FLIGHT OF ANIMALS — GRAY 



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to meet over the bird's back; and (2) they no longer swing forward 

 at the end of their downward movement: the bird's head remains 

 clearly in sight at all stages of the flight. Changes also occur in the 

 upstroke; the backward flick of the wrist and primary feathers has 

 almost, if not completely, disappeared : the inner surface (radio-ulnar) 

 of the wing begins to rise first and is followed by the wrist and pri- 

 maries. What these characteristic features of open flight may signify 

 is not altogether clear. During the downstroke, the whole wing travels 

 downward and forward at a relatively small angle to its path of 

 motion, and it seems likely that the resultant "lift" is directed both 

 forward and upward, and can therefore propel the bird as well as 

 support its weight. On the other hand, the upstroke of a bird in full 

 flight appears to be of a more passive kind than the upstroke im- 

 mediately after a takeoff ; perhaps during free flight the wing is pas- 

 sively raised by the pressure of air against its under surface, and its 



Figure 9. — A bat in flight. The wing action is similar to that of a bird. 



rate of upward movement controlled by the braking effect of the 

 pectoralis major muscle. If this is so, the wings, during the upward 

 stroke, act as kites to lift the body merely at the expense of some 

 diminution of forward speed- — rather like an umbrella opened down- 

 wind on a windy day. Wliether this suggestion will bear further 

 scrutiny remains to be seen, but at least it seems to explain one way 

 by which rapid free flight can be made much less tiring than a take- 

 off from the ground — for if our explanation is correct, the relatively 

 small pectoralis minor muscle would be in action only for a little while 

 after each takeoff. After that the only muscle concerned would be the 

 large pectoralis major. 



Among vertebrate animals there is only one other — the bat (fig. 9) — 

 whose powers of flight come anywhere near those of a bird. At pres- 



326511 — 55 21 



