80 STRUCTURE AND COMPARISON. 



are not necessary. The first thing absolutely required is 

 strength to stand all strains put upon the wing ; the second is 

 surface enough to fasten the muscles, tendons, and strong 

 flight feathers firmly. We can imagine the wing-bones as 

 being made of the right size and shape, and then bored out 

 inside until the weight is reduced as much as is safe. We can 

 see thus that a long and very slender wing-bone like the swift's 

 or swallow's might not perhaps be bored out at all without 

 making it liable to break. 



Lightness is only an advantage, not a necessity in the wing. 

 Safety is the prime essential. To secure this the weight is 

 never reduced to the danger point, and a number of neat 

 devices are arranged to guard against accidents. If you 

 examine the wing of a chicken you will see that while there 

 is considerable freedom of movement in the joints as they 

 lie by the side in the closed wing, as soon as the wing is 

 extended the joints lock and become rigid, so that the wing 

 cannot be twisted back by any sudden flaw. Only at the 

 shoulder is there any flexibility, and this is guarded by the 

 strong muscles that draw the wing up and down. 



Thus we see how perfectly the bird's wing is planned to 

 secure speed and safety with the least exertion. It is not 

 so simple as at first appeared, and there is still more which 

 we shall have no time to study. 



