READJUSTMENTS OF REGULATION 37 
tions between hydrogen ion concentrations and CO, 
pressure in blood. A difference in CO, pressure which 
would be sufficient to double the breathing, or to cause 
apnoea, produced a difference in hydrogen ion concen- 
tration which was just measurable by the method, so 
the method is very rough as compared with the deli- 
cacy of discrimination by the respiratory centre. By 
varying the diet from alkaline to acid-producing 
Hasselbalch succeeded in producing a variation of 
several millimetres in the alveolar CO, pressure. He 
then found that with the blood saturated with CO, at 
the existing alveolar CO, pressure the hydrogen ion 
concentration as measured was sensibly the same on 
either diet ; whereas if the blood was saturated in both 
cases at the same CO, pressure the hydrogen ion con- 
centration was markedly different on the two diets. 
The difference in alveolar CO, pressure was thus just 
sufficient to keep the hydrogen ion concentration, in so 
far as it could be measured by the electrometric 
method, constant in the two samples of blood, although 
there was presumably a slight difference as indicated 
by the difference in the breathing. Other similar ex- 
periments had a similar result, and there seems now 
to be no doubt that it is true that what the respiratory 
centre responds to is hydrogen ion concentration, and 
not mere CO, pressure. 
The delicacy of the response of the respiratory 
centre to change in the reaction of the blood is very 
extraordinary ; but what is still more marvellous is the 
fact that in spite of this delicacy the alveolar CO, 
pressure is so steady during rest. The respiratory 
