CHAP. XXIX.] THEORY OF RESPIRATION. 415 



animal heat, and the other depending upon the union of the oxygen 

 with the carbon of those substances which are produced in the 

 disintegration of tissues during the performance of their functions ; 

 in this combination also heat is developed. 



In purely carnivorous animals, the greater portion of the carbonic 

 acid results from the disintegration of the muscular and nervous 

 tissues ; while, in the herbivora, much of the food, rich in carbon 

 and poor in nitrogen, is at once converted into carbonic acid. 



Although the action of the oxygen upon the carbon of the com- 

 pounds, from which the carbonic acid is formed, is a strictly chemical 

 process, the application of this oxygen to the substance to be de- 

 composed, and the removal of the resulting carbonic acid, are 

 dependent solely upon the physical relations which these gases 

 bear to each other, to the membrane through which they pass, and 

 to the fluids in which they are dissolved. 



The blood, loaded with carbonic acid, at length returns to the 

 respiratory surface, where it parts with this gas, and absorbs oxygen 

 in obedience to the physical laws above referred to. 



More oxygen is usually absorbed than is necessary to convert the 

 carbon into carbonic acid. This is required for the oxidation of 

 other elements, as sulphur and phosphorus, by which compounds 

 are produced which are eliminated by other emunctories. 



The amount of oxygen inhaled will depend, in great measure, 

 upon the quantity and nature of the food, and upon the activity 

 of the vital functions, and is intimately associated with the pro- 

 duction of animal heat, as will appear in the next chapter. 



It has been shown that the activity of the respiratory function 

 is materially influenced by various external and internal conditions. 

 Temperature, a dry or moist state of the atmosphere, the period of 

 the day, the digestive process, rest, exercise, etc., all exert an in- 

 fluence on the amount of oxygen inhaled and of carbonic acid 

 exhaled. These are points which must be borne in mind by the 

 careful practitioner in the treatment of such diseases as phthisis, 

 pneumonia, emphysema, and the like. 



Respiration is, therefore, partly a physical and partly a chemical 

 process; chemical, as far as regards the results; physical, with 

 reference to the means by which these results are produced. The 

 introduction of the restorative oxygen, and the removal of the 

 deleterious carbonic acid, are effected solely by a physical process ; 

 while the formation of the carbonic acid is essentially a chemical 

 process, and in its production many complicated chemical decom- 

 positions take place. 



