250 DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 



direction in which the body of the bird is being urged. This additional difference is to be 

 noted : the blade of the oar is applied vertically, or with its maximum of surface, during 

 the effective stroke, and horizontally, or with its minimum of surface, during the non- 

 HVoctiYe one; the wing, on the contrary, is inclined more or less obliquely both during 

 the down or effective and the up, non-effective, or return stroke. This arises from 

 the fact that the oar simply impels, whereas the wing both impels and elevates. 



Function of individual Feathers. The Wing rigid in extension. — While the bones of the 

 forearm and hand are revolving on their own axes during extension, and causing the 

 primary and secondary feathers to make an angle of 30°, or thereabouts, with the horizon 

 (PI. XIV. figs. 32, 35 & 42), the feathers themselves are likewise rotating on their axes, 

 these being placed obliquely during flexion, to admit of the air passing between them, 

 but gradually presenting their flat surfaces to form an unyielding phalanx and prevent 

 its escape during extension. In extension the wing is rigid, its bones and soft parts 

 being, as it were, screwed home or fixed (PL XIV. figs. 30, 31, 41 & 42). The feathei 



tribute to this result by individually overlapping each other, the primaries and 

 secondaries overlapping each other more and more completely in a direction from with- 

 out inwards (PL XV. figs. 63 and 67, b a), these being overlapped by their respective 

 coverts and subcovcrts from above downwards (d e). The wing in extension is thus 

 balustrad. 1 or propped up in every direction; and I am of opinion, as I have already 



of the elbow and wrist is the elevating and depressing of the wing during extension and flexion ; the mere act of 

 externum raising the pinion preparatory to its making the downward stroke, the act of flexion gathering the pinion up 

 or off the wind preparatory to a second extension. These movements are analogous in every respect to those made by 

 corresponding parts in the Bat, and by the arms in swimming. 



The Elbow, Wrist., and other Joints always at nearly right angles.-^ long axis of the elbow-, wrist-, and other 

 joints during extreme extension is always the reverse of what it is during extreme flexion ■ hut whatever the direction of 

 the ax,s of one joint the axis of the joint next to it is always nearly at right angles. The joints in this manner 

 coordinate and complement each other. The conformation of the elbow-, wrist-, and other joints, and the movements 

 of the bones of the hand on the forearm, and of the forearm on the humerus, account for the wing of the bird and bat 

 being twisted upon themselves, and for their peculiar spiral mode of action. In the insect, where the wing, as a rule, 



rj m l W 7 r, 1 , n° *' ^ *« ^ «* is ^^ ° n the P**» * ** and retained. 



Lateral Moments * the El Low JJ r ist-, and other Joints.-ln addition to the movements described by the elbow- 



ml wnst-joints during extension and flexion, a considerable degree of lateral motion is permitted. This is best seen by 



l7t£ ^TI',* 1 ^ TZf* f0ream UP ° n % " *** ^ f — a » d ***** ^e hand upon that" 

 It the bird be placed with its head away from the snertAtnr an A tk a * i » , 



.* tl,P ,dNn«- will h* f nnn i ( ( . . spectator, and the forearm be rotated towards him, the motion 



at the elho« will be found most extensive in an unwarrJ nr,ri /,«,./.,„„ ^ i- 



tl,P W n S t inint Tl„ . v, • i / , backward direction, the reverse of this holding true of 



the wrist-joint. 1 he same may be said of the metfleamnl -,n,i r>K„i„ i • • . 



nnnn the r^nam,! th» ™ , ' i 1 ™tacarpal and phalangeal joints ; so that the bones of the hand twist 

 upon the metaca pal, the metacarpal on the carpal, the carpal on the radios and ulna, and the radius and ulna upon 

 t ehnmeru, The alternating spiral arrangement is also manifest in the disposition of the bones of the wing, -the 

 ulna, which is the principal bone of the forearm, curving from below u pW ard S) while the metacal 1 and Xlangeal 

 bones curve from above downwards. Advantage is taken of th;, „;„ L tnetacai pal and phalangeal 



... iW ° e 1S iaKen ot thl3 circumstance to transfer the sniral arrangement ob- 



taining ,n the osseous structures to the feathers principally employed in fl-ht fh. 7 i i I i u 



supporting the primary feathers, the ulna the secondary "ones Tht coW ' T^ ^'^ 



of the wing and the feathers thereof is necessary, 1^^^^^^ TTt " ^ d' 

 revolve upon their axes during extension and flexion, and in so doing hey olZZl ' "f 7 17 



on and off the wind, and give them the precise angles necessary for fl^ht * ' "^ 



It is a curious circumstance that the movements described bv th?fnr ao » j u i , , 

 those described bv the mf ^ m _:„„ „„, 1 J 7 the forearm ™d hand of the bird are analogous to 



6) 



