XVI] THE MECHANISM OF FLIGHT 963 



All this is part of the automatic mechanism of the wing, than which 

 there is nothing prettier in all anatomy. ' The triceps muscle, massed 

 on the shoulder, extends the elbow- j oint in the usual way. But another 

 muscle (a long flexor of the wrist) has its origin above the elbow and 

 its insertion below the wrist. It passes over two j oints ; it is what 

 German anatomists call a zweigelenkiger Muskel. It transmits to the 

 one joint the movements of the other; and in birds of powerful flight 

 it becomes less and less muscular, more and more tendinous, and so 

 more and more completely automatic. The wing itself is kept hght; 

 its chief muscle is far back on the shoulder ; a contraction of that re- 

 mote muscle throws the whole wing into gear. The little ligaments we 

 have been speaking of are so linked up with the rest of the mechanism 

 that we have only to hold a bird's wing by the arm-boue and extend 

 its elbow- joint, to see the whole wing spring into action, with every 

 joint extended, and every feather tense and in its place*. 



Again on the fore-edge of the wing there lies a tiny mobile " thumb," 

 whose little tuft of stiff, strong feathers forms the so-called "bastard 

 wing." We used to look on it as a "vestigial organ," a functionless 

 rudiment, a something which from ancient times had "lagged 

 superfluous on the stage"; until a man of genius saw that it was 

 just the very thing required to break the leading. vortices, keep the 

 flow stream-lined at a larger angle of incidence than before, and 

 thereby help the plane to land. So he invented, to his great profit 

 and advantage, the "slotted wing." 



We learn many and many another interesting thing. How a 

 stiff "comb" along the leading edge, a broad soft fringe along 

 the trailing edge (the fringe acting as a damper and preventing 

 "fluttering"), and a soft, downy upper surface of the wing, all help 

 as silencers, and give the owl her noiseless flight. How the wing- 

 loading of the owls is lower than in any other birds, lower even 

 than in the eagles; and how owl and eagle have power to spare to 

 carry easily their prey of mouse or mountain-hare. How the deep 

 terminal wing-slots aid the heavy rook or heron in their slow 



* The mole's forelimb has a somewhat similar action, by which, as the arm and 

 hand are pulled violently backward, the claws are powerfully and automatically 

 flexed for digging. The "suspensory ligament " of the horse, which is, or was, a short 

 flexor of the digit, is an analogous mechanism. See a paper of mine On the nature and 

 action of certain ligaments, Journ. of Anat. and Physiol, xviii, pp. 406-410, 1884. 



