles. Below the head and tuberosities the bone decreases in size, and this is called the "surgical neck," as it is the part most easily fractured.

The shaft is cylindrical above, but gets flatter lower down.

Fig. 3. - The Right Humerus (Anterior View).

The bicipital groove passes down its anterior surface, directed slightly to the inner side, and gradually fades away. About the middle of the inner and outer surfaces are rough surfaces for the insertions of the coraco-brachialis and deltoid respectively. On the posterior surface a shallow groove - the musculo-spiral groove-winds from within outwards, separating the origins of two heads of the triceps. The lower end of the shaft is flattened and expanded into two condyles, of which the inner is larger than the outer. From each of these condyles a ridge runs up for about one-third of the shaft. Between the two condyles are the trochlear and capitellar surfaces for the articulation of the ulna and radius respectively. The trochlea is a grooved surface, which winds spirally round the inferior end of the bone and shows on the posterior surface. The capitellum is on the outer side of the trochlea, a small, rounded surface which shows only on the anterior surface. Above the trochlea, both anteriorly and posteriorly, are small, round fossae, the coronoid and olecranon respectively, for articulation with the processes of the-ulna in extreme flexion and extension.

The humerus articulates, by means of its rounded head, with the glenoid cavity of the scapula, and at its inferior end with the ulna and radius.

Ossification

The primary centre for the shaft appears before birth. Secondary centres for the two tuberosities and the head appear during the first few years of life, and these three first unite, forming an epiphysis, which unites with the shaft as a whole in adult life. A similar arrangement is observed with the condyles and articular surfaces of the lower end, which also form a separate epiphysis.