262 SOURCE OF CARBONIC ACID EXHALED. 



tion of carbon ; and this agreement in tlie results naturally 

 leads to the belief that the causes of both are the same. It 

 is to be borne in mind, however, that the substitution of 

 carbonic acid for oxygen is not the only change produced by 

 respiration in the air ; for there is nearly always a disap- 

 pearance of oxygen (to an amount sometimes equal to one- 

 third of that exhaled in combination with carbon), which 

 is taken into the system to be applied to other uses 

 ( 343). 



306. It was at one time supposed that the oxygen of the 

 inspired air combines, in the lungs themselves, with the carbon 

 brought there in the blood ; and thus produces the carbonic 

 acid which is expired, occasioning at the same time the 

 development of heat. But this theory is inconsistent with 

 experiment ; for it has been proved that the carbonic acid, is 

 not formed in the lungs, but that it is brought to them in 

 the venous blood of the pulmonary artery ; and that their 

 office is to disengage or get rid of it, whilst they at the same 

 time introduce oxygen into the arterial blood. For in the 

 first place, it can be shown that a considerable quantity of 

 carbonic acid exists in venous blood, from which it may be 

 removed by drawing it into a vessel filled with hydrogen or 

 nitrogen, or by placing it under the vacuum of an air-pump ; it 

 can also be shown that arterial blood contains a consider- 

 able quantity of oxygen. Again, if Frogs, Snails, or other 

 cold-blooded animals, be confined for some time in an atmo- 

 sphere of nitrogen or hydrogen (neither of which gases in itself 

 exerts any injurious effect upon them), they will continue for 

 some tune to give off nearly as much carbonic acid as they 

 would have done in common air; thus proving that the 

 carbonic acid is not formed in the lungs by the union of carbon 

 brought in the venous blood with the oxygen of the air, since 

 here no oxygen was supplied ; and showing that the carbonic 

 acid must have been brought ready-formed. This process, , 

 however, could not be continued for any great length of time, 

 even in cold-blooded animals ; since a supply of oxygen is 

 necessary to the performance of their various functions. And 

 in warm-blooded animals, a constant supply of this element 

 is so much more important, that they will die if cut off from 

 it, even for a short time. 



307. The quantity of oxygen thus taken in, and of carbonic 



