96 THE STRUCTURE AND LIFE OF BIRDS CHAR 



speaking, equal to the amount of oxygen absorbed. 

 M. Milne Edwards records some experiments of this 

 nature, the results of which are very striking. 1 The 

 amount of carbonic acid gas exhaled by various 

 animals during a given time was exactly measured, 

 and then equated to one standard, so that the different 

 sizes of the subjects of the experiments might cause 

 no confusion. Thus, the figures that follow enable us 

 to compare animals of the most widely separated 

 classes, in respect of the amount of carbonic acid which 

 they breathe out ; and since, as I have said, this roughly 

 corresponds to the amount of oxygen absorbed, it is a 

 measure of the excellence, or the reverse, of their 

 breathing apparatus. 



The pigeon is a good deal ahead of the guinea-pig, 

 the only other warm-blooded animal in the list. The 

 cold-blooded creatures are far behind the guinea-pig. 



To sum up, then, a bird's respiratory system is, as 

 far as we know, much more active than that of any 

 mammal. As evidence of this we have — (1) the greater 

 amount of carbonic acid given off; (2) the more rapid 

 breathing, the effect of which is much increased by the 

 air-sacks ; (3) high temperature, which could not exist 

 without thorough oxidation of the blood. 



1 Physiologie et Anatomie comparee, vol. ii., p. 534. 



