76 THE FLIGHT OF BIRDS 
feathers (i.e. those which arise from the hand) point 
mainly outward and@fly 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. 
