VI FORM AND FUNCTION 95 
mainly to the more rapid breathing caused, and the 
consequent more rapid oxidation of the blood. Now, 
we have seen that in birds the air rushes in through 
the lungs into the air-sacks behind, and that the latter 
have a capacity many times as great as that of the 
lungs. Not only, therefore, does the fresh air pene- 
trate all the bronchial passages on its way to the air- 
sacks, but expiration also will bring to the lungs a 
supply of air only slightly vitiated, since it will drive 
into them the as yet unused air in the sacks. This 
fact must be viewed in connection with the known 
rapidity of a bird’s breathing. According to M. 
Milne Edwards big birds, when inactive, breathe 
20—30 times a minute, small birds 30—6o times. ! 
The thick coating of feathers makes it difficult to 
count a bird’s respirations. In ducks, which I have 
watched closely, they have been from 18 to 22. Even 
this lower estimate makes a bird breathe more rapidly 
than we do ourselves, for an adult man, when sitting 
still, averages only 13—15 respirations per minute. 
In the case of a young horse, according to M. Milne 
Edwards, the average is 10—12 per minute, of an 
adult horse 9g—10. In comparing a bird’s rate of 
breathing with that of other animals, we must bear in 
mind the fact, that exhalation brings air that is 
practically fresh to the lungs, so that a duck’s 18 
breaths per minute, taking the lowest estimate, ought 
to be counted as nearly 36. 
It is possible to obtain more accurate evidence of an 
animal’s respiratory activity by measuring the amount 
of carbonic acid gas given off, for this is, roughly 
1 Physiologie et Anatomie comparée, vol. ii., p. 487. 
