66 THE NEW PHYSIOLOGY. 



regulation ; but of the marvellous accuracy of the regu- 

 lation, physiologists had, till the recent work on the 

 physiology of breathing, no clear conception. 



It is not merely the hydrogen ion concentration of the 

 blood that is accurately regulated, but also its capacity 

 for taking up a constant amount of CO 2 in presence of 

 a constant partial pressure of this gas. This capacity 

 depends on the concentration of and balance between 

 alkaline salts and albuminous substances in the blood. 

 Recent investigations by Christiansen, Douglas and 

 myself have shown that this concentration and balance 

 are so accurately maintained day by day, and month by 

 month, that, under normal conditions, no deviations can 

 be detected by the most delicate existing method of 

 blood gas analysis. The balance can be temporarily 

 upset by what may be called violent means ; but within 

 an hour it is back again to normal. It is, of course, 

 evident that if the carrying-power of blood for CO 2 did 

 not remain normal, the breathing and circulation would 

 not, without special adjustment, remain normal. 



Now let us look back for a moment, and see where we 

 stand. The experimental study of the physiology of 

 breathing has led us to the discovery of four normals, 

 the maintenance of which furnishes the interpretation 

 of a mass of what would otherwise be isolated and un- 

 intelligible observations. We have first of all the normal 

 alveolar CO 2 pressure. This turns out to be directly 

 subordinate to the normal regulation of the hydrogen 

 ion concentration of the blood, the normal reaction of 

 the respiratory centre to hydrogen ion concentration, 

 and the normal regulation of the capacity of the blood 

 for carrying CO 2 . With the discovery of each of these 



