DP PETTIGPEW ON THE PHYSIOLOGY OF WINGS. 445 



Plate XIV. 



Figures 18 and 19 represent the several positions assumed by the wing of the gull during extension 

 and flexion, and during the down and up strokes. Figure 1 9 also shows how the wing 

 during its ascent and descent rotates upon two axes. At 4 of figure 19 the wings are 

 represented as they appear at the middle of the down stroke. They are fully extended, and 

 affording their maximum of support. At 5 of this figure the wings are slightly flexed, and 

 more deeply arched than at 4. They are also on a lower level. At 6 the wings are represented 

 as they appear at the end of the down stroke. They are now fully flexed and form short levers. 

 They are also more deeply arched than at 5, a circumstance which prepares them for making 

 the up stroke, as the arching renders the upper or dorsal surfaces of the wings very markedly 

 convex. The wings, when in the positions indicated by 6 of figure 19, are elevated as 

 short levers, until they assume the positions indicated by 1, 1' of figure 18. The wings 

 are then pushed away from the body, and extended and elevated, as shown at 2, 2' 

 and 3, 3' (fig 18). At 3 3' the wings are fully extended and fully elevated, and ready to make 

 the down stroke. They descend as long levers, until they assume the positions indicated 

 by 4 of figure 1 9, the changes in position just described being repeated in rapid succession 

 as the wings vibrate. The wings are flexed towards the termination of the down stroke 

 (5 and 6 of figure 19) to convert them into short levers, to destroy the momentum acquired 

 by them during their descent, and to prepare them for making the up stroke. They are 

 extended towards the termination of the up stroke (2, 2'; 3, 3' of figure 18) to convert 

 them into long levers, and to prepare them for making the down stroke. Figure 18 repre- 

 sents the bird when it is flying vigorously, or when it is rising or picking up garbage 

 from the surface of the sea. In leisurely flight the wings do not rise much above the level 

 of the body, as shown at figure 19. In this case the wings are made to play rather under 

 than above the body {vide p. 374). The compound rotation of the wing is shown at figure 

 1 9, the wing rotating at its root (a) and along its anterior margin (c b), the tip of the 

 wing describing the arc of one circle (e b f ), and the posterior margin of the wing the arc 

 of another circle (g d h). The compound rotation of the wing is further illustrated at 

 figure 45, page 376. 



Figure 20. Wing of the piet in the extended position. — In this figure the under lapping of the 

 primary (12345678 9) and secondary (j Jc I m n op q r s) feathers are shown, and how 

 the axis of each primary feather occupies a more and more central position in proportion 

 as it is placed nearer the secondary feathers. This want of symmetry in the primary 

 feathers is necessary to their valvular action during flexion and extension. The wing 

 during its vibration forces a certain portion of the air in waves along its under surface in 

 the direction of its root, as indicated by the arrows and dotted lines ; the greater portion 

 of the air, however, is urged from the tip and posterior margin of the wing in a backward 

 and downward direction, the reaction propelling the body upwards and forwards. The 

 commotion produced in the air by the tip and posterior margin of the wing is on all 

 occasions very great, as the exposure of a flame behind or to the outside of the wing will 

 readily satisfy. 



Plate XV. 



Figures 21 and 22 represent the muscles and elastic ligaments of the wings of the snipe, as seen on 

 the ventral and dorsal aspects. In figure 21 (ventral aspect) the wing to the right of the 

 observer is fully extended, and the elastic ligaments put upon the stretch. The wing to 

 the left of the observer is represented as flexed, the elastic ligaments being in a state of 

 contraction. The same points are illustrated at figure 22, which represents the dorsal 

 aspect of the bird. The wing is flexed principally by the action of the elastic ligaments. 

 It is extended chiefly by voluntary muscular efforts. Those figures show the difference in 

 the length of the wing in the extended and flexed condition, the pinion being a long lever 

 in extension, and a short one in flexion. That the elastic ligaments are subsidiary, and to 

 a certain extent under the control of the muscular system, is evident from the fact that 

 voluntary muscular fibres run into the ligaments in question. Thus the voluntary muscular 

 slip marked a in figure 21 terminates in the fibro-elastic band k ; this, again, being geared 

 to voluntary muscle x, and to certain musculo-fibrous bands j. Their conjoined action is 

 to flex the forearm upon the arm, the arm being drawn towards the body by a musculo- 



