158 THE FLIGHT OF BIRDS 



possible for his breast since he is resting his weight 

 upon it. There is every reason to believe that he 

 breathes in the same way during flight, his backbone 

 rising and falling while his breastbone remains 

 steady. It is difficult for the breastbone to move 

 freely, since the pressure inwards of the wings tends 

 to hold it fixed. Were it to move easily it would 

 form a very unsteady pivot for the wings. The 

 lowering of the wing helps to lift the back, for as it 

 descends it hauls upon a muscle which passes from 

 the upper armbone to the backbone, and sometimes 

 even to the pelvis. Attempting to get direct evi- 

 dence of this method of breathing, I suspended a 

 freshly-killed pigeon by its wings and inflated its 

 air-sacs by means of a blowing-tube inserted in the 

 windpipe. The backbone, a little anterior to the 

 thigh-joint, moved rather more than half an inch, the 

 movement of the breastbone being almost too slight 

 to measure. Of course, the conditions that obtain 

 during flight were not reproduced ; there was no 

 pressure inwards. But the only result of such 

 pressure would be to render the breastbone and the 

 bones united with it still less ready to move. 



The spacious air-sacs are useful not only for 

 breathing. The bird regulates his temperature, but 

 not by the machinery that is most effective in human 

 beings and most mammals, for, like his reptilian 

 ancestors, he does not perspire at any part of his 

 surface. During hard exercise he prevents a rise to 

 fever heat by giving off aqueous vapour from his 

 lungs, and besides this the great amount of air that 

 he breathes out when respiration is rapid has pre- 

 sumably a temperature not much below that of the 



