370 DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 



bird and bat, as compared with the very powerful depressor muscles, is thus 

 accounted for. The elevation of the wing, as will be inferred, is to a certain 

 extent a mechanical act, and is due to the reaction of the air, the contraction 

 of the elastic ligaments, and the downward and forward fall of the body. It 

 is, however, not altogether mechanical, the wing, as I shall show subsequently, 

 being perfectly under control both during the down and up strokes. 



Lax Condition of the Shoulder Joint in Birds, §c. — The great laxity of the 

 shoulder joint readily admits of the body falling downwards and forwards during 

 the up stroke. This joint, as has been already stated, admits of movement in 

 every direction, so that the body of the bird is like a compass set upon gimbals, 

 i.e., it swings and oscillates, and is equally balanced, whatever the position of 

 the wings. The movements of the shoulder joint in the bird, bat, and insect, 

 are restrained within certain limits by a system of check ligaments and pro- 

 minences ; but in each case the range of motion is very great, the wing being 

 permitted to swing forwards, backwards, upwards, downwards, or at any degree 

 of obliquity. It is also permitted to rotate along its anterior margin, or to 

 twist in the direction of its length to the extent of nearly a quarter of a turn. 

 This great freedom of movement at the shoulder joint enables the insect, bat, 

 and bird, to rotate and balance upon two centres — the one running in the 

 direction of the length of the body, the other at right angles, or in the direc- 

 tion of the length of the wings. 



The Wings Elevated Indirectly by the Operation of Gravity. — I have explained 

 that during the up stroke the body falls, and the wings are elevated. Let us now, 

 for the sake of argument, advocate an opposite view. Let us take for granted 

 that the body is fixed in space, and that the wings are elevated by a purely 

 vital act. From this it follows that the wings during their ascent will of necessity 

 experience much resistance from the superimposed air, the rounded form of the 

 upper or dorsal surfaces of the pinions diminishing, but not removing the evil. 



The resistance experienced by the wings during their ascent is obviated in the 

 simplest manner possible, the movement, as has been explained, being dex- 

 terously transferred from the wings to the trunk in such a manner that the under 

 or concave surfaces of the wings are made to act in lieu of the upper or convex 

 surfaces. The body, in a word, is dragged downwards by the inexorable power of 

 gravity ; but the descent of the body involves the ascent of the wings. The body and 

 wings, therefore, reciprocate, the body being elevated by the descent of the wings 

 in conjunction with other means, while the wings are elevated to a great extent 

 by the descent of the body, as shown at figures 20, 21, and 22, pages 352 and 353."" 

 The wings are also partly elevated by the reaction elicited from the air — the 

 contraction of the elevator muscles and elastic ligaments and the forward travel 



* The alternate ascent and descent of the wings and body during the down and up strokes are 

 well seen in the butterfly and in all animals whose wings are large for their bodies. 



