34 THE NEW PHYSIOLOGY. 



ing varies considerably in both depth and frequency, 

 and becomes very irregular during irregular exertions 

 such as speaking or singing. There are many people 

 who think that the lungs may not be getting enough 

 oxygen, and who in order to remedy this imaginary 

 defect recommend increased breathing. 1 Now exact 

 investigation shows that the breathing in each person 

 is so regulated as to maintain during rest a certain abso- 

 lutely definite mean percentage, or more correctly, 

 partial pressure, of carbon dioxide in the air of the 

 lung alveoli. This percentage depends on the balance 

 between the varying amount of carbon dioxide coming 

 off from the venous blood, and the amount of fresh 

 air introduced into the lung alveoli by the breathing. 

 By an effort of will we can temporarily increase the 

 breathing so as to dilute the carbon dioxide further ; 

 but a dilution of as little as one-thirtieth in the 

 mean alveolar carbon dioxide percentage causes the 

 disappearance of all spontaneous impulse to breathe, 

 until the carbon dioxide balance has been restored ; 

 while an increase from any cause of one-twentieth in 

 the mean alveolar carbon dioxide percentage is accom- 

 panied by an increase to more than double in the volume 

 of air breathed. The average breathing is thus regulated 

 with almost incredible exactitude in accordance with 

 the varying rate at which carbonic dioxide is produced 

 in the body, and brought to the lungs by the blood. 



The. percentage or partial pressure of carbon dioxide 

 in the alveolar air determines the partial pressure of 



1 By one of Nature's ironies, one effect of forced breathing is 

 to produce dizziness, and even loss of consciousness, due possibly 

 to want of oxygen induced indirectly. 



