EXCHANGE OF GASES OF MINOR IMPORTANCE 53 



eggs and chrysalides, to the liberation of nitrogen which had previously 

 been dissolved in the fats catabolized, and in the case of the mice per- 

 haps to ammonia liberated as such and converted into free nitrogen 

 by combustion in the respiration apparatus. The sources of error of 

 the earlier investigations were inquired into, and it was shown that the 

 figures of Regnault and Reiset, when rightly interpreted, were in ac- 

 cordance with the result that the excretion of gaseous nitrogen, if 

 any, must be extremely slight. 



There are, however, certain conditions in which gaseous nitrogen 

 will be excreted and may constitute a serious source of error with 

 regard to the nitrogen balance. The alimentary canal of herbivorous 

 animals always contains denitrifying bacteria and if nitrates or nitrites 

 are present in the food (beetroot) these substances disappear, as shown 

 by Rohmann [1881], more or less completely. Rohmann concludes 

 that they are probably reduced and perhaps yield free nitrogen. Un- 

 published experiments by the writer have shown that nitrates when 

 added in the mercury pump to the contents of the caecum of a rabbit 

 are at least partially converted into free nitrogen, but the excretion of 

 free nitrogen from a living animal after feeding with nitrates has still 

 to be demonstrated by respiration experiments. 



HYDROGEN AND METHANE. 



Hydrogen and methane are formed by fermentation processes in 

 the alimentary canal. In carnivorous animals the quantity is usually 

 small but in herbivorous it may be considerable. The presence of 

 these gases in the expired air was demonstrated by Regnault and 

 Reiset, and it was shown by Tacke [1884], who gave a good biblio- 

 graphy of the earlier literature, that they are chiefly excreted through 

 the lungs and not through the anus. In herbivorous animals they 

 must be taken into account in metabolism experiments, amounting 

 for instance in rabbits to 2 to 8 c.c. per kilogram and hour (Tacke) 

 and in goats to 10 to 30 c.c. (Boycott and Damant [1907]). 



Carbon dioxide is formed together with hydrogen and methane in 

 the gut. Boycott and Damant found on goats that the ratio of CO 2 

 to H 2 + CH 4 produced in the intestine was variable, but on an 

 average probably above 2. The intestinal CO 2 must therefore amount 

 to at least 10 per cent, of the total, and as this carbon dioxide is pro- 

 duced by fermentation and not by oxidation the true respiratory 

 quotient is much lower than the apparent. This may have been a 



