1875. MECHANISM OF THE BIRD’S WING. 83 
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 sug- 
gested 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 neces- 
sary to attach a very projecting hinge at the portion of the model of 
the humerus which represents the humero-ulnar articulation, other- 
wise, 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 consoli- 
dated 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 
pataguvi 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. 
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