378 



DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 



figures 5 and 18, Plates XI. and XIV., it is suddenly pushed away from the 

 body, extended and elevated, as shown at 2 and 3 of the same figures ; the angles 

 made by the several portions of its under surface with the horizon being in- 

 creased, while those formed by the under surfaces of the rowing feathers are 

 decreased (12 3 4 5 6 7 8 9 of fig. 47). The wing thus comes to form a kind 

 of natural parachute, as shown at 2, 2' and 3, 3' of figure 18, Plate XIV. 

 This completes the up or return stroke. While the wing is ascending, the 

 primary, secondary, and tertiary feathers rotate upon their long axes, and 

 present their thin margins to the air, into which they cut like so many knives. 

 The feathers are most widely separated at the beginning of the up stroke, and 

 least at the termination of that act, as they then flap together to make the 

 wing impervious, and prepare it for making the down stroke. The individual 

 primary, secondary, and tertiary feathers are so arranged and so rotated that they 

 open up, and close, and present the precise angles required for flight, whatever 

 the shape and whatever the position of the wing. 



Fig. 46. 



Fig. 47. 



Fig. 48. 



Figure 46 shows the tips of the primary (r) and secondary (s) feathers in the 

 wing of the piet during flexion, and it will be observed that the angles made 



