DR PETTIGEEW ON THE PHYSIOLOGY OF WINGS. 443 



one downwards and forwards. The rotation of the wing on its long axis during exten- 

 sion increases the angles made hy the under surface of the wing with the horizon, 

 but decreases the angles made by the individual primary and secondary feathers, these 

 being made to flap together, and to assume a more or less horizontal position, as is well 

 shown at a b c d efg hij klmn op q of figure 48, page 378. This napping together of the 

 primary and secondary feathers during extension effectually prevents the air from passing 

 between them. The power of the wing is greatly augmented during the down stroke — 

 1st, by its being converted into a long lever ; 2d, by the flapping of the feathers together ; 

 3d, by its under surface being rendered deeply concave (page 378, figure 48); and 4th, 

 by the various angles of inclination made by the several portions of the under surface 

 of the wing with the horizon being increased. These points are further illustrated at 

 figures 16 and 17, Plate XIII. At figure 17 the margins of the primary (o p) and 

 secondary (q) feathers, as seen in flexion, are given; whereas in figure 16 the flat of the 

 feathers {op q), as seen in extension, are shown. These figures also show that, as the 

 angles made by the under surface of the wing with the horizon increase, the angles 

 made by the individual primary and secondary feathers (o p q) decrease, and vice versa. 

 The angles made by the primary and secondary feathers are increased during the up 

 stroke, when the speed of the wing is slowed, and decreased during the down stroke, 

 when the speed is increased, an inclined surface, which forms a large angle with the horizon, 

 giving, when forced against the air at a low speed, the same amount of buoying power 

 as an inclined surface, which forms a smaller angle when urged at a lower speed. 



figures 9, 10, and 11 (Plate XII.) show the wing of the gannet in the flexed, semiflexed, and extended 

 condition. Those figures are also intended to illustrate how the various inclined surfaces 

 made by different portions of the under surface of the wing in extension and flexion are 

 directed forwards, backwards, outwards, and inwards. Thus in flexion and semiflexion 

 (figures 9 and 10), the portions of the wing marked g h and c d, are inclined upwards and 

 inwards (vide arrows), whereas the portions marked e/and a b are inclined upwards and 

 outwards. "When the wing is being extended, as in figure 10, the portions marked ef and 

 a b produce or draw after them a current, on which the portions marked c d and g h operate 

 when the wing is being flexed, and vice versa. When the wing is fully extended, as at figure 

 1 1 , the inclined surfaces indicated by g h, c d, e f, and a b of figures 9 and 1 disappear, 

 the under surface of the wing making a variety of inclined surfaces, which are directed 

 principally upwards and forwards, as shown at figure 16, Plate XIII. It is in this way 

 that the wing is capable of change of form in all its parts, and it will be observed that 

 those changes are induced irrespectively of any resistance experienced from the air. 

 When the wing ascends, it draws after it a current on which it operates when it descends; 

 and when the wing descends, it produces a current which assists in the elevation of the 

 wing. By the acts of flexion and extension, and by the down and up strokes, the wing of the 

 bird and bat produces the whirlwind on which it depends for support and progress. The 

 tip of the wing rotates upon t of figures 9 and 10 (Plate XII.) as a centre, and by its alter- 

 nately darting in and out in flexion and extension, it describes the segment of a circle 

 (m n), and contributes to the stability of the bird by increasing the area of support. 



The letter x in figures 9, 1 0, and 1 1 indicates the shoulder joint ; 6', the elbow joint ; t, the wrist 

 joint; v and w, the hand and finger joints; op (fig. 11), the primary feathers; p q, 

 the secondary feathers ; r the tertiary feathers ; x stvw, the anterior margin of the wing; 

 and o p qr the posterior margin. 



Figure 12 shows how the wing is twisted upon itself structurally, and how the tip of the wing forms 

 an inclined surface, which is directed upwards and outwards (see arrows marked a and b). 

 x, m, n, anterior margin of wing ; o p q, posterior margin. 



Plate XIII. 



Figures 13, 14, and 15 represent the flight of the gull with the wings in the flexed, semi-flexed, and 

 extended conditions. The letters indicate the same parts of the wing in all the figures, 

 x representing the shoulder joint, s the elbow joint, t the wrist joint, and v and w the 

 hand and finger joints ; o p the primary feathers, and q the secondary ones. At figure 1 5 

 the wings are fairly twisted upon themselves, and form true screws. In this figure the 

 pinions are extended to their utmost, and affording their maximum of support. They 

 are represented as they are seen at the middle of the down stroke. At figure 14 the 



VOL. XXVI. PART II. 5 Y 



