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, 



