The humerus, or arm-bone, is a bone of great thickness and density, and is situated between the scapula or blade-bone above and the radius and the ulna below. Externally the body of the bone is deeply grooved by the furrow of torsion, and presents at about its upper third a somewhat bold pointed projection, the external tuberosity. On the internal surface it presents a rounded prominence, the internal tuberosity, into which important muscles gain insertion.

On the upper extremity are the broad rounded head and several bony prominences. The former, situated behind, is coated with cartilage, and articulates with the glenoid cavity of the scapula or blade-bone to form the shoulder-joint. The latter comprise a double projection of bone on the outer side, termed the great trochanter. On the inner side is the small trochanter or tubercle, and between them a prominence which divides the upper front portion of the humerus into two grooves (bicipital groove). The grooves and intervening tubercle are covered with fibro-cartilage, and over them plays, pulley-like, the broad tendon of the flexor brachii muscle, between which and the bone there is a synovial membrane.

Fig. 297. - Scapula (Tnner Surface).

1 Coracoid Process. 2 Glenoid Cavity. 3 Cartilage of Prolongation.

Fig. 298. -Humerus (Front Aspect).

1 Bicipital Groove. 2 Small Trochanter. 3 Great Trochanter. 4 External Tuberosity. 5 Shaft with Furrow of Torsion. 6 Epitrochlea or External Condyle. 7 Supra-condyloid Fossa. 8 Epicondyle or Internal Condyle.

Fig. 299. - Humerus (Outer Aspect).

1 Great Trochanter. 2 Deltoid Tuberosity.

3 External Tuberosity. 4 Epitrochlea. 5 Epi-condyle. 6 Supra-condyloid Fossa. 7 Shaft of Bone with the Furrow of Torsion. 8. Articular Surface or Head.

The lower extremity is smaller than the upper, and in front presents two unequal rounded portions separated by a superficial groove. These are the internal and external condyles. Behind is a deep pit (supra-condyloid fossa), which separates two prominent ridges from each other, and receives into it a pointed process (peak) on the elbow when the joint is in extreme extension.

Fig. 300. - The Forearm: Radius and Ulna.

1 Ulna. - Point of Ulna.

3 Beak of Ulna. 4 Radio-ulnar Arch. 5 Radio-ulnar Articulation. 6 Bicipital Tuberosity.

7 Shaft or Body of Radius.

8 Radio-carpal Articulation.