44*2 DR PETTIGREW ON THE PHYSIOLOGY OE WINGS. 



of the wing vary- — the angle made by the part q of figure 16 with the horizon being 

 greater than that made by the part p — this, again, being greater than that made by the tip 

 of the wing o. Those points are also illustrated at figure 8, Plate XII. The letters in 

 figures 1, 2, and 3 (Plate XL) represent the same parts of the wing — x, shoulder joint ; 

 s, elbow joint ; t, Avrist joint ; v, w, hand and finger joints ; x s t v ic, anterior margin of 

 wing ; o p q, posterior margin. 



Figure 4 represents the very oblique and almost horizontal direction of the stroke of the wing in the 

 flight of the insect (wasp) — how the wing is twisted upon itself at the end of the up (a) 

 and down (b) strokes, and how the tip of the wing, during its vibration, describes a figure 

 of 8 track in space (a, c, b). 



Figures 5 and 6 show the more or less perpendicular direction of the stroke of the wing in the flight of 

 the bird (gull) — how the wing is gradually extended as it is elevated (1, 2, 3 of figure 5) — 

 how it descends as a long lever until it assumes the position indicated by 4 of figure 6 — 

 how it is flexed towards the termination of the down stroke, as shown at 4, 5,6 of figure 

 6, to convert it into a short lever (a b), and prepare it for making the up stroke. The dif- 

 ference in the length of the wing during flexion and extension is indicated by the short 

 and long levers a b and c d of figure 6. The sudden conversion of the wing from a long into a 

 short lever at the end of the down stroke is of great importance, as it robs the wing of its 

 momentum, and prepares it for reversing its movements. Those same points are illus- 

 trated at figures 18 and 19, Plate XIV. At 4 of figure 19 the wing is represented as 

 fully extended, and in the middle of the down stroke. At 5 of the same figure the wing 

 is being flexed and slowed, and at 6 it is fully flexed, and its momentum destroyed. The 

 wing is then elevated as a short lever until it assumes the position indicated at 1' of figure 

 18. It is subsequently elevated and extended, as shown at 2' and 3' (fig. 18). At 3 it is 

 transformed into a long lever, and in a condition to make a second down stroke. Figure 1 9 

 also shows the compound rotation of the wing — the tip of the wing rotating upon the axis 

 c d, and describing an arc of a circle, e f — the posterior margin of the wing rotating upon 

 the axis (a b). and describing the arc cj h. The compound rotation of the wing occurs 

 simultaneously with the down and up strokes, and it is to it that the great variety of 

 inclined surfaces made by the under surface of the wing is principally due. 



Plate XII. 



Figure 7 is designed to show that the angle made by the under surface of the wing (more particularly 

 at its root) with the horizon, is much greater than is generally supposed. This arises from 

 the fact that the body of the bird is inclined in an upward direction in flight, and 

 because the anterior margin of the wing (a) curves in a downward direction in such a 

 manner as to conceal the actual angle made. Thus, if e f be taken to represent the horizon, 

 the angle apparently made by the under surface of the wing with it is a b d. The real 

 angle, however, is c b d. 



Figure 8. The lapwing, or green plover (Vanelh/s eristattis, Meyer), with one wing fully extended (c b, 

 d' e' f), the other being in a semiflexed condition (d ef, c IJ). In the extended wingtbe 

 anterior or thick margin (d' e'/') of the pinion is directed upwards and forwards (n't le 

 arrow), the posterior or thin margin (c b) downwards and backwards. The reverse of this 

 happens during flexion, the anterior or thick margin (d e f) of the pinion being directed 

 slightly downwards and forwards (vide arrow), the posterior or thin margin bearing the 

 rowing feathers slightly upwards and backwards. The wings, therefore, twist in opposite 

 directions during extension and flexion. In the flexed condition of the wing the anterior 

 (def) and posterior (b c) margins are nearly on the same level, and the wing acts as 

 a short lever. In this condition of the pinion the primary or rowing feathers (b) are 

 separated from each other, and inclined obliquely upwards and outwards. (These feathers 

 are also shown at 1, 2, 3, 4, 5, 6, 7, 8, 0, 9 of figure 46, page 378.) When, therefore, 

 the wing ascends, the feathers in question (as well as the secondary feathers) cut into 

 the air like so many knives. They thus diminish the resistance experienced from the 

 superimposed air during the up stroke, a result to which the flexing or folding of the 

 wing and its conversion into a short lever contributes. From this account it will be 

 seen that when the wing is flexed the angles made by its under surface with the horizon 

 are diminished, whereas those made by the individual primary and secondary feathers are 

 increased. When the wing is extended it rotates in the direction of its length, the 

 anterior margin (d ef) being gradually inclined upwards and backwards, the posterior 



