is a third series of long, almost “ 
quill-like ” feathers running 
from the elbow to the body, thus closing up what would other- 
wise be a gap between the wing surface and the body, rendering 
flight impossible. 
The most important muscles of the wing are those which have 
to provide the power for the down-stroke of the wing. And 
these are the ‘ 
‘pectoral ” or “* breast-muscles ”—which form such 
dainty meat in a roast fowl. Owing to their great bulk the breast- 
bone itself would be insufficient to afford them attachment. 
This is furnished by the development of a deep, median keel, 
so that the breast-bone of a bird, such as a pigeon, bears a fanciful 
resemblance, when seen in profile, to the hull of a ship—unusually 
shallow—with a very deep keel. The front end of the breast- 
bone supports two slender rods of bone, and these in their turn 
support the long, sword-like blade-bone, and the “* merry-thought.” 
The general appearance of this frame-work for the support 
of the wing and its muscles can be seen in the adjoining illustra- 
tions. But it must be remembered that in their relative sizes 
and disposition these various parts present a very considerable 
range of differences. That these differences are correllated 
with different forms of flight goes without saying, but, be it noted, 
no one, as yet, has attempted to discover in what way they are 
related. Some of the readers of this book may, perhaps, be 
tempted to try and solve the problems which these differences 
