38 ORGANISM AND ENVIRONMENT 



centre is responsible for neutralising, by getting rid 

 of excess of CO 2 , the changes in hydrogen ion concen- 

 tration which would occur in the blood if the excess 

 of CO 2 were not got rid of ; but its action in regulating 

 the breathing does not explain why, apart from the 

 disturbing influence of CO 2 , the reaction of the blood 

 remains so marvellously constant, as shown by the 

 constancy during rest of the alveolar CO 2 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 2 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 



