SECRETION 



51 



secretion in the ordinary sense each play a part in the activity 

 of the renal tubules of the Vertebrate, and evidence relating to 

 one constituent of the urine is not necessarily valid as regards 

 another. 



Nitrogenous Excretion (after v. der Heyde, J. Biol. Chem. 46). 



Animal. 



Mammal (Homo) 

 Amphibian (Rana) 

 Teleost (Lophius) 

 Elasmobranch (Mustelus) 



Looking at the alternative of secretion and reabsorption 

 from the standpoint of comparative physiology, it seems likely 

 that a thorough comparison of the state of affairs in fishes with 

 that which exists in land vertebrates would well repay investiga- 

 tion. For in fishes the blood from the heart encounters the 

 resistance of the gill capillaries before reaching the kidneys by 

 way of the dorsal aorta ; the pressure of blood in the dorsal 

 aorta is therefore extremely low even as compared with that of 

 the frog. Exact data are unfortunately lacking, but it seems 

 unlikely that there is ever a blood-pressure in the renal vessels 

 of the fish sufficient to overcome the osmotic pressure of the 

 blood colloids. If this is really the case, filtration clearly plays 

 no part in the renal function of fishes ; and the hypothesis of 

 reabsorption cannot be applied to them. The amount of 

 urine secreted is, according to Denis, about i c.c. per kilo 

 body weight per hour in the elasmobranch ; and destruction 

 of the cord, which would presumably lower the blood-pressure, 

 does not reduce the output. At the same time, the distribution 

 of nitrogenous (non-protein) materials in blood and urine is 

 not very different in Teleosts from the condition found in land 

 vertebrates (see table). In Elasmobranchs an anomalous 

 feature is the large amount of urea present in the blood and 



