158 VITAL FUNCTIONS. 



bined with a portion of carbon from the dark blood in 

 the lungs, and that while carbonic acid is thus formed, 

 the blood becomes of a lighter color in consequence of 

 parting with a portion of carbon, and this undoubtedly 

 is the true theory of respiration. 



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 qualifi- 

 ed to be transmitted to the different parts of the system, 

 for their nourishment and growth. 



It is found by analysis, that the venous blood contains 

 a greater proportion of carbon than the arterial blood, 

 and also a greater proportion than the animal solids or 

 fluids. Now the elements of blood 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 

 accumulation 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. 



Respiration analogous to combustion. The process 

 of respiration has long since been considered analagous 

 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. Mr. 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 pulmo- 

 nary 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 7 What are the elements of 

 blood 1 How do you account for the accumulation of carbon in the 

 blood * 



