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 — 60 times. x 

 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 9 — 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 Physiologic ct Anatomic comparcc, vol. ii., p. 487. 



