PKOGRESSION IN OR TIIKOUGH THE AIR. 



147 



insect it is due to a twisting which occurs at the root of the 

 wing and to the reaction of the air. 



Fig. 76. Fig. 77. Fig. 78. 



Fig. 76.— Curves seen on the anterior {d e f) and posterior (c a h) margin in 

 the wing of the bird in flexion. — Origrma^. 



Fig. 77.— Curves seen on the anterior margin {d ef) of the wing in semi-exten- 

 sion. In this case the curves on the posterior margin (6 c) are obliter- 

 ated. — Original. 



Fig. 78.— Curves seen on the anterior (d e f) and posterior (c a h) margin of 

 the wing in extension. The curves of this fig. are the converse of those seen 

 at fig. 76. Compare these figs, with fig. 79 and fig. 32, p. 68.— Original. 



The Tip of the Bat and Bird's Wing describes an Ellipse. — 

 The movements of the wrist are always the converse of those 

 occurring at the elbow- joint. Thus in the bird, during ex- 

 tension, the elbow and boues of the forearm are elevated, and 

 describe one side of an ellipse, while the wrist and bones of 

 the hand are depressed, and describe the side of another 

 and opposite ellipse. These movements are reversed during 

 flexion, the elbow being depressed and carried backwards, 

 while the wrist is elevated and carried forwards (fig. 79). 



Extension (elbow). ^ Flexion (wrist). 



Flexion (elbow). Extension (wrist). 



Fig. 79.— (a h) Line along which the wing travels during extension and flexion. 

 The body of the fish in swimming describes similar curves to those described 

 by the wing in flying.— (Fic^e fig. 32, p. 68.) 



The Wing capable of Change of Form in all its Parts, — From 

 this description it follows that when the different portions of 

 the anterior margin are elevated, corresponding portions of 

 the posterior margin are depressed ; the different parts of the 

 wing moving in opposite directions, and playing, as it were, 

 at cross purposes for a common good ; the object being to 

 rotate or screw the wing down upon the wind at a gradually 

 increasing angle during extension, and to rotate it in an 



