12 Physiology of the Kidney 



possibility of some, even if slight, tubular reabsorption of 

 inulin, which is a carbohydrate, even if a large and very inert 

 one? This question was answered in the dog by Shannon,^"' ^® 

 who showed that inulin and creatinine are concentrated to 

 precisely the same degree. Similar identity in the concentra- 

 tion ratios of creatinine and inulin has been demonstrated in 

 the rabbit,^^ the seal,^* the sheep^' and, as Forster has recently 

 shown, in the frog/° Since it is implausible that two sub- 

 stances of such different natures as inulin and creatinine 

 should be either excreted or reabsorbed by the tubules to pre- 

 cisely the same extent in these different species of animals, it 

 may be accepted that in these species both substances are ex- 

 creted by filtration without tubular participation. 



But the concentration ratio of creatinine is very much 

 higher than that of inulin in the dogfish,^^ the red grouper,^^ 

 the chicken,^^ and the anthropoid apes.^^ This discrepancy is 

 also evident in man; when creatinine is freshly injected into 

 the blood it is concentrated by the human kidney about 40 

 per cent more than is inulin ;^^' ^^' ^* here either 40 per cent of 

 the inulin is reabsorbed, or considerable creatinine is excreted 

 by the tubules in addition to that which is filtered. This is 

 an important question since, on the basis of Rehberg's"^ early 

 investigations, the creatinine clearance is widely accepted as 

 a measure of the rate of filtration in man. The difference be- 

 tween the creatinine and inulin clearances may be as much as 

 70 per cent. Shannon^' in his original study on man decided 

 that creatinine was excreted in part by the tubules. Though 

 I feel that the evidence on this point, when considered criti- 

 cally, is conclusive, this evidence has not been such as to con- 

 vince all investigators and we have never abandoned our 

 efforts to reinforce it. It is gratifying that we are able to add 

 supplementary facts at this time. 



