444 DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 



wings are slightly flexed and deeply concave on their under surfaces, the greater 

 concavity of the wings compensating in part for the diminution in their length. They 

 are also further depressed than in figure 15.' At figure 13 the wings are represented 

 as seen at the end of the down stroke, the concavity of their under surfaces heing still 

 more increased, and their length still more diminished. The wings are now short 

 levers, and prepared to make the up stroke, the great convexity of their upper sur- 

 faces diminishing the resistance which they experience from the superimposed air during 

 their ascent. Figures 13, 14, and 15 illustrate very clearly how the downward and for- 

 ward fall of the body during the up stroke contributes to the elevation of the wings. 

 Thus in figure 1 3 the body is up and the wings down. At figure 1 4 the body has fallen 

 a little, and the wings are elevated and spread out more than in fig. 13. At figure 15 

 the body has fallen further, and the wings are spread out to their utmost, and on a level 

 with the body. If we now turn to figure 1 8 of Plate XIV. we will see that the body 

 continues to fall and the wings to rise, as shown at 1, 2, 3 ; 1' 2' 3'. At 3, 3' of this 

 figure the wings are elevated to their utmost, and the body depressed to its utmost. The 

 wings are consequently in a position to make a new down stroke. Erom these figures it 

 will be evident that the wings and body rise and fall alternately, the fall of the body con- 

 tributing to the elevation of the wings, and the descent of the wings necessitating the 

 ascent of the body. It is in this way that the weight of the body comes to play an 

 important part in flight. The alternate waved tracks described by the wings and body in 

 flight are given at figure 14, page 344 ; a, c, e, g, i giving the undulations made by the 

 wings ; 1, 2, 3, 4, 5, those made by the body. 

 Figures 1 6 and 1 7 (Plate XIII.) show the wing in the extended and flexed condition in the gannet. 

 In these figures the body of the bird is exactly in the same position. When the wing is 

 flexed, as in figure 1 7, it is crushed together, the tip of the wing (s, p, v, w) folding 

 beneath the central portion (p, q, t), the central portion and root (x s r) flapping together 

 on nearly the same plane. It is by this means that the wing is converted from a long 

 into a short lever. The flexing of the wing reduces the angles of inclination formed by 

 the several portions of the under surface of the wing with the horizon, and causes the 

 anterior (x, s, t, v, w) and posterior (op q) margins of the pinion to occupy nearly the 

 same level. It, however, increases the angles of inclination made by the primary and 

 secondary feathers, these changes being necessary to reduce the resistance experienced 

 from the air during the up stroke. When the wing is flexed, all its parts are in a lax 

 condition, the wing being principally under the control of the elastic ligaments, the muscles 

 acting more especially during extension. When the wing is pushed away from the side of 

 the body, and extended as represented at figure 1 6, the angles of inclination made by the 

 several portions of the under surface of the pinion with the horizon are increased, while 

 those made by the primary (o p>) an( l secondary (p q) feathers are diminished. The 

 pinion, moreover, is rendered more or less rigid. When the wing is fully extended, it 

 acts as a long lever (compare length of wing in figures 1 6 and 1 7). By increasing its 

 length, the wing also increases its power and speed towards the tip. It therefore attacks 

 the air with great violence during the down stroke, and insures a corresponding upward 

 recoil of the body. The angles of inclination made by the several portions of the under 

 surface of the wing with the horizon vary. Thus the angle made by the portion q s is 

 greater than that made by the portion^ v, and th.it made by p v greater than that made 

 by o w. The diminution and increase of the angles bears a fixed relation to the speed at 

 which the different portions of the wing travel, the angle always being greatest when the 

 speed is lowest, and vice versa. The change in the angles is principally due to the rota- 

 tion of the wing in the direction of its length, the posterior margin of the pinion rotating 

 round the anterior one in a downioard direction during extension (figure 1 6, vide arrows), 

 and in an upward direction during flexion (figure 1 7, vide arrows). It is this rotation of 

 the wing upon its long axis which presents the upper or dorsal surface of the pinion to 

 the spectator in flexion (figure 1 7), and the under or ventral surface in extension (figure 

 16.) These points are further illustrated at figure 8, Plate XII. (see description of figure 

 8.) In figures 16 and 17 the same letters are affixed to the same portions of the wing 

 in both ; x representing the shoulder joints ; s, the elbow joint ; t the wrist joint ; r, w, 

 the hand and finger joints ; op, the primary feathers ; p, q, the secondary feathers; r, 

 the tertiary feathers ; x, s, t, v, w, the anterior margin of the pinion ; o p q, the posterior 

 margin. 



