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SCIENCE 



[N. S. Vol. XLIV. No. 1140 



duced into the body the percentage in the 

 blood remains very steady, as excess is 

 taken up by the liver and other organs, 

 or excreted by the kidneys. Voit's obser- 

 vations on the relative constancy of the 

 sodium chloride in the blood, and the man- 

 ner in which the kidneys regulate this per- 

 centage, are of a similar character. If 

 food freed from chloride is administered 

 the elimination of chloride by the urine 

 diminishes to almost nothing, though the 

 high percentage of chloride in the blood- 

 plasma remains about the same. As Voit 

 also showed, the blood during prolonged 

 starvation retains its normal composition, 

 and its volume remains proportional to 

 body weight, while other tissues (e. g., 

 muscle) are reduced. 



Dr. Priestley and I have recently inves- 

 tigated the excretion of water by the 

 kidneys. By simply drinking large quan- 

 tities of water one can produce an enor- 

 mous increase in the secretion of urine, and 

 this urine is almost pure water. What we 

 wished to observe was the degree of water- 

 ing down of the blood which was necessary 

 to produce the huge increase in excretion 

 of water. We did not doubt that the 

 watering down would be very small, but 

 when we attempted to measure the dilu- 

 tion by determining the percentage of 

 hemoglobin we found that there was no 

 dilution at all, though the method used 

 was one of extreme accuracy. When, how- 

 ever, the plan of measuring the electrical 

 conductivity of the serum was adopted, a 

 slight, but quite distinct, diminution in the 

 conductivity could be detected during, and 

 ending with, the diuresis. This showed 

 that there was a slight diminution in the 

 salt-concentration, and to this diminution 

 the secreting cells were reacting. Here, 

 then, we are in presence of another exactly 

 but elastically regulated normal, the slight- 

 est deviation from which produces, in the 



kidneys, a reaction comparable in its ex- 

 quisite delicacy with the reaction of the 

 respiratory center or liver or kidneys to a 

 change in hydrogen ion concentration. 



The physiology of the kidneys has, in 

 accordance with prevalent physiological 

 conceptions, been attacked from the side 

 of "causal" explanation. I know nothing 

 more hopeless than the attempts to explain 

 the outstanding features of secretion of 

 urine on the lines of ordinary physics and 

 chemistry. So far as the facts are yet 

 known we can, however, get a practical 

 grasp of the kidney activities if we attack 

 the subject from the standpoint of the 

 active maintenance of the normal blood 

 composition. 



Let me turn now to the general physiol- 

 ogy of nutrition. In the preliminary 

 stages of investigation of this subject physi- 

 ology has owed much to the pure chemists, 

 and this debt is constantly increasing. We 

 have only to think of the work of such men 

 as Black, Priestley, Lavoisier and Liebig. 

 Like Wohler, who synthesized urea, Liebig 

 was a convinced vitalist. For him there 

 was a central kernel of vital activity under 

 the control of the "vital force" ; but outside 

 this central kernel he interpreted the phe- 

 nomena of nutrition on purely chemical 

 lines. He thought of oxygen as something 

 free to oxidize anything oxidizable within 

 the body, except what is protected by the 

 vital force ; and he assumed that the greater 

 the concentration of oxygen in the lungs, 

 and the greater the supply of food-material 

 to the body, the greater will be the amount 

 of oxidation, since only a limited amount 

 of oxidation is under the direct control of 

 the vital force. He gave special attention 

 to the elimination of urea and other prod- 

 ucts of nitrogenous oxidation, and intro- 

 duced methods of measuring the nitroge- 

 nous waste. It was found, apparently in 

 direct confirmation of his general ideas, 



