98 THE STRUCTURE AND LIFE OF BIRDS cuap, 
Thus, air-sacks are 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. They 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 
