The automating skeletal and muscular mechanisms of the avian wing (Aves)

Abstract

The avian wing possesses the ability to synchronize flexion or extension of the elbow and wrist joints automatically. Skeletal and muscular mechanisms are involved in generating this phenomenon. The “drawing-parallels” action of the radius and ulna coordinates the movements of the forearm with the carpus. Movement of the radius along the length of the forearm isnot dependent on the shape disparity between the dorsal and ventral condyles of the humerus, nor is it generated by the shape of the dorsal condyle itself. Instead, shifting of the radius toward the wrist occurs during humeroulnar flexion when the radius, being pushed by muscles toward the ulna, is deflected off theIncisura radialis toward the wrist. Movement of the radius toward the elbow occurs during the latter stages of humeroulnar extension when, as the dorsal condyle of the humerus and the articular surface of the ulna's dorsal cup roll apart, the radius gets pulled by the humerus and its ligaments away from the wrist. Synchronization of the forearm with the manus is accomplished by twojoint muscles and tendons.M. extensor metacarpi radialis and the propatagial tendons act to extend the manus in unison with the forearm, whileM. extensor metacarpi ulnaris helps these limb segments flex simultaneously.M. flexor carpi ulnaris, in collaboration with the “drawing-parallels” mechanisms, flexes the carpus automatically when the elbow is flexed, thereby circumducting the manus from the plane of the wing toward the body. In a living bird, these skeletal and muscular coordinating mechanisms may function to automate the internal kinematics of the wing during flapping flight. A mechanized wing may also greatly facilitate the initial flight of fledgling birds. The coordinating mechanisms of the wing can be detected in a bird's osteology, thereby providing researchers with a new avenue by which to gauge the flight capabilities of avian fossil taxa.