10 Physiology of the Kidney 



visable to draw conclusions from a single species, these in- 

 vestigations, as indeed nearly all our investigations since, were 

 extended to several species, principally the dogfish, the dog, 

 and man. The dogfish was chosen because it is the lowest of 

 the vertebrates which lends itself to experimental investiga- 

 tion; it has, moreover, certain unique features such as the 

 capacity to actively conserve urea which make it a critical 

 testing-ground for any theory. The dog kidney very closely 

 resembles the human kidney in its functional capacities, and 

 very accurate quantitative observations can be made upon 

 trained, unanesthetized dogs. And in the last analysis every 

 conclusion reached from a study of the lower animals must, 

 of course, be put to its final test in man. In addition to these 

 three species, numerous supplementary observations have 

 been made by investigators in New York University College 

 of Medicine on the sculpin, the chicken, the rabbit, the sheep, 

 the seal, the monkey and the anthropoid apes (See 45). I 

 feel that this wide foundation of comparative physiology is 

 essential if we are to interpret with confidence details of 

 function in the human kidney. 



Among the first compounds with which we worked were 

 xylose, sucrose and rafiinose. Xylose and sucrose are not ex- 

 creted by the aglomerular kidney, while in the dogfish, dog 

 and man these three sugars show in simultaneous experiments 

 almost identical concentration ratios (i.e., urine/plasma, or 

 U/P ratios) ; i.e., they are concentrated in the tubule to the 

 same extent.*^ From this fact we concluded that they are 

 excreted by glomerular filtration without the participation 

 of tubular excretion, and with negligible passive back-dif- 

 fusion across the tubule. Subsequent investigations have sub- 

 stantiated both of these conclusions. From comparisons of 

 the concentration ratios of the sugars with the concentration 



