Bird Flight 181 



presenting a larger surface to the supporting air with 

 very little additional weight. The tail, too, with its long, 

 closely woven quills spread out like a fan, not only serves 

 the purpose of a rudder for guiding the aerial craft, but 

 is still more useful in helping to sustain the bird's weight 

 in the up-buoying element. 



It is interesting to note that the feathers on the bodies 

 of the flying birds are arranged in tracts, with intervals 

 here and there of quite, or almost, bare skin, called 

 " apteria." Now, when a bird is carefully skinned, it will 

 be seen that the feathered spaces have their own special 

 slips of muscles inserted into the roots of the feathers, 

 and when these muscles are contracted, they serve to 

 raise the feathers, and must, therefore, be of some sub- 

 sidiary value in flying, by making the bird's body more 

 buoyant. Suggestive, indeed, is the fact that the plumes 

 of the non-flyers are not arranged in tracts, but are evenly 

 distributed over the body. 



Nor is that all that Nature has done to carry out her 

 evident purpose of making the bird a natural "flying 

 machine. 1 ' The body of the bird contains numerous air 

 sacs, all connected with the lungs, and these, when inflated, 

 are a great help in flying by making the bird light. More 

 than that, many of the bones, though strong, have thin 

 walls and are hollow, the cavities being connected with 

 the lungs and air sacs, from which they are also filled 

 with air, contributing another element of lightness to the 

 aerial navigator. That the bird's bones are capable of 

 being permeated with air can be demonstrated by actual 



