64 THE NEW PHYSIOLOGY. 



When the barometric pressure is varied it becomes 

 evident that the normal which dominates the regulation 

 of breathing is not the percentage of carbon dioxide 

 in the alveolar air, but the partial pressure or molecular 

 concentration. At the normal atmospheric pressure of 

 30 inches there is about 5-6 per cent, of carbon dioxide 

 in the alveolar air, but only 2-8 at 60 inches barometric 

 pressure, and 1-4 at 120 inches. In these three cases 

 the percentage of CO* varies widely, but the partial 

 pressure is the same. It is only with constant baro- 

 metric pressure that the normal percentage is steady. 



When the breathing is increased by excess of CO 2 

 in the inspired air, or increased production of CO 2 in 

 the body, there is, as might be expected, a slight rise 

 in the alveolar CO 2 percentage. It is this slight rise 

 that is the stimulus to increased breathing. Roughly 

 speaking, a rise of 0-2 per cent, increases the resting 

 breathing by 100 per cent., while a fall of 0-2 per cent, 

 produces apnoea. The stimulus of the increased CO 2 

 percentage is conveyed to the respiratory centre by the 

 blood. Under ordinary average conditions the centre 

 responds with normal breathing when the blood leaving 

 the lungs is saturated with air containing 5-6 per cent, 

 of CO 2, but does not respond at all when the blood is 

 saturated with 5-4 per cent, of CO 2 or less. The thres- 

 hold value of CO 2 is, however, greatly lowered by ex- 

 cessive administration of acids or in any condition of 

 so-called acidosis, and is raised by alkalies or an alkaline 

 diet. This and other evidence points to the fact that 

 CO 2 acts on the respiratory centre in virtue of its acid 

 properties when in solution. 



According to modern ideas, the acidity or alkalinity 



