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Show 1875.] MECHANISM OF THE BIRD'S WING. S3 The explanation of this mechanism is not difficult. The arm consists of one bone only, the humerus ; the forearm of two, the ulna and radius ; the manus of the two carpals together with the metacarpals and phalanges. The mutual relations of these two bones are such that the radius and ulna move one above the other like the two limbs of a pair of drawing-parallels, each being fixed proximally to the humerus and distally to the carpus. The plane common to the radius and ulna is the same as that in which flexion and extension of the elbow is performed, so that one of the two bones of the forearm, the radius, articulates with the humerus at a point nearer the shoulder, or further from the elbow, than its companion, the ulna. At the wrist the radius is consequently superior, articulating with the carpal bone on the pollex side; whilst the ulna articulates with the other element of the carpus. This condition maintaining the parallel movements of the radius on the ulna must necessarily be attended by a parallel movement of the humerus on the manus. When the humerus bends upon the ulna, the manus therefore similarly bends upon the forearm; and the triceps muscle is able, unassisted, to maintain the whole limb in a rigid state during extension. In making a wooden model of these bones to illustrate the above described mechanism, one or two points of mechanical detail suggested a reference to the shape of the distal end of the humerus. The wing in the living bird, when at rest, is completely folded; and when fully extended forms but a slightly angular rod. To allow of this considerable range of movement of the bones of the forearm on the humerus, and of their being completely folded up, it is necessary to attach a very projecting hinge at the portion of the model of the humerus which represents the humero-ulnar articulation, otherwise, when fully flexed, the model radius would not be able to be included between the then parallel humerus and ulna; especially as the radius, to get in its fully flexed position, must rotate on a hinge which itself projects its semidiameter at least beyond the humerus. These requirements explain the characteristic shape of the distal end of the humerus in birds. It is curved towards its flexor side, and sharply so at its extremity where it comes in contact with the ulna. At the same time the radius articulates with it on a well-developed knob, situated above the similar surface for the ulna, and to its outer side (which allows of a less extensive joint). The similar arrangement required at the wrist-joint is arrived at by the interpolation of the carpal bones between the forearm and consolidated metacarpus. In some wings, when all the muscles are removed, this movement is not so manifest as in others, there being a certain amount of independent power of movement in the manus in all positions. This is much reduced in the living bird by the tendon of the tensor patagii longus muscle, which runs from the shoulder, along the free margin of the patagium, to the wrist, where, in being attached to the metacarpal mass on the pollex side, it aids the extension of the manus during the extension of the forearm. 6* |