CHANGES IN THE BLOOD DURING RESPIRATION. 249 



to the' superior displacing power of hydrogen for carbonic acid. 

 For while 15,500 grains' weight of frogs exhaled about 1.13 grain 

 of carbonic acid per hour in atmospheric air, they exhaled during 

 the same time in pure hydrogen as much as 4.07 grains. The same 

 observer found that frogs would recover on the admission of air 

 after remaining for nearly half an hour in a nearly complete 

 vacuum ; and that if they were killed by total abstraction of the 

 air, 15,500 grains' weight of the animals were found to have 

 eliminated 9.3 grains of carbonic acid. The exhalation of carbonic 

 acid by the tissues does not, therefore, depend directly upon the 

 access of free oxygen. It cannot go on, it is true, for an indefinite 

 time, any more than the other vital processes, without the presence 

 of oxygen. But it may continue long enough to show that the 

 carbonic acid exhaled is not a direct product of oxidation, but that 

 it originates, on the contrary, in all probability, by a decomposi- 

 tion of the organic ingredients of the tissues, resulting in the pro- 

 duction of carbonic acid on the one hand, and of various other 

 substances on the other, with which we are not yet fully acquainted : 

 in very much the same manner as the decomposition of sugar 

 during fermentation gives rise to alcohol on the one hand and to 

 carbonic acid on the other. The fermentation of sugar, when it has 

 once commenced, does not require the continued access of air. It 

 will go on in an atmosphere of hydrogen, or even when confined in 

 a close vessel over mercury; since its carbonic acid is not produced 

 by direct oxidation, but by a decomposition of the' sugar already 

 present. For the same reason, carbonic acid will continue to be 

 exhaled by living or recently dead animal tissues, even in an atmo- 

 sphere of hydrogen, or in a vacuum. 



Carbonic acid makes its appearance, accordingly, in the tissues, 

 as one product of their decomposition in the nutritive process. 

 From them it is taken up by the blood, either in simple solution or 

 in loose combination as a bicarbonate, transported by the circulation 

 to the lungs, and finally exhaled from the pulmonary mucous mem- 

 brane in a gaseous form. 



The carbonic acid exhaled from the lungs should accordingly be 

 studied by itself as one of the products of the animal organism, and 

 its quantity ascertained in the different physiological conditions of 

 the body. The expired air usually contains about four per cent, of 

 its volume of carbonic acid. According to the researches of Yier- 

 ordt, 1 which are regarded as the most accurate on this subject, an 



1 In Lehmann, op. cit., vol. ii. p. 439. 



