The strength of the pigeon's wing bones in relation to their function.

In the pigeon, as in other members of the three groups of vertebrates which have developed advanced powers of flight—the birds, bats and pterosaurs—the bending and torsional strength of the proximal part of the wing lies in the humerus and radio-ulna. The humerus runs obliquely back from the shoulder joint to the elbow, and from there the radio-ulna runs obliquely forward to the wrist. In flight the radio-ulna is generally set at a large angle to the humerus, often about a right angle (Fig. 8). The elbow joint is free to bend in the plane of the wing, but transmits bending moments at right angles to this plane, and twisting moments. Because of the large angle between the radio-ulna and the humerus, bending moments applied by the lift force to the radio-ulna are transmitted to the humerus largely as twisting moments, and vice versa. At the shoulder joint, the head of the humerus is free to rotate through large angles in all of the six possible ways—up, down, forward, back, and rotation in the nose-up and nose-down senses: bending and twisting moments cannot, therefore, be transmitted to the body without the help of muscles inserting on the humerus. In this paper the ultimate strengths of the humerus and radio-ulna under bending and twisting loads are examined. The question is then asked—How much lift would be needed to break the bones in various ways? This can be answered subject to making some assumptions about the distribution of lift over the wing, and, this done, the limitations imposed by the structural strengths of the wing bones on the pigeon's performance can be considered.