RESPIRATION. 



Since the blood in the capillaries contains a larger proportion of 

 carbonic acid than the air in the air- vesicles, a portion of this gas 

 leaves the blood and passes out through the pulmonary membrane; 

 while the oxygen, being more abundant in the air of the vesicles 

 than in the circulating fluid, passes inward at the same time, and is 

 condensed by the blood. 



In this double phenomenon of exhalation and absorption, which 

 takes place in the lungs, both parts of the process are equally 

 necessary to life. It is essential for the constant activity and nutri- 

 tion of the tissues that they be steadily supplied with oxygen by the 

 blood ; and if this supply be cut off, their functional activity ceases. 

 On the other hand, the carbonic acid which is produced in the body 

 by the processes of nutrition becomes a poisonous substance, if it 

 be allowed to collect in large quantity. Under ordinary circum- 

 stances, the carbonic acid is removed by exhalation through the 

 lungs as fast as it is produced in the interior of the body ; but if 

 respiration be suspended, or seriously impeded, since the production 

 of carbonic acid continues, while its elimination is prevented, it 

 accumulates in the blood and in the tissues, and terminates life in a 

 few moments, by a rapid deterioration of the circulating fluid, and 

 more particularly by its poisonous effect on the nervous system. 



The deleterious effects of breathing in a confined space will 

 therefore very soon become apparent. As respiration goes on, the 

 oxygen of the air constantly diminishes, and the carbonic acid, 

 mingled with it by exhalation, increases in quantity. After a time 

 the air becomes accordingly so poor in oxygen that, by that fact 

 alone, it is incapable of supporting life. At the same time, the 

 carbonic acid becomes so abundant in the air vesicles that it pre- 

 vents the escape of that which already exists in the blood ; and the 

 deleterious effect of its accumulation in the circulating fluid is 

 added to that produced by a diminished supply of oxygen. An 

 increased proportion of carbonic acid in the atmosphere is therefore 

 injurious in a similar manner, although there may be no diminution 

 of oxygen ; since by its presence it impedes the elimination of the 

 carbonic acid already formed in the blood, and induces the poison- 

 ous effects which result from its accumulation. 



Examination of the blood shows furthermore that the interchange 

 of gases in the lungs is not complete but only partial in its extent. 

 It results from the experiments of Magendie, Magnus, and others, 

 that both oxygen and carbonic acid are contained in both venous 



