DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 393 



Hoiv the Wing is Attached to the Body — Movements of the Shoulder, Elbow, 

 Wrist, and other Joints. — Having endeavoured to prove, in a variety of ways, 

 that insects, bats, and birds have their wings thoroughly under control both 

 during the down and up strokes, I now proceed to show that the configuration 

 of the wing, its structure, its attachments to the body, its joints, its muscles 

 (voluntary in their nature), and its elastic ligaments, many of which have 

 muscular fibres running into them, all tend to confirm this belief. 



While, however, saying so much, I take this opportunity of stating that the 

 structure of the living wing and its relations and attachments to the body are 

 such that if it moves at all it must move in such a manner as shall contribute 

 to flight. In other words, the wing is mechanically perfect ; and if it be made 

 to vibrate, even by artificial means, all its movements will tend in the direction 

 of flight. This, however, is a very different thing from asserting that the move- 

 ments of the living wing are purely mechanical in their nature. By mechanical 

 I mean such movements as would be produced by the elasticity of the wing 

 and the reaction of the air, minus volition, minus the voluntary muscles — mus- 

 culo-elastic ligaments and nerves of the wing. Flight is vito-mechanical in its 

 nature and intelligence, or that form of action which results from the habitual 

 use of intelligence, is necessary to its production. 



All wings are constructed upon a common type. They are in every instance 

 carefully graduated, the wing tapering from the root towards the tip,. and from 

 the anterior margin in the direction of the posterior margin. They are of a 

 generally triangular form, and twisted upon themselves in the direction of their 

 length, to form a helix or screw. They are convex above and concave below, 

 and more or less flexible and elastic throughout, the elasticity being greatest at 

 the tip and along the posterior margin. They are also movable in all their 

 parts. In all the wings which I have examined, whether in the insect, bat, or 

 bird, the wing is recovered, flexed, or drawn towards the body by the action of 

 elastic ligaments, these structures, by their mere contraction, causing the wing, 

 when fully extended and presenting its maximum of surface, to resume its posi- 

 tion of rest and plane of least resistance. The principal effort required in flight 

 is, therefore, made during extension and at the beginning of the down stroke. 

 The elastic ligaments are variously formed, and the amount of contraction which 

 they undergo is in all cases accurately adapted to the size and form of the wing 

 and the rapidity with which it is worked, the contraction being greatest in the 

 short-winged and heavy -bodied insects and birds, and least in the light-bodied 

 and ample-winged ones, particularly in such as skim or glide. The mechanical 

 action of the elastic ligaments, I need scarcely remark, ensures an additional 

 period of repose to the wing at each stroke ; and this is a point of some im- 

 portance, as showing that the lengthened and laborious flights of insects and 

 birds are not without their stated intervals of rest. 



