38 ORGANISM AND ENVIRONMENT 
centre is responsible for neutralising, by getting rid 
of excess of CO,, the changes in hydrogen ion concen- 
tration which would occur in the blood if the excess 
of CO, were not got rid of ; but its action in regulating 
the breathing does not explain why, apart from the 
disturbing influence of CO,, the reaction of the blood 
remains so marvellously constant, as shown by the 
constancy during rest of the alveolar CO, pressure. 
Acid-forming and alkali-forming substances are con- 
stantly being taken into the body in more or less irreg- 
ular quantities. For instance the sulphur in albumin- 
ous food is oxidised to form sulphuric acid, and the 
phosphorus to form phosphoric acid; while on the 
other hand the organic acids contained as salts in 
vegetable foods are oxidised to CO, and thus intro- 
duce alkaline carbonates into the body. Acid or 
alkaline secretions, such as the gastric or pancreatic 
juice, are also being formed at intervals. Yet the 
reaction of the blood hardly varies even when tested 
by such an exquisitely sensitive indicator as the res- 
piratory centre, while no other indicator shows any 
variation. 
It is thus evident that to understand the physiology 
of breathing we must consider the regulation of the 
blood alkalinity. Two means are already known by 
which the blood-reaction is regulated. One of these 
is by regulation of the formation of ammonia in the 
body. It was discovered by Schmiedeberg of Strass- 
burg and his pupils that when mineral acids are ad- 
ministered to dogs or to men the amount of ammonia 
salts eliminated in the urine increases greatly, at the 
