SAFETY DEVICES IN BIRDS WINGS GRAHAM 



299 



seen in faster-flapping birds, such as pigeons. It is quite probable 

 that this " doing away with the need for the whole wing to twist " 

 is one of the most important duties of wing-tip slots. 



X. THE WRIST SLOT 



In addition to the wing-tip slots already described, all birds are 

 the fortunate possessors of another antistalling device which is even 

 more like the Handley-Page gear. This is the alula or bastard wing. 





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FiGURB 35. — Probable flow of air through the separated flight feathers 

 of a partridge in the down-beat 



It consists of one main feather overlaid by two or more auxiliary ones 

 which give it strength and thickness. These all spring from a small 

 limb which corresponds in the anatomy of a bird to the thumb of the 

 human hand. In Figure 36 a wing is shown with all the feathers 

 removed except those of the bastard wing and the primaries and 

 secondaries. The relationship to a thumb is unmistakable. 



sAc<y/</er 



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e/ioi- 



— -^-'"-V 



FiGCEB 36. — Under surface of the left wing of a dove, with the covert 

 feathers removed 



This limb has a set of nerves and muscles all of its own. Headley, 

 in The Flight of Birds (1912), remarks that it has more muscles 

 than one would expect to be at the service of so insignificant a piece 

 of machinery. Nowadays (1930) we know that it is not so insignifi- 

 cant, except perhaps in size. 



Shufeldt, in his Myology of the Raven, says that the muscles and 

 tendons that serve the bastard wing are so arranged that when the 



