402 A MANUAL OF PHYSIOLOGY 



Tried by the latter test, the mechanical theory breaks down 

 for the kidney, as it does for other glands. 



In the first place, the absence from urine of the proteids 

 and sugar of the blood under normal circumstances if 

 infinitesimal quantities of these substances, as some have 

 asserted, are really to be found in healthy urine, it makes 

 no difference to the argument and the elimination by the 

 kidney of egg-albumin, peptone, and other bodies when 

 injected into the veins, show a selective power inexplicable 

 except by reference to the vital activity of cells. Urea and 

 sugar, both highly diffusible substances, circulate side by 

 side in the bloodvessels of the kidney. The one is taken 

 and the other left. The urea is a waste-product of no 

 further use in the economy. The sugar is a valuable food- 

 substance. The kidney selects with unerring certainty the 

 urea, of which only 4 parts in 10,000 are present in the 

 blood, but rejects the sugar, of which there is five times 

 as much. 



Egg-albumin injected into the blood passes through the 

 renal circulation side by side with the serum-albumin of 

 the plasma. Both are indiffusible through membranes, and 

 to the chemist the differences between them may appear 

 superficial and minute. But the kidney does not hesitate 

 for an instant. A large part of the egg-albumin is promptly 

 excreted as a foreign substance; the serum-albumin passes 

 on untouched. 



Not only does the kidney exercise a power of qualitative 

 selection ; it also takes cognizance of the quantitative com- 

 position of the blood. So long as there is less sugar in the 

 plasma than about 3 parts in 1,000, it is refused passage 

 into the renal tubules. But when this limit is passed, and 

 the proportion of sugar in the blood becomes excessive, the 

 kidney begins to excrete sugar, and continues to do so till 

 the balance is redressed. 



The advocates of the theory of filtration, driven from one 

 position to another, have made their firmest stand on the 

 excretion of the inorganic constituents of the urine. But 

 even here the theory has at length become untenable ; and 

 there is little more reason to believe that the copious flow of 



