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 time 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 
