THE AMERICAN NATURALIST [Vol. XLIX 



vidua!. The reaction of the body fluids is likewise de- 

 pendent, in some degree at least, upon the tension of 

 oxygen and carbon dioxide in the blood. 



The reaction of the body fluids, particularly the blood, 

 remains remarkably constant during the life of the ani- 

 mal. This is not so much the peculiarity of the higher 

 organisms as is the constant temperature. The process 

 of regulation of neutrality, as Henderson has shown, is a 

 physico-chemical process and depends upon the proper- 

 ties of carbon dioxide, bicarbonates and phosphates in so- 

 lution. The changes in concentration of hydrogen and 

 hydroxyl ions in the blood are, in their turn, related to 

 the respiratory rhythm. 



The whole subject has been so well summarized by 

 Haldane 30 that I venture to quote his statement entire. 



To illustrate this point I may perhaps refer to a subject which we 

 have recently been investigating at Oxford. We have found that the 

 respiratory center is so extremely sensitive to any increase or diminu- 

 tion of the partial pressure of carbon dioxide in the blood that a 

 diminution of 0.2 per cent, of an atmosphere, or 1.5 mm. of mercury will 

 cause apnea, while a corresponding increase will double the breathing. 

 The recent researches of Hasselbach have afforded experimental evi- 

 dence of what had already seemed very probable — that the stimulus to 

 which the center responds is the difference in hydrogen ion concentra- 

 tion, or acidity, brought about by the very slight deficiency or excess of 

 carbon dioxide. He has also investigated quantitatively the effect on 

 the hydrogen ion concentration of the blood of varying the partial 

 pressure of carbon dioxide. From his results and ours it follows that 

 the hydrogen ion concentration of the blood during rest is extraor- 

 dinarily constant, and remains so day by day and year by year. As 

 the amount of acid and alkali passing into the blood from the food and 

 other sources is constantly varying, it follows that the regulation of 

 hydrogen ion concentration is mainly brought about by the kidneys. 

 It has been known for long that the urine varies in acidity or alkalinity 

 according to the diet; but Hasselbach has measured the actual varia- 

 tions in hydrogen ion concentration. Putting together his conclusions 

 and ours, it appears that durum' ordinary resting conditions the varia- 



