RESPIRATION 139 



elastic catheter introduced via the trachea and bronchus. It has the 

 further advantage that it can be applied to the human subject. 



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 in men 5'16, in women and children 4'77 per 

 cent, of an atmosphere. 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 con- 

 ditions. 



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 ill 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 thus the changes in the lung alveoli are trans- 

 mitted to the respiratory centre. They found that a rise of 0'2 per 

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

 the amount of alveolar ventilation during rest. During work the 

 alveolar carbon-dioxide pressure increases slightly, and the pulmonary 

 ventilation is consequently increased. Changes in the oxygen pressure 

 within wide limits have no such influence ; the normal chemical 

 stimulus to respiration is therefore the presence of an increase of carbon 

 dioxide, and not diminution of oxygen. If these limits are exceeded, 

 as when the oxygen pressure falls below 13 per cent, of an atmosphere, 

 the respiratory centre begins to be excited by want of oxygen, for 

 the alveolar carbon-dioxide pressure is lower than normal under such 

 circumstances. ' Apncea 'is the name given to the cessation of breathing 

 which temporarily follows excessive ventilation of the lungs, as when 

 one takes a number of deep breaths in rapid succession. The deep 

 and rapid breathing clears out the carbon dioxide in the alveoli until 

 it is so small in quantity that it is insufficient to excite the respiratory 

 centre via the blood to action, the oxygen pressure at the same time 

 remaining sufficiently high not to excite the centre either. Hence 

 breathing ceases. The old idea that apnoea is due to over-oxygena- 

 tion of the blood has been abundantly disproved, but Head and others 

 have gone to an extreme in assuming that apnoea is purely nervous 



