360 RESPIRATION [CH. XXVI. 



self taking a number of deep breaths rapidly. This condition, called 

 apncea, is not due, as at one time supposed, to over-oxygenation of the 

 blood, but is, according to Head, produced reflexly ; for under normal 

 circumstances arterial blood is almost fully oxygenated. Apnoea is 

 observed if inert gases, such as nitrogen or hydrogen, are used 

 instead of air. The pause, however, is then shorter, as the blood 

 becomes venous, and in a short time stimulates the respiratory centre 

 to activity. 



Under abnormal circumstances, namely, after division of the vagi, 

 apnoea obviously cannot be due to such reflex action. Fredericq 

 holds that even ordinary apnoea has a chemical rather than a nervous 

 origin. He attributes it, however, not to over-oxygenation, but to a 

 lessening of the carbonic acid in the blood. 



3. The Chemical Factor in Respiration. 



A consideration of apnoea thus leads us to the study of the 

 chemical stimuli that play their part in the respiratory process. 

 Their importance has been recently demonstrated by Haldane and 

 Priestley. 



In the first place, they introduced the new and simple method 

 of obtaining the composition of the air in the alveoli, described 

 on p. 372. They found that, under constant atmospheric pressure, 

 in man the alveolar air contains a nearly constant percentage of 

 carbon dioxide in the same person. In different individuals this 

 percentage varies somewhat, but averages 51 per cent, of an atmos- 

 phere in men, and 4'7 in women and children. 



With varying atmospheric pressures, the percentage varies 

 inversely as the atmospheric pressure, so that the pressure or tension 

 of the carbon dioxide remains constant. The oxygen pressure, 

 however, varies widely under the same conditions. 



These observations and the next to be immediately described 

 furnish the chemical key to the cause of the amount of pulmonary 

 ventilation, and play an important part in conjunction with the 

 respiratory nervous system in the regulation of breathing. For the 

 respiratory centre is not only affected by the impulses reaching it by 

 the vagi and other afferent nerves, but it is also very sensitive to 

 any rise in the tension of carbon dioxide in the blood that supplies 

 it. The changes in the tension of this gas in the arterial blood are 

 normally proportional to the changes in the carbon dioxide pressure 

 in the alveoli, and the changes in the lung alveoli are transmitted to 

 the respiratory centre by the blood. They found that a rise 0'2 per 

 cent, in the alveolar carbon dioxide pressure is sufficient to double 

 the amount of alveolar ventilation during rest. During sudden 

 muscular work the alveolar carbon dioxide pressure increases slightly, 

 and the pulmonary ventilation is consequently increased. Most 



