The humerus and the femur are corresponding bones of the arms and legs, respectively. While their parts are similar in general, their structure has been adapted to differing functions. The head of the humerus is almost hemispherical, while that of the femur forms about two-thirds of a sphere. There is a strong ligament passing from the head of the femur to further strengthen and ensure its position in the acetabulum.

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The anatomical neck of the humerus is only a slight constriction, while the neck of the femur is a very distinct portion, running from the head to meet the shaft at an angle of about 125°. Actually, the femoral neck is developmentally and functionally a part of the shaft. The entire weight of the body is directed through the femoral heads along their necks and to the shaft. The structure of the bone within the head and neck and the upper part of the shaft of the femur would do credit to an engineer who had worked out the weight-bearing problems involved in the maintenance of upright posture.


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(Left) The radius and the ulna, bones of the forearm; (right) the fibula and the tibia, bones of the lower leg.

At the elbow, the ulna forms with the humerus a true hinge joint, in which the actions are flexion and extension. In this joint a large projection of the ulna, the olecranon, fits into the well-defined olecranon fossa, a depression of the humerus.

The radius is shorter than the ulna. Its most distinctive feature is the thick disk-shaped head, which has a smoothly concave superior surface to articulate with the head, or capitulum, of the humerus. The head of the radius is held against the notch in the side of the ulna by means of a strong annular, or ring-shaped, ligament. Although thus attached to the ulna, the head of the radius is free to rotate. As the head rotates, the shaft and outer end of the radius are swung in an arc. In the position of the arm called supination, the radius and ulna are parallel, the palm of the hand faces forward, and the thumb is away from the body. In the position called pronation, the radius and ulna are crossed, the palm faces to the rear, and the thumb is next to the body. There are no actions of the leg comparable to the supination and pronation of the arm.

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The radius and ulna (bones of the forearm), shown in supination (the arm rotated outward so that the palm of the hand faces forward).