272 
regards the air, the substitution of an equal bulk 
of carbonic acid for the oxygen gas that is lost, com- 
prises the only essential change which the atmosphere 
experiences during the performance of this animal 
process. We have before maintained (74 et seq.), 
that the oxygen of the air does not enter the animal 
system, either by the living function of absorption, or 
by the operation of chemical affinity ; and have con- 
sequently concluded, that the union of this substance 
with the animal carbon takes place exterior to the 
vessels of the living animal. 
