THE ESSENTIAL PHENOMENA OF RESPIRATION. 17 



internal air. According to well-known physical prin- 

 ciples a quantity of air then enters the lungs. 



Expiration. Soon, however, the ribs and the sternum 

 fall ; the diaphragm ceases to contract, and is forced up- 

 ward by the abdominal viscera; the thorax resumes its 

 original volume, and the compression exerted on the lungs 

 forces out a quantity of air corresponding to that which 

 had entered just before. 



The alternate movements of inspiration and expiration 

 are repeated in man on an average sixteen times a min- 

 ute. The capacity of the lungs, when completely filled, 

 is about two and a half quarts, and the actual renewal 

 of air by each respiratory movement corresponds to 

 about thirty cubic inches. If this last number be mul- 

 tiplied by the number of movements, it will be found 

 that in every twenty-four hours there pass through 

 the lungs about three thousand gallons, or four hun- 

 dred cubic feet. This volume, of course, varies ac- 

 cording to numerous circumstances. When the air enters 

 the pulmonary vesicles it is separated from the blood 

 only by the thickness of the walls of the vesicles and 

 the capillary vessels which cover their surfaces. The 

 walls of the vesicles, like those of the capillary blood- 

 vessels, are so thin that the thinnest fabrics could give no 

 idea of them, and the exchange between the oxygen of 

 the air and the carbonic acid gas of the blood takes place 

 through an almost impalpable gauze. 



These phenomena of exchange constitute only a part 

 of respiration, and, in fact, the least important part. The 

 most important phenomena take place in the depths of 

 the tissues to which passes the arterial blood charged 

 with oxygen. They are chemical reactions. The oxygen 

 seizes on the carbon and combines with it to form car- 

 6 2* 



