CHAPTER III. 



RESPIRATION. 



OF all the functions of living animals, respiration perhaps is the most 

 general. The union of oxygen with the tissues and the removal of the 

 consequent oxidation-products is at the basis of metabolism. We may 

 define respiration then as essentially an interchange of oxygen and of 

 carbon dioxide between the tissue-protoplasm and the atmosphere. 

 So long as these requirements are fulfilled, whatever the means, how- 

 ever superficial or deep the tissues lie, respiration is going on. In man it 

 is obvious that almost all of .his tissue-cells are so far away from the 

 atmosphere that there can be no direct interchange between them and the 

 oxygen] and carbon dioxide in the air. Even^ the skin in the case of 



FIG. 49 



Head and gills ot the mud-puppy (Necturus), to show direct respiration in a relatively 

 highly developed animal. (Dalton.) 



man performs no respiration directly with the atmosphere, for its outer 

 layers are essentially dead, while its inner layers are supplied much as 

 are the inmost tissues of the body. Francke estimates that an average 

 human body is made up of 400,000,000,000,000 cells, while another esti- 

 mate makes the number 26,500,000,000,000. Every one of these requires 

 oxygen, and exhales carbon dioxide. The biological problem then in the 

 case of man is how to supply an abundance of oxygen to all these cells and 

 how to remove the products of their combustion. Thus, in practice the 

 conspicuous part of respiration is the mechanical means of taking in 

 and giving out respiratory gases. The essential process, however, is 

 the interchange of these gases between the living protoplasm and the 

 atmosphere. The pulmonary part of the process is called external 

 respiration, while the essential intercellular interchange is called internal 

 respiration. 



The Chemistry of Respiration Proper. Before taking up a systematic 

 description of the respiratory process as a unified system of events, the 



