DR. PETT1GREW ON THE MECHANISM OF FLIGHT. 245 



as in partial flexion (PL XV. fig. 73 b), or to the elevation of the posterior distal portion 

 of the wing (PI. XV. fig. 69 b) and the depression of the corresponding anterior portion 

 (PL XV. fig. 69/), as happens when the posterior margin of the wing is rotated in an 

 upward direction at the beginning of the up or return stroke. In this case, the posterior 

 margin (Plate XV. fig. 69 b) is lost to view, the anterior margin (/), which is bent upward* 

 towards the extremity of the wing, being alone seen. Finally, as has been explained, it 

 may be referred to the upward direction communicated to the primary feathers by the 

 metacarpal and phalangeal bones of the wing rotating or twisting on their Ion- axes in 



the direction indicated. 



Natural Outlet for the Air towards the root of the pinion; the bending upward* <>/ 



the feathers for this purpose unnecessary .—The bending upwards of the shafts ol' the 

 feathers during the descent of the wing would, I opine, impair its eflicacy by permitting 

 more air to escape along its posterior or thin margin than is necessary, the air by this 

 arrangement being diverted from its real outlet, which occurs towards the root of the 

 pinion, where the secondary and tertiary feathers approach each oilier. In this locality 

 a very beautiful provision is made for its exit, it so happening that the secondary featherg 

 with their coverts and subcoverts, on both the dorsal and ventral aspects, run obliquely 



from without inwards, or towards the body of the bird ; while the corresponding tertiary 

 feathers proceed in a slanting direction from within outwards, i. e. in the direction of the 

 tip of the wing. The result of this arrangement is obvious : the secondary and tertiary 

 feathers, proceeding obliquely in opposite directions, meet at a certain angle, and form 

 by their union on the ventral or under surface of the wing, to which the present descrip- 

 tion more particularly applies, a conical depression, or hollow, the base of which « 

 directed upwards and outwards, towards the anterior margin and body of the wing- 

 apex downwards and inwards, towards the posterior margin and root of the pinion. 

 in this latter situation that the air on which the wing has acted escapes in M~*»^ 

 form, the recoil which it elicits urging the body of the bird m an upward and forward 



th 



It 



direc'tion, and acting at the very point where its elevating- and TO^P"" 



greatest. Compare tie axillary and distal «£— «£ Sj.Sffi 



of the wing: with those made by the bones ol tto^Q * 



flexion, as shown in Diagrams from 7 to J"*-** £ JJJ* £ £J hmo %M 



Framework or Support for the Wings of the Bird. The 1™9"V 

 in Fli 9 ht.-In order clearly to comprehend the structure and £""^^T™3 

 it will he necessary to advert hrieny at this stage to the osseous = m WJjJ ">, £ 

 to those portions of the general skeleton more immedmtely «™ e ■> 



Class Aves, the thoracic vertehr* are more or less completely anch losed to we r r 



rigidity, while the rihs are supplied with ^J^^^ S 



together. The cylinder of the thor« ^^^Co^TusclL, tiJ, * their 



shaped sternum, which gives origin to the powerful P«*° 



action upon the wings, heing principally «■*■»*"£ ££*£* great weight 



bird. It may appear a paradox ; hut those -^-^^^ ten d likewise to 



(they are sometimes equal to two-thirds of the ent.re , muwuto * 



depress it ; and it is only hy their heing made to operate upon g . 



VOL. XXVI. 



