368 DR PETTIGKEW ON THE PHYSIOLOGY OF WINGS. 



of the wing with the horizon considerably exceeds that made by the tip, and 

 is much greater than a casual observer would be inclined to admit. It 

 is obscured by the curving downwards and forwards of the anterior mar- 

 gin of the wing towards the root, as seen at a of figure 7, Plate XII. In 

 this figure the apparent angle made by the root of the wing with the horizon 

 (ef) is a b d, the real angle being c b d. The wing of the bird rotates in 

 opposite directions during extension and flexion. The various angles of 

 inclination made by the wing of the gannet in extension and flexion is well 

 shown at Plate XIII. figures 16 and 17. 



In figure 17 (flexion) the posterior margin of the wing (s q p o) is on a 

 level with the body of the bird ; whereas in figure 16 (extension) the posterior 

 margin {qp o) is directed downwards and forwards, as indicated by the arrows. 

 The same thing is seen in the pea- wit, at Plate XII. figure 8. In this figure 

 the wing to the right of the observer is flexed, and in the act of making the 

 up stroke, the anterior margin of the pinion being slightly directed down- 

 wards {vide arrow). The wing to the left of the observer is, on the contrary, 

 extended, and in the act of making the down stroke, the anterior margin of 

 the pinion being directed upwards {vide arrow). 



The rotation of the posterior margin around the anterior as an axis during ex- 

 tension, is occasioned by the points of insertion of the pectoralis major and other 

 muscles, by the attachments and directions of the elastic and other ligaments, 

 and by the spiral nature of the articular surfaces of the bones of the wing — 

 the mere act of extension on all occasions involving the rotation in question. 



The Wing of the Bird Descends as a Long Lever. — Let us imagine the wing 

 fully extended and elevated, and making a certain angle with the horizon, as 

 indicated at c b d of figure 1, Plate XL, at 3 of figure 5, Plate XL, and at 3' of 

 figure 18, Plate XIV. The wing is now prepared to make the down stroke, and 

 descends in a spiral swoop, successively assuming the position 4 in figure 19, 

 Plate XIV., and 4 in figure 6, Plate XL It acts with extreme energy as a long 

 lever {vide c d of figure 6, Plate XL), the purchase which it has on the body 

 being much greater than is usually anticipated. 



During its descent the angle which the wing makes with the horizon is 

 increased, as shown at a b c of figures 16 and 17 (page 349), the horizon in these 

 figures being indicated by the straight line x x' . 



In the bird, therefore, as in the insect, the posterior or thin flexible margin 

 of the wing is screwed down upon the air while the wing is descending. 



The Rotation of the Posterior Margin of the Wing in a Downward Direction 

 increases the Elevating, but diminishes the Propelling Power of the Wing. — The 

 additional hold which the bird can cause its wing to take of the air by resorting 

 to a greater or less degree of rotation, is truly surprising. If the wing is 

 depressed minus the rotation, it darts forward, but takes no very decided catch 



