132 RESPIRATION 



not if oxygen could be introduced into the blood and carbon 

 dioxide removed from it in some other equally efficient way. 

 So far as the cell is dependent on the acquisition of oxygen 

 and the removal of carbon dioxide, it would make no differ- 

 ence if there were no respiration and no circulation if these 

 materials could be acquired and removed in some other 

 equally efficient way. 



On the other hand, it would be useless to keep up artificial 

 respiration or to inject oxygen into the lungs if the cells, 

 through some disability, cannot take up the oxygen fur- 

 nished, or if the circulation cannot absorb or convey the 

 oxygen. 



It is seen that, from the standpoint of the blood, the inter- 

 change of gases on the lungs is exactly opposite to that in the 

 tissues ; that is to say, in the lungs it loses carbon dioxide and 

 gains oxygen, while in the tissues it loses oxygen and gains 

 carbon dioxide. The pulmonary interchange is properly 

 termed external respiration in contradistinction to that in 

 the tissues which is termed internal respiration. 



It is needless to comment upon the universal necessity of 

 oxygen to the life of cells. Its appropriation is to be looked 

 upon as a part of the nutritive process; and, indeed, while 

 in the long run, cells are certainly dependent upon the nu- 

 triment furnished by the ordinary aliments, they will retain 

 their vital activity for a longer time when deprived of any 

 or all of these than when deprived of oxygen alone. This 

 gas is more immediately necessary to the maintenance of life 

 than is any other substance. 



Since, in order to bring about internal respiration in the 

 human being, the lungs and circulation happen to be nec- 

 essary, attention will have to be directed to the respiratory 

 phenomena taking place in both. 



ANATOMY OF THE RESPIRATORY ORGANS. 



It will be considered that the air has passed through the 

 posterior nares into the pharynx and is ready to enter the 

 larynx. 



