DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 365 



such cases the air can exert no influence whatever, as the wing is depressed 

 gently, expressly to avoid recoil. 



We have therefore the conditions of flight developed to nearly as great an 

 extent in the insects as in the bats and birds. That distinct elevator and de- 

 pressor muscles exist in the bat and bird, and that these act in conjunction with 

 elastic ligaments there can be no doubt whatever, see pages 395, 396, and 397. 



Wings Mobile and Flexible as well as Elastic — Elasticity, Flexibility, and 

 Mobility not to be confounded — Mobility and Flexibility necessary to Flight. — 

 Much importance has been attached by ancient and modern authors to the 

 elastic properties of the wing, and not a few recent investigators are of opinion 

 that flight is mainly due to the yielding of the wing to the impact of the air on 

 its under surface during the down stroke. That, however, the mere elasticity 

 of the pinion, if regarded apart from its mobility and flexibility, avails little 

 may be proved in a variety of ways. By mobility I mean that power which 

 the wing enjoys of moving at its root in an upward, downward, forward, 

 backward, or oblique direction, and likewise the remarkable property which 

 it possesses of rotating or twisting in the direction of its length. I also include 

 under the term mobility the additional power possessed by bats and birds of 

 opening and closing, i.e., of flexing and extending the wings during the up and 

 down strokes, as well as the power enjoyed by the bat of moving its fingers, 

 and by the bird of moving its individual primary, secondary, and tertiary 

 feathers at their roots. By the flexibility of the wing, I mean that power 

 which the wing possesses of throwing itself into a great variety of curves during 

 its action — these curves being formed, reversed, or obliterated at the will of the 

 flying animal. It is necessary to distinguish between mobility, flexibility, and 

 mere elasticity, because any rotation of the wing along its anterior or thick 

 margin is at once followed by an elevation or depression of its posterior or thin 

 margin, which elevation or depression is almost invariably and wrongly attri- 

 buted to elasticity. That the wing is elastic throughout, and that its posterior 

 or thin margin yields slightly (to prevent shock) when it attacks the air there 

 can be no doubt. The yielding, however, is very slight, and it is always accom- 

 panied by a certain degree of rotation or torsion. If it were otherwise — if 

 the posterior margin of the wing yielded to any marked extent in an upward 

 direction when the wing descended, it is evident that the air on which the wing 

 depended for support would escape from under it, and flight as a consequence be 

 rendered abortive. It is the air more than the wing which yields or gives way 

 in flight, and the yielding that occurs in the wings, is to be traced for the most 

 part, to a rotation of the wing along its anterior margin — to movements occur- 

 ring in the muscles and ligaments, and in the bones and feathers when present, 

 particularly at the root of the feathers. These remarks are true of living wings. 

 It is not, however, to be inferred from what is here stated that natural wings 



