INNERVATION OF THE RESPIRATORY MOVEMENTS. 703 



confined space (provision being made for the absorption of the 

 CO 2 ), the effect on breathing may be so slight as to cause no dis- 

 tress to the individual so that he may continue the rebreathing until 

 a sudden loss of consciousness ensues from the lack of oxygen. A 

 similar experience is recorded by balloonists for very high ascen- 

 sions, 25,000 to 30,000 feet (see p. 714) Zuntz's experiments, in 

 which the CO 2 in the air breathed was increased, while the oxygen 

 remained normal, gave quite different results, as follows: 



Normal air volume breathed per minute, 7,433 c.c. 



Air of 20.2 per cent. O, 0.95 per cent. 



CO 2 " " " " 9,060 " 



Air of 18.06 per cent. O, 2.97 per cent. 



CO 2 " " " " 11,326 " 



Air of 18.42 per cent. O, 11.5 per cent. 



CO 2 " " " " 32,464 " 



These and similar results* show that small differences in the 

 amount of the carbon dioxid in the blood have a distinct effect 

 upon the activity of the respiratory center. The trend of recent 

 work favors the view that the normal stimulus to the respiratory 

 center is the carbon dioxid. When this substance is present above 

 a certain amount or tension it acts as a stimulus, directly or indi- 

 rectly, and gives rise to the moderate movements of normal in- 

 spiration. If the tension of the carbon dioxid is increased, the 

 stimulus becomes stronger and leads to the production of a condi- 

 tion of hyperpnea and dyspnea. On the other hand, if for any 

 reason, such as active ventilation of the lungs, the tension of the 

 carbon dioxid in the blood falls below a certain value, estimated by 

 Zuntz as lying between 19 and 24 mms., no stimulation occurs, the 

 center is in a condition of apnea and respiratory movements cease. 

 Haldanef has laid great emphasis upon the delicacy of the reaction 

 of the respiratory center to any change in the carbon dioxid press- 

 ure. The reaction, in fact, is so sensitive that under quite extreme 

 conditions the concentration of carbon dioxid in the alveolar air is 

 kept nearly constant. In muscular exercise, for example, there may 

 be a large output of carbon dioxid to the blood, but the resulting 

 increase in carbon dioxid pressure so stimulates the respiratory 

 center that there is a corresponding augmentation in the breathing, 

 the alveolar space is more thoroughly ventilated, and the concen- 

 tration of C0 2 in the alveolar air, and consequently in the arterial 

 blood, shows but little change. The average concentration of CO 2 

 in the alveolar air is 5.5 per cent. An increase of as little as 0.2 

 per cent, will be attended by a doubling of the volume of respira- 

 tion. Accepting the view that the carbon dioxid of the blood con- 



* See Haldane and Priestley, "Journal of Physiology," 32, 225, 1905. 

 t Haldane, " Organism and Environment," 1917. 



