62 THE NEW PHYSIOLOGY. 



great an effect on the breathing as an increase to 1 per 

 cent, from the normal of 0*03 per cent. Observations 

 of this kind suggested that the breathing is so regulated 

 as to maintain a certain normal percentage of carbon 

 dioxide in the air within the lungs, and that as the 

 percentage in the inspired air rises, a greater and greater 

 increase in the breathing is required to maintain this 

 normal. It is, moreover, excess of carbon dioxide that 

 excites the breathing. A corresponding deficiency of 

 oxygen has no such effect. 



It was found by Mr Priestley and myself that a 

 sample of the air in contact with the blood in the lungs 

 could easily be obtained by catching the latter part of 

 the air expired in a deep inspiration. As we expected, 

 the percentage of carbon dioxide in this air turned out to 

 be on an average practically constant for each individual. 



If the frequency of breathing is voluntarily varied, 

 even as widely as from three a minute to sixty a minute, 

 the depth adjusts itself so as to keep the average alveolar 

 percentage of carbon dioxide almost absolutely steady ; 

 and conversely if the breath is varied. With resist- 

 ance to breathing there is a similar effect. The effort 

 put into inspiration and expiration is so increased 

 as to overcome the resistance and keep the alveolar 

 carbon dioxide almost steady. If the breathing is 

 temporarily interrupted or abnormally increased, the 

 time is made up afterwards, so that the average alveolar 

 carbon dioxide percentage is practically steady. If, 

 finally, the inspired air is vitiated by carbon dioxide, 

 the breathing is so increased as to keep, if possible, the 

 alveolar percentage approximately steady. 



The effects discovered by Hering and Breuer appeared 



