178 THE STRUCTURE AND LIFE OF BIRDS  cHap. 
out upon the wing, the fulcrum is at a point not far 
from the tip. As a fact, of course, the fulcrum is dis- 
tributed over the whole wing, but since, owing to its 
more rapid motion, the end meets with far more 
resistance than the base, we may consider it to be 
not far from the tip. Here it will be well to mention 
something that often makes living machinery puzzling. 
The different parts are not distinct. For instance, 
when a man breathes, his chest is a suction pump. 
But there is no separate piston. The walls of the 
chest, that is, the walls of the pump itself, expand and 
so cause a vacuum. In the same way we have been 
speaking of the bird’s body as the weight to be raised, 
of the wings as levers, and of the power as residing in 
certain muscles. But the muscles in question form 
part of the body, and they and also the wings go to 
make up the weight. Nor have we yet done with the 
complications in which we get involved when we 
study the wing as a lever. When it is being moved 
rapidly through the air in order to gain a fulcrum, by 
the help of which to move the body, the weight is, at 
first, at the extremity in the shape of the resistance 
of the air that has to be overcome, while the fulcrum 
is at the shoulder-joint. When the fixed point has been 
gained, then the end of the wing becomes the fulcrum, 
and the body is the weight. But it is only in imagina- 
tion that we can divide the down-stroke into two such 
periods. During the whole of it we have at the near 
end both a weight and a fulcrum, during the whole of 
it both a weight and a fulcrum at the further end. 
The body is always suspended from the wings, the 
ends of the wings never cease to move as they strive, 
