368 RESPIRATION. 



more active than that of mammals, but this is by no means an 

 invariable law, and may very probably depend upon the mode of 

 life of the animals ; since hens, for in stance, which seldom fly, con- 

 sume very little more oxygen than rabbits, and not even so much 

 as dogs ; while the ratio of the absorbed oxygen to the oxygen 

 re-appearing in the carbonic acid is very nearly the same in hens 

 which have been fed upon oats as in rabbits. Very different 

 respiratory relations exist in those more active birds which sing 

 much, are constantly flying about, and seldom at rest except 

 during sleep. Birds of this kind consume more than ten times 

 the amount of oxygen absorbed by proportionally more inactive 

 birds, such as hens, whilst they also exhale nearly ten times more 

 carbonic acid. The experiments of Regnault and Reiset also 

 exhibit a great difference in this respect, that in the more active 

 birds far less oxygen (only three-fourths) is employed in the 

 formation of carbonic acid ; but this ratio may, however, probably 

 be dependent upon the fact that in the experiments in question 

 the birds were fasting, being alarmed and off their feed. Regnault 

 and Reiset, moreover, refer the great absorption of oxygen and 

 exhalation of carbonic acid to the smallness of these animals, and 

 connect it with the greater necessity for heat in the smaller 

 animals. Although* I was long since led by my own experiments 

 to express the view, that the excretion of carbonic acid in birds 

 stood in an inverse ratio to their size, it appears to me that the 

 necessity for heat may afford the most available ground on which 

 to explain this fact. The cause must undoubtedly be sought in 

 the greater activity and in the consequently more rapid metamor- 

 phosis in the more active birds, although it is unfortunately only 

 the smaller varieties which can be employed for such experiments. 

 If we were able to investigate the respiratory equivalents of 

 vultures and other large birds of prey, which continue for a long 

 time on the wing, and if we could examine them under their 

 natural relations, we should most certainly find them much greater 

 than in the case of hens, ducks, geese, &c. Nature may, however, 

 have endowed smaller birds with greater energy and a more rapid 

 metamorphosis, in order to enable them to maintain the same 

 temperature of body as larger birds. 



The smaller birds whose respiratory equivalents we have given 

 in the following table from the mean results of Regnault and 

 Reiset, were green-finches, crossbills, and sparrows. 



* Jahresber. der ges. Med. 1843 n. 1844, S. 39. 



