153 RESPIRATION. 



Some physiologists have attempted to fix certain hours of the day when the exhalation 

 of carbonic acid is at its maximum or at its minimum ; but the respiratory activity is 

 influenced by such a variety of conditions, that it is impossible to do this with any degree 

 of accuracy. 



Relations between the Quantity of Oxygen consumed and the Quantity of 



Carbonic Acid exhaled. 



Oxygen unites with carbon in certain proportions to form carbonic-acid gas, the vol- 

 ume of which is precisely equal to the volume of the oxygen which enters into its com- 

 position. In studying the relations of the volumes of these gases in respiration, we have 

 a guide in the comparison of the volumes of the inspired and expired air. It is now 

 generally recognized that the volume of air expired is less, at an equal temperature, than 

 the volume of air inspired. Assuming, then, that the changes in the expired air, as re- 

 gards nitrogen and all gases except oxygen or carbonic acid, are insignificant, it must be 

 admitted that a certain quantity of the oxygen consumed by the economy is unaccounted 

 for by the oxygen which enters into the composition of the carbonic acid exhaled. We 

 have already noted that from T V to ^, or about 1*4 to 2 per cent, of the inspired air is 

 lost in the lungs ; or it may be stated, in general terms, that the oxygen absorbed is equal 

 to about five per cent, of the volume of air inspired, and the carbonic acid exhaled, only 

 about four per cent. A certain amount of the deficiency in volume of the expired air is 

 to be accounted for, then, by a deficiency in the exhalation of carbonic acid. 



The experiments of Regnault and Reiset, to which frequent reference has been made, 

 have a most important bearing on the question under consideration. As these observers 

 were able to accurately measure the entire quantities of oxygen consumed and carbonic 

 acid produced in a given time, the relation between the two gases was kept constantly in 

 view. They found great variations in this relation, mainly dependent upon the regimen 

 of the animal. The total loss of oxygen was found to be much greater in carnivorous 

 than in herbivorous animals ; and, in animals that could be subjected to a mixed diet, by 

 regulating the food, this was made to vary between the two extremes. The mean of 

 seven experiments on dogs showed that, for every 1,000 parts of oxygen consumed, Y45 

 parts were exhaled in the form of carbonic acid. In six experiments on rabbits, the 

 mean was 919 for every 1,000 parts of oxygen. 



In animals fed on grains, the proportion of carbonic acid exhaled was greatest, some- 

 times passing a little beyond the volume of oxygen consumed. 



" The relation is nearly constant for animals of the same species which are subjected 

 to a perfectly uniform alimentation, as is easy to realize as regards dogs ; but it varies 

 notably in animals of the same species, and in the same animal, submitted to the same 

 regimen, but in which we cannot regulate the alimentation, as in fowls." 



When herbivorous animals were entirely deprived of food, the relation between the 

 gases was the same as in carnivorous animals. 



The final result of the experiments of Regnault and Reiset was, that the "relation 

 between the oxygen contained in the carbonic acid and the total oxygen consumed, varies, 

 in the same animal, from 0*62 to 1-04, according to the regimen to which it is subjected." 

 These observations on animals have been confirmed in the human subject by M. Doyere, 

 who found a great variation in the relations of the two gases in respiration ; the volume 

 of carbonic acid exhaled varying between 1*087 and 0*862 for 1 part of oxygen consumed. 



The destination of the oxygen which is not represented in the carbonic acid exhaled 

 is obscure. Some have thought that it unites with hydrogen to form water ; but there is 

 no satisfactory evidence of the formation of water in the economy, and researches have 

 failed to show that there is more thrown off from the body than is taken in with food and 

 drink. 



The variations in the relative volumes of oxygen consumed and carbonic acid produced 



