DE PETTIGREW ON THE PHYSIOLOGY OF WINGS. 369 



of the air. As a consequence, the recoil is feeble. If, however, the rotation is 

 added, the wing seizes the air with such avidity as in all cases to produce a very 

 violent reaction. The tendency of the wing to dart forward is diminished by the 

 rotation, but the actual elevating power of the pinion is greatly augmented. 

 This point can be readily ascertained by depressing and screwing, in the manner 

 described, the wing of the swan or of any other large bird, previously dried, in 

 the extended position. In preparing the wing for the experiment care should 

 be taken not to destroy the curves peculiar to the natural extended wing. I 

 mention this fact because, of many swans' wings prepared by me for this purpose, 

 I found one had been inadvertently flattened, and gave quite an indifferent result. 



The Importance to be attached to the Concavo-Convex Form of the Wing in 

 Birds.— The downward screwing of the concave or under surface of the wing, 

 which is so efficacious in securing a powerful hold of the air during the down 

 stroke, is followed during the up stroke by an upward screwing of the convex or 

 upper surface, which is not less effective in evading the air. In fact, when the 

 wing ascends it is drawn towards the body, and deeply arched, so that it is 

 literally made to roll upwards, its convex or dorsal surface being directed 

 upwards throughout the entire up stroke. It is thus the wing evades the super- 

 incumbent air during the return stroke. This account will be readily under- 

 stood by a reference to figures 13, 14, and 15, Plate XIII. 



At figure 15, Plate XIII., the wing is represented as seen in the middle of the 

 down stroke. It is widely spread out, and finely arched. At figure 14, Plate 

 XIII., the wing is shown as observed towards the end of the down stroke — the 

 wing being partly flexed or drawn towards the body, and the arch rendered more 

 abrupt, particularly towards the root of the pinion. . At figure 13, Plate XIII., 

 the wing is seen quite at the termination of the down stroke. It is fully flexed, 

 and drawn still closer to the body. It is, moreover, more deeply arched than in 

 either of the other figures. It has, in fact, assumed the shape which offers 

 least resistance in an upward direction, and is prepared to make the up stroke. 



The Under or Concave Biting Surface of the Wing of the Bird effective both 

 during the Down and Up Strokes. — If, instead of believing that the wing is 

 elevated, we believe what, as I have already stated is actually the case, viz., that 

 the body of the bird falls downwards and forwards, we at once transfer the 

 resistance from the dorsal or convex non-biting surface of the wing to the ven- 

 tral concave or biting surface — the body being supported while the wings are 

 being elevated by a beautifully arched natural parachute formed by the wings. 

 The elevation of the wings is, in short, in a great measure a consequence of 

 the falling of the body. It is in this way that the air comes to assist in elevating 

 the wings. The air, in short, caught under the wings is instrumental in elevat- 

 ing and extending them in proportion as the body falls [vide figures 13, 14, 

 and 15, Plate XIII.) The small size of the elevator muscles of the wing of the 



