76 THE FLIGHT OF BIRDS 



feathers (i.e. those which arise from the hand) point 

 mainly outward and only slightly backward. Hence 

 the length of a bird's wing is due largely to feathers, 

 and they are proverbially light things. Their 

 strength is no less remarkable than their lightness. 



The Scaffolding of the Wing. Pneumatic Bones. 



The scaffolding of the wing is itself very light. 

 The thickest of the bones, the humerus, is hollow in 

 big birds that are strong on the wing. In some the 

 bones are hollow right on to the finger-tips ; there 

 is an opening in each bone at the near end (see fig. 

 22) ; a thin pulmonary membrane enters there, and 

 thus they are filled with air that has passed through 

 the lungs. The Gannet is a good example of this 

 complete aeration. Many small birds, however, 

 though first-rate flyers, have all their wing-bones 

 solid ; the Swallow, for example. The Swift has 

 only the humerus pneumatic. The differences be- 

 tween nearly related birds are remarkable. The 

 Gannet's remarkable pneumaticity I have already 

 mentioned ; his near relative, the Cormorant, has 

 only the humerus pneumatic. Such examples seem 

 to make it clear that aeration is an adaptation to 

 the life and habits of the particular species, not an 

 unvarying character firmly established in certain 

 orders of birds.* 



The fact that large birds have more aeration than 

 small demands explanation. But the explanation 



* Besides the wing-bones, the Gannet has the following bones 

 pneumatic : The vertebrae, the greater part of the sternum 

 (not the keel), the ribs, the coracoid, the ischium and the femur. 

 There are also ample air-cushions beneath the skin that covers 

 the breast. 



