THE PLACE OF BIOLOGY. 37 



salts are absorbed more or less quickly by the intestines ; 

 and it is therefore to the kidneys that we must mainly 

 look for the explanation of this marvellous regulation 

 of the composition of the blood. 



Now, all we know of the mode of action of the kidneys 

 goes to show that they have the same delicate and 

 exact power of regulating the liquid and crystalloid 

 constituent parts of the blood as the respiratory centre 

 possesses in regulating the reaction or hydrogen ion 

 concentration. We actually find that salts and other 

 soluble crystalloids introduced into the body by the 

 mouth or in other ways in excess are promptly excreted 

 by the kidneys. On the other hand, we also find that 

 the kidneys have an equally marked power of holding 

 back substances which are not in excess. If, for 

 instance, a diet free from sodium chloride is given, 

 the chloride in the urine, normally abundant, practically 

 disappears, although there is still nearly the usual 

 high proportion of sodium chloride in the blood-plasma. 



Let me give another illustration. We know that when 

 a person who is not thirsty drinks a large amount of 

 water, the consequence is an approximately equal 

 secretion of urine by the kidneys. Further investigation 

 shows that the urine secreted is practically nothing but 

 pure water. When, during the enormous kidney secre- 

 tion produced by this means, the blood was examined, 

 we found that it was sensibly undiluted by the water 

 passing through it; for the percentage of haemoglobin 

 in the blood remained sensibly the same during, before, 

 and after the secretion. 



Now, it has already been found, through exact obser- 

 vations, that minute alterations in the alkalinity of the 



