PETTIGREW ON THE MECHANISM OF FLIGHT. 211 



arrangement is necessary because the strain on the feathers during flight 

 proportion to their distance rom the trunk. 



The Wing of the Bird twisted upon itself; its shape to.— The feathers, as will he 



from this description, become longer and stronger from before backwards and ii 



within outwards; and the length and shape of the various Bets of fathers, particularly 

 the primaries, determine to a groat extent the shape of the wing. Thus, if the first 

 primary be longest, as in the Golden Plover, Curlew (PL XTV. fig. 86), the wing is 

 acuminate; whereas if the third or even the fourth be longest, as happens in the 

 Partridge (PL XIV. fig. 32) and Pheasant, the wing is rounded or circular. Between 

 these extremes every variety of form may be encountered*. 



Although the feathers are longest and strongest towards the extremity of the wing, 

 it does not follow that the wing itself is strongest in this direction. On the contrary, 

 it is strongest towards the axilla or shoulder, the unwieldy or weighty parts being in 

 this manner placed nearer the centre of gravity (which in birds falls between the (rings, 

 somewhere in the pectoral region f), where they are more immediately under control. 

 The wing of the bird in this respect closely resembles the wing of the insect, both 

 tapering from before backwards, and from within outwards, and presenting on section 

 a conical surface in either direction. The wing of the bird, like that of the insect and 

 bat, is concavo-convex (Plate XIV. figs. 28 & 32), and more or less twisted upon itself 

 when extended (p. 253, Diagram 18 a. d' e' 'f ', a V), so that the upper or thick margin of 

 the pinion presents a different degree of curvature from that of the nether or thin 

 margin, the curves presented by the two margins in the maj ority of instances i n 1 ersect- 



■5 



each other (PL XV. fig. 70 dfeb; also Diagram 18 a. d' e'f, a //). This 



great measure owing to the arrangement of the primary, secondary, and tertiary feathers, 

 to the manner in which the bones of the wing of the bird are twisted upon themselves, 

 and to the spiral nature of their articular surfaces,— the long axes of the joints, as in the 

 bat, always intersecting each other at nearly right angles. 



The Wing of the Bird moveable in all its parts 



formed by 



sion and flexion.— The double twisting-process to which I refer is particularly well 

 seen in PL XIV. fig. 29 ; and to this I would specially direct the attention of the reader, 

 as it shows how, during flexion, the anterior or thick margin of the wing is slightly 

 directed downwards (ef), the pinion presenting a narrow cutting edge to the wind; 

 whereas during extension, the anterior or thick margin is very decidedly directed up- 

 wards (d' ef), the wing in this case presenting its maximum of surface. The kite- 



* " The wing 



is snort, oroau, uuutw a»u iuu«u».i* "« "-"—i -■ o — » - — - - *» - 



r ^ost Pigeons. In the Peregrine Falocn it is acuminate, the second quill being longest, and the first 



little^horter - and in the Swallows this is still more the case, the first quill being long -t. the rest rapidly diminishing 



in length." — Macgillivray, Hist 



c< 



gnoble 



wing 



H.Salvin and IV, 



the short-winged ones by the second feather of the wing being either the longest or equal in length jo^the third, and 

 by the nature of the stoop made in pursuit of their prey." 



Brodrick, Lond. 1855, p. 28. 



t " In flight the centre of gravity is brought forward beneath the origin of the wings ; but in stand.ng it is carried 



various degrees backwards."— Macgillivray, Hist, of Brit. Birds, p. 47. 



