98 THE STRUCTURE AND LIFE OF BIRDS chap. 



Thus, air-sacks arc not peculiar to birds, though they 

 have turned them to account in a way that is quite 

 unique. How important a part they play in 

 respiration I have already described. But they are 

 far larger than is necessary for this, and we shall now 

 have to consider what other purposes they serve. 



Regulation of Temperature. 



Our investigations have already made it clear why 

 a bird is warm-blooded. Thorough oxidation of the 

 blood and a rapid circulation bring about the burning 

 of the tissues which is the cause of animal warmth. 

 It must not be imagined that high temperature can 

 be due to a thick coating of feathers. The)- no doubt 

 help to retain heat, but they cannot produce it. Wrap 

 a lizard up in blankets, and he will still remain cold- 

 blooded. He has not* the digestion, the heart, or the 

 lungs that mark the warm-blooded vigorous animal. 



We must now try to understand by what means 

 warm-blooded creatures in general, and birds in par- 

 ticular, regulate their temperature. This power is one 

 of the most wonderful things in the constitution of the 

 higher animals. In Parry's Polar Expedition, a wolf 

 was shot, and its temperature was found to be 104 F., 

 while the thermometer was nearly 33 below zero. 

 The greater the cold to which the body is exposed, 

 the more rapid the combustion that is always going 

 on within it, so that its temperature does not rise and 

 fall with the thermometer. But it must always be 

 remembered that extremes, whether of cold or heat, 

 must reduce the vigour of the body by obliging it to 



