212 DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 



like action of the fully extended pinion, and the feathering principle (to use a nautical 

 phrase) of the partially flexed one, may readily be inferred from this figure. It is 

 further illustrative as showing the great facility with which a bird may elevate or 

 depress its body, and how it may turn and wheel without the possibility of a mishap. 

 The anterior or thick margin of the wing and the posterior or thin margin present 

 different degrees of curvature, so that under certain conditions the two margins 

 cross each other and form a true helix, as represented at Figs. 70, 73, and 74, PL 

 XV. The anterior margin presents two well-marked curves, a corresponding number 

 being found on the posterior margin. These curves may for the sake of clearness 

 be divided into axillary curves and distal curves, the former occurring towards the 

 root of the wing, the latter towards its extremity. The curves (axillary and distal) 

 found on the anterior margin of the wing are always the reverse of those met with 

 on the posterior margin; i. e., if the convexity of the anterior axillary curve be 

 directed downwards (PL XV. fig. 73^), that of the posterior axillary curve is directed 

 upwards (PL XV. fig. 73 a cf), and so of the anterior (/) and posterior (b) distal 

 eurves. The two curves occurring on the anterior margin of the wing (axillary 

 and distal) are likewise antagonistic, the convexity of the axillary curve being always 

 directed downwards (PL XV. fig. 73 e) when the convexity of the distal one is directed 

 upwards (PL XV. fig. 73/), and vice versd. The same holds true of the axillary and 

 distal curves occurring on the posterior margin of the wing (PL XV. fig. 73 a c b). In 

 some birds, pari icnlarly where the wings are large and there is a considerable degree of 



increment permitted between the phalangeal or finger-bones bearing the primary feathers, 



there is a tendency to multiply or repeat the curves towards the tip of the wing. This 

 was particularly the case in the wing of a Wild Swan, a fine specimen of which I had 

 the good fortune to purchase at one of the London markets. In the win- of this 

 magnificent bird, the movement permitted between the phalanges was such that in 

 extreme extension the feathers were rotated in a slightly upward direction towards their 

 roots, the movement extending to the tip of the feathers, and producing an additional 

 or bird curve at the extremity of the wing. The third curve is smaller than the others 

 and has its concavity directed upwards. At first sight it appears as if it were produced 

 Itl^^^L^l^ •" others, which I have satisfied myself is not 



The yielding of the joints towards the extremity of the 



preventm 



viceable 



. . , • .. J Sh °° k when the P inion ^ forcibly depressed, and, secondly 



introducing, as it were, thewino-to the niv wWt, ™i- •* r , 



The *ZL nJZ '*_ T* ^^l 1 ;'^ 11011 ^ 8 ^ its fulcrum or pivot of support 



axillary and distal curves completely reverse themselves during the 

 sion and flexion, and so of the posterior axillary and distal 



of the 



acts of exten- 

 curves. This reversal 



the M„l hv ft. t0 m ° St '**** in the Potior margin of the wing, formed, 

 PI XV d 'i £ ] ~ll T*7L-- ^/eathers. It is well represented 



ty 



at PI. XV. figs. 73 



74 & 75 In fig. 73 the wing is partia% flexed> the 



of the diatal curve (4) being directed *>_,& thai tf ftTLZT TT 



»«*: In fig. 74 the wing is rather more than half ^ T^ T™ t C) * 

 dition of rhr> nini™ +v,„ , ,-, alf tended; and in this con- 



of the pmmn the curves are obhterated, the posterior margin becoming straight, 











