154 VITAL FUNCTIONS 



with a portion of carbon from the dark blood in the lungs 

 and that while carbonic acid is thus formed, the blood be 

 comes of a lighter color in consequence of parting with a 

 portion of carbon, and this undoubtedly is the true theory 

 of respiration. 



462. The blood having thus parted with its super- 

 abundant carbon, which escapes in the form of carbonic 

 acid gas, acquires its natural vermillion, color, and is again 

 qualified to be transmitted to the different parts of the sy.s 

 tern, for their nourishment and growth. 



463. It is found by analysis that the venous blood con- 

 tains a greater proportion of carbon than the arterial 

 blood, and also a greater proportion than the animal solids 

 or fluids. Now the elements of bloojl are oxygen, hydro- 

 gen, and nitrogen, and it is from the blood that all the 

 other parts are formed. Hence in the formation or 

 growth of. other parts, if there is employed a greater 

 proportion of the other elements, and a less proportion of 

 the carbon than the blood contains, the effect is an ac- 

 cumulation of the latter in the blood. And as an excess 

 of carbon appears to be noxious to the animal economy, 

 this excess is removed by combining with the oxygen 

 of the atmosphere as the blood circulates through the 

 lungs. 



464. Respiration analogous to Combustion. The pro- 

 cess of respiration has long since been considered analo- 

 gous to that of combustion, which is certainly the case. 

 In ordinary combustion, the carbon of the combustible 

 body unites with the oxygen of the atmosphere, and 

 while heat is evolved, carbonic acid escapes, being the 

 joint product of the carbon and oxygen. Dr. Roget has 

 carried this idea so far as to turn the respiratory appa- 

 ratus into a regular furnace, with bellows, &c. " The 

 food," says he, " supplies the fuel, which is prepared for 

 use by the digestive organs, and conveyed by the pul- 

 monary arteries, to the place where it is to undergo com- 

 bustion : the diaphragm is the bellows which feeds the 



What is said of the quantity of oxygen, consumed by animals? What is 

 the composition of carbonic acid? What are the elements of blood? How 

 do you account for the accumulation of carbon in the blood ? 



