November 3, 1916] 



SCIENCE 



623 



I observed that when the air breathed is 

 gradually and increasingly vitiated by re- 

 breathing it, or by what is known to miners 

 as "black damp," the breathing is also in- 

 creased, but not in any simple relation to 

 the extent of the vitiation. With a steady 

 increase in the vitiation the breathing at 

 first increases only a little, but as the vitia- 

 tion increases further the effect on the 

 breathing is greater and greater. Thus an 

 increase from 4 per cent, to 5 per cent, in 

 the percentage of CO, in the inspired air 

 produces about 20 times as 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 main- 

 tain 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 cor- 

 responding 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 ex- 

 pired in a deep inspiration. As we ex- 

 pected, 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 volun- 

 tarily varied, even as widely as from three 

 a minute to 60 a minute, the depth adjusts 

 itself so as to keep the average alveolar per- 

 centage of carbon dioxide almost absolutely 

 steady; and conversely if the depth is 

 varied. With resistance to breathing there 

 is a similar effect. The effort put into in- 

 spiration 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 to them to depend simply 

 on the state of mechanical distention of 

 the lungs, and to have no relation to the 

 chemical regulation of breathing. Mr. 

 Mavrogordato and I have quite recently re- 

 investigated these phenomena in man. The 

 results showed that the amounts of infla- 

 tion or deflation needed to produce the 

 Hering-Breuer effects depend entirely on 

 the chemical stimulus of carbon dioxide. 

 When this stimulus is absent, as in apnea, 

 a very slight inflation or deflation will suf- 

 fice, so that the breathing is, as it were, 

 jammed during apnea; while if the chem- 

 ical stimulus is strong it needs a great in- 

 flation or deflation to produce the Hering- 

 Breuer effect. The vagi prevent useless 

 prolongation of inspiratory or expiratory 

 effort and consequent waste of time in 

 breathing, or damage to the lung structure. 

 They also coordinate the discharges of the 

 center with actual inflations or deflations of 

 the lungs. When the vagi are cut the 

 breathing becomes slow, and, as Scott 

 showed, can only imperfectly respond to an 

 increased chemical stimulus, since the fre- 

 quency can not be increased. The influ- 

 ence of the vagi is entirely in the direction 

 of keeping the alveolar air normal. Per- 

 haps nothing illustrates more clearly the 

 dependence of nervous reactions on more 

 fundamental physiological conditions than 

 the varying response of the respiratory 

 center to the stimulus of inflation or defla- 

 tion of the lungs. , 



When excessive ventilation of the lungs 



