1 10 RESPIRATION 



diminution caused apnoea, just as when pure air is breathed. It 

 was evident, therefore, that the CO 2 pressure, though at a lower 

 level, was controlling the breathing still. The primary marked 

 increase in the breathing was due to the alveolar CO 2 pressure 

 and the CO 2 pressure in the whole of the body being above the 

 new level, and the quieting down of the breathing was due to 

 the gradual washing out of CO 2 from the whole body till the at- 

 tainment of the new normal level, which was itself determined 

 by the alveolar oxygen pressure. 



A fuller discussion of these facts, and of the ultimate physio- 

 logical response to long-continued slight anoxaemia, must be 

 postponed to Chapter VII, but meanwhile it is evident that they 

 throw a new light on the physiology of breathing. Hitherto we 

 have considered the amount of lung ventilation as if it were de- 

 termined solely by a certain excess of partial pressure of CO 2 in 

 the arterial blood; but now we see that the excess is something 

 variable and dependent, for one thing, on the pressure of oxygen 

 in the arterial blood, just as the action of the Hering-Breuer re- 

 flex depends, not merely on the amount of distention or collapse 

 of the lungs, but also on the pressure of CO 2 in the arterial blood. 

 Similarly the action of want of oxygen on the breathing depends 

 on the CO 2 pressure. On how many other factors which together 

 make up "normal conditions" the action of CO 2 or want of oxy- 

 gen on the respiratory center depends we do not know. We always 

 find normal conditions in a healthy organism, and we are there- 

 fore apt to overlook their unknown complexity. If we represented 

 the relation between arterial CO 2 pressure, oxygen pressure, and 

 lung ventilation in the form of an equation, this equation would 

 only be valid under conditions otherwise normal. In other words 

 an unknown constant C would have to be set down in the equation. 



That this constant exists during life in other words that living 

 organisms maintain fundamental normals of structure and ac- 

 tivity representing the <ris of Hippocrates is one basis of 

 biological science. Apart from this basis physiology would be a 

 mere chaos of unconnected "bio-physical" and "bio-chemical" 

 fragments. 



The effect produced on the breathing by a given reduction in 

 the oxygen pressure of the inspired air or alveolar air varies con- 

 siderably in different individuals. Some respond much more 

 readily by increased breathing than others, and for this reason 

 seem to be better protected against the other and more serious ef- 

 fects of want of oxygen, since the increased breathing raises the al- 



