DB, PETTIGREW ON THE PHYSIOLOGY OF WINGS. 377 



are three in number, the outermost band breaking up at the root of each 

 feather, and giving off two processes (a d, b e, cf of figure 25, Plate XVI.), the 

 one of which coils round the root of the feather in a spiral manner from right 

 to left ; the other coiling in an opposite direction, or from left to right (m, n of 

 figure 26, Plate XVI.) The root of each feather is consequently enveloped by a 

 fibrous investment, capable of rotating it in opposite directions. The fibrous 

 bands referred to are arranged with much precision, and as they are geared to 

 each other at stated intervals, they cause the feathers (right wing) to rotate 

 at nearly the same instant from right to left, and from below upwards, during 

 extension ; and from left to right, and from above downwards, during flexion. 

 The arrangement of the fibrous bands is much the same on the dorsal and 

 ventral aspects of the wing (compare figs. 24 and 28). It varies slightly in dif- 

 ferent species of birds, but the function of the bands is the same in all. 



The tips of the primary, secondary, and tertiary feathers are prevented from 

 rising too high during the descent of the wing by the oblique overlapping of the 

 feathers forming the primary, secondary, and tertiary coverts (m, n, o of figure 

 28, Plate XVI.), those feathers acting as buffers and limiting the action. 



The Up or Return Stroke of the Wing of the Bird — Diminution of Area of 

 Wing — Valvular Action, §c. — Towards the termination of the down stroke, the 

 wing is suddenly flexed and drawn towards the body, as shown at 4, 5, 6 of 

 figures 6 and 19, Plates XL and XIV. This is necessary to convert the wing 

 from a long (Plate XL figure 6, c d) into a short lever (Plate XL figure 6, a b), 

 and to destroy the momentum acquired by the wing during its more or less 

 vertical descent. While the wing is being shortened, the angles which the 

 several portions of its under surface make with the horizon are being diminished 

 (c d ef of figures 16 and 17, page 349) ; the angles made by the under surfaces 

 of the rowing feathers from within outwards being increased (123456789 

 of fig. 46, p. 378). These changes prepare the wing of the bird for making 

 an effective up or return stroke, and are necessitated by the more vertical 

 play of the bird's wing, as compared with that of the insect. But for the 

 diminution of the actual area of the wing during the up stroke, the upper or 

 dorsal surface of the pinion would experience much resistance from the 

 air during its ascent. This difficulty is in a great measure obviated by the 

 wing being drawn close to the side of the body, and by its being made to 

 assume a somewhat crippled appearance, the tip of the wing folding upon 

 the root in a direction from below upwards, and in such a manner as to displace 

 comparatively little air (vide 4, 5, 6 of figure 6, Plate XL) The pinion is 

 then, as a rule, elevated as a short lever (a b of figure 6, Plate XL), until it 

 attains the position indicated at 1 of figures 5 and 18, Plates XL and XIV. 

 In these situations the wing is for the most part deeply arched (vide figure 13, 

 Plate XIII.) When the wing has assumed the position indicated by 1 of 



