1 278 ON THE MECHANISM OF THE BIRD’S WING. 
Page 83. 
in the living bird, when at rest, is completely folded; and when fully 
extended forms but a slightly angular rod. To allow of this consider- 
able 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 semi- 
diameter 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 inter- 
polation 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 in- 
dependent power of movement in the manus in all positions. This is 
much reduced in the living bird by the tendon of the tensor patagit . 
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. 
The mechanism above deseribed is stated by Dr. Alix* to have 
been first indicated by Bergmann, as far as the anatomical arrange- 
ment is concerned, although Strauss-Diirckheim, in his ‘Théologie 
de la Nature’ was the first to explain it fully. Dr. Alix himself 
has alsot entered into the detail of the movement “of elongation ” 
of the radius, which is well explained in his large work above referred 
to.t My object in bringing the subject before the Society is to draw 
special attention to so important a point, and to illustrate its action by 
a wooden model, which demonstrates its accuracy in a very striking 
manner, 
It may be here mentioned that the movement of the general plane 
of the wing during both the up and down stroke, which by Borelli and 
his followers is ascribed to the elastic yielding of the feathers in birds, 
* ‘ Essai sur l'appareil locomoteur des Oiseaux,” Paris, 1874, p. 230. 
+ “ Bulletin de la Société Philomathique,” 1864. 
t Loe. cit. p. 330 et seq. 
