330 DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 



vated, corresponding portions of the posterior margin are depressed, the dif- 

 ferent 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 opposite direction and withdraw it at a gradually 

 decreasing angle during flexion. It also happens that the axillary and 

 distal curves co-ordinate each other and bite alternately, the distal curve 

 posteriorly seizing the air in extreme extension with its concave surface 

 (while the axillary curve relieves itself by presenting its convex surface), 

 the axillary curve, on the other hand, biting during flexion with its con- 

 cave surface (while the distal one relieves itself by presenting its convex 

 one). The wing may, therefore, be regarded as exercising a fourfold func- 

 tion, the pinion in the bird being made to move from within outwards, and 

 from above downwards during extension, in the effective or down stroke ; and 

 from without inwards, and from below upwards, during flexion in the up or 

 return stroke." 



The Wing during its Vibration produces a Cross Pulsation.- — "This oscillation 

 of the wing on two separate axes — the one running parallel with the body of the 

 bird, the other at right angles to it — is well worthy of attention, as showing that 

 the wing attacks the air on which it operates in every direction, and at almost the 

 same moment, viz., from within outwards, and from above downwards, during 

 the down or effective stroke ; and from without inwards, and from below upwards, 

 during the up or return stroke. As a corollary to the foregoing, the wing may 

 be said to agitate the air in two principal directions, viz., from within outwards, 

 or the reverse, and from behind forwards, or the reverse, the agitation in question 

 producing two powerful pulsations — a longitudinal and a lateral ; the longitu- 

 dinal running in the direction of the length of the wing, the lateral in the 

 direction of its breadth. As, however, the curves of the wing glide into each 

 other when the wing is in motion, so the one pulsation merges into the other by 

 a series of intermediate and lesser pulsations. 



The longitudinal and lateral pulsations occasioned by the wing in action 

 may be fitly represented by wave-tracks running at right angles to each other, 

 the longitudinal wave track being the more distinct." 



Analogy between the Wing in Motion and the Sounding of Sonorous Bodies.— 

 " It is a remarkable circumstance that the undulation or wave made by the wing 

 when the insect and bird are fixed or hovering before an object, and when they 

 are progressing, corresponds in a marked manner with the track described by 

 the stationary and progressive waves in fluids,"' and likewise with the waves of 

 sound.t This coincidence would seem to argue an intimate relation between 



* Handbook of Natural Phil. (vol. on Electricity, Magnetism, and Acoustics), by Dr Labdneii 

 (Lond. 1863), pp. 366-7. f Op. c#.,pp. 378, 379, 380. ' 



