148 



ANIMAL LOCOMOTION. 



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 concave surface 

 (while the distal one relieves itself by presenting its convex 

 one). The wing may therefore be regarded as exercising a 

 fourfold function, the pinion in the bat and bird being made 

 to move from within outwards, and from above downwards 

 in the down stroke, during extension; and from without 

 inwards, aiid from below upwards, in the up stroke, during 

 flexion. 



The TVing during Us Vibration produces a Cross Pulsation. — 

 The 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 (fig. SO, a h, c d) — is well worthy of atten- 

 tion, 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 stroke; and from without inwards, and 

 from below upwards, during the up stroke. As a corollary 

 to the foregoing, the wing may be said to agitate the air 

 in two principal directions, viz. from within outwards and 

 downwards, or the converse ; and from behind forwards, or 

 the converse ; the agitation in question producing two power- 

 ful pulsations, a vertical and a horizontal. The wing when 

 it ascends and descends produces artificial currents which 

 increase its elevating and propelling power. The power of 

 the wing is further augmented by similar currents developed 

 during its extension and flexion. The movement of one part 

 of the wing contributes to the movement of every other part 

 in continuous and uninterrupted succession. As 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 vertical and horizontal pulsations occasioned by the 

 wing in action may be fitly represented by wave-tracks running 



