DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 397 



voluntary muscle than those of the crane, and this is no doubt owing to the 

 fact that the wings of the swan are driven at a much higher speed than those 

 of the crane. In the snipe the wings are vibrated very much more rapidly than 

 in the swan, and as a consequence we find that the fibro-elastic bands are not 

 only greatly increased, but they are also geared to a much greater number of 

 voluntary muscles, all which seems to prove that the elastic apparatus employed 

 by nature for recovering or flexing the wing towards the end of the down stroke 

 become more and more highly differentiated in proportion to the rapidity with 

 which the wing is moved.* The reason for this is obvious. If the wing is to 

 be worked at a higher speed, it must, as a consequence, be more rapidly flexed 

 and extended. The rapidity with which the wing of the bird is extended and 

 flexed is in some instances exceedingly great ; so great, in fact, that it escapes 

 the eye of the ordinary observer. The rapidity with which the wing darts in 

 and out in flexion and extension would be quite inexplicable, but for a know- 

 ledge of the circumstance that the different portions of the pinion are disposed 

 at various angles of inclination (vide x, s, t, w of figures 9 and 10, Plate XII.), 

 these angles being instantly increased or diminished by the slightest quiver of 

 the muscular and fibro-elastic systems. If we take into account the fact that 

 the wing of the bird is recovered or flexed by the combined action of voluntary 

 muscles and elastic ligaments ; that it is elevated to a considerable extent by 

 voluntary muscular effort ; and that it is extended and depressed entirely by 

 muscular exertion, we shall have difficulty in avoiding the conclusion that the 

 wing is thoroughly under the control of the muscular system, not only in flexion 

 and extension, but also throughout the entire down and up strokes. 



An arrangement in every respect analogous to that just described is found 

 in the wing of the bat, the covering or web of the wing in this instance forming 

 the principal elastic ligament. In fact, the bones and muscles of the bat's 

 wing, and the inclined surfaces made by its different portions with each other 

 and with the horizon during flexion and extension, and during the down and up 

 strokes, so closely resemble those of the bird that a separate description is un- 

 necessary. From the foregoing description it will be obvious that the wing of 

 the bird and bat is a highly differentiated organ, endowed with independent 

 movements, which enable it to direct and control the air for a purpose. 



How Balancing is Effected in Flight. — The manner in which insects, bats, 

 and birds balance themselves in the air has hitherto, and with reason, been 

 regarded a mystery, for it is difficult to understand how they maintain their 

 equilibrium when the wings are beneath their bodies. Figures 3 and 4, page 338, 

 throw considerable light on the subject in the case of the insect. In those 

 figures the space (a, g) mapped out by the wing during its vibrations is entirely 



* A careful account of the musculo-elastic structures occurring in the wing of the pigeon is given 

 by Mr Macgillivray in his admirable " History of British Birds," pages 37 and 38. Lond. 1837. 



