CHAPTEE XII. 



CHANGES WHICH THE AIR UNDERGOES IN RESPIRATION. 



General considerations Discovery of carbonic acid Discovery of oxygen Com- 

 position of the air Consumption of oxygen Influence of temperature In- 

 fluence of sleep Influence of an increased proportion of oxygen in the atmos- 

 phere Temperature of the expired air Exhalation of carbonic acid Influence 

 of age Influence of sex Influence of digestion Influence of diet Influence 

 of sleep Influence of muscular activity Influence of moisture and tem- 

 perature Influence of seasons Relations between the quantity of oxygen 

 consumed and the quantity of carbonic acid exhaled Exhalation of watery 

 vapor Exhalation of ammonia Exhalation of organic matter Exhalation 

 of nitrogen. 



FROM the allusions which have already been made to the 

 general process of respiration, it is apparent, that before the 

 discovery of the nature of the gases which compose the air 

 and those which are exhaled from the lungs, it was impossible 

 for physiologists to have any correct ideas of the nature of 

 this important function. It is not surprising that the ancients, 

 observing the regular introduction of air into the lungs, and 

 noting the fact that the air is generally much cooler than the 

 body, supposed the great object of respiration to be the cool- 

 ing of the blood. It is also evident that no definite knowl- 

 edge of any of the processes of respiration could exist prior 

 to the discovery of the circulation of the blood. 



Though it is foreign to our purpose to treat historically 

 of the theories concerning any of the functions of the body, 

 the facts relating to changes in the respired air, which from 



