158 THE FLIGHT OF BIRDS 
possible for his breast gince 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 
