354 DR PETTIGREW ON THE PHYSIOLOGY OE WINGS. 



forwards in a curve to g, the pinion (/) by this act being made to describe the 

 segment of a circle h % j, its under concave surface being applied to the air like 

 a kite all the time. (It is thus that the wing elevates and sustains during the 

 up stroke.) The wing (j) is made to descend in the direction k I, and forces 

 the body (g) along an upward curve until it arrives at m, its subsequent fall 

 elevating the wing (n) in the direction o p. Here again, the body and wing 

 play alternately on either side of a given line x x. 



A careful study of figs. 20, 21, and 22 (pages 352, 353) shows the great im- 

 portance of the twisted configuration and curves peculiar to the natural wing. 

 If the wing was not curved in every direction it could not be rolled on and off 

 the wind during the down and up strokes, as seen more particularly at fig. 22. 

 This, however, is a vital point in progressive flight. The wing (b) is rolled on to 

 the wind in the direction c d, its under concave or biting surface being crushed 

 hard down with the effect of elevating the body to e. The body falls to g, 

 and the wing (/) is rolled off the wind in the direction h i, and elevated partly 

 by the action of the elevator muscles and elastic ligaments, and partly by the 

 reaction of the air, operating on its under or concave biting surface, until it 

 assumes the position j. The wing is therefore to a certain extent resting 

 during the up stroke. The concavo-convex form of the wing is admirably 

 adapted for the purposes of flight. In fact, the power which the wing possesses 

 of always keeping its concave or under surface directed downwards and more 

 or less forwards enables it to seize the air at every stage of both the up and 

 down strokes so as to supply a persistent buoyancy. The action of the natural 

 wing is accompanied by remarkably little slip — the elasticity of the organ, the 

 resiliency of the air, and the contraction and relaxation of the elastic ligaments 

 and muscles all co-operating and reciprocating in such a manner that the 

 descent of the wing elevates the body, the descent of the body aided by the 

 reaction of the air and the contraction of the elastic ligaments and muscles 

 elevating the wing. The wing during the up stroke arches above the body after the 

 manner of a parachute, and in turn prevents the body from falling. The 

 sympathy which exists between the parts of a flying animal and the air on 

 which it depends for support and progress is consequently of the most intimate 

 character. 



The up stroke (B of figures 16 and 17, page 349), as will be seen from the fore- 

 going account, is a compound movement due in some measure to recoil or resist- 

 ance on the part of the air — to the contraction of the muscles, elastic ligaments, 

 and other vital structures, to the elasticity of the wing, and to the falling of the 

 body in a downward and forward direction. The wing may be regarded as rotating 

 during the down stroke upon 1 of figure 16, page 349, which may be taken to 

 represent the long and short axes of the wing, and during the up stroke upon 

 2, which may be taken to represent the yielding fulcrum furnished by the air. 



