THEORIES OF RESPIRATION. 105 



Dr. B. About all the physical changes produced in 

 the blood by respiration can be more satisfactorily ex- 

 plained by admitting the presence of oxygen, than mere- 

 ly a loss of carbon. The fact then that they are owing 

 to a fresh supply of oxygen is nearly all we know of it ; 

 how it should have this effect, we cannot explain. 



Emily. That carbon in some form or other is emit- 

 ted from the blood in respiration, is one fact at least in 

 which they all agree. The decarbonizatipn of the blood 

 then, is one object of the function, whether its oxygena- 

 tion be another or not ; but I do not understand where 

 all this carbon comes from. 



Dr. B. You must recollect that it is into veins that 

 the nutritious aliment prepared by digestion, is poured, 

 as well as a considerable portion of those particles of the 

 body that have become unfit for nutrition and require to 

 be removed from the system. These materials all abound 

 with carbon, and it is from this source that the carbon in 

 the venous blood is derived. 



Emily. It has just occurred to me, Dr. B., that if 

 these chemical changes are actually going on in the 

 lungs, they ought to be accompanied with their usual re- 

 sults. The union of carbon with oxygen, so as to form 

 carbonic acid gas, is attended with a disengagement of 

 heat, as any one may know who ever sat by a charcoal 

 fire. 



Dr. B. And so you would expect that the lungs, not 

 being made of exactly fire-proof materials, must inevita- 

 bly get scorched ? 



Emily. Rather an unpleasant conclusion indeed, but 

 really, I see not how it can be avoided. 



Dr. B. No sooner were the chemical changes that 

 are effected in the lungs, brought to light, than physiolo- 

 gists began to connect the production of animal heat with 

 the function of respiration. At first it was imagined that 

 the lungs themselves were the grand furnace in which 

 the caloric was manufactured, that was to be taken up by 

 the blood vessels and carried to every portion of the syS- 



