THE SECRETION OF THE URINE 501 



secretory activity of cells may be supposed to consist. When we 

 see a barge passing through a lock, and being gradually lifted to 

 the proper level by the inrush of water, we never dream of saying 

 that the whole thing is an affair of the laws of hydrostatics. We 

 know that the part played by the lock-keeper, the opening and 

 closing of the gates and sluices at the proper time, is all-important, 

 although he does not lighten by one ounce the weight which the 

 water must lift. He uses the head of water for a specific purpose 

 namely, to lift the barge. In like manner it is to be expected 

 that the glomerular epithelium, when the difference of pressure 

 on its two surfaces is increased by hydraemic plethora, will use the 

 increased facility of nitration to rapidly excrete a portion of the 

 water. But who will believe that the addition of a tumbler of 

 water, absorbed from the alimentary canal, to 4 or 5 litres of blood 

 circulating in a system of vessels whose capacity can and does vary 

 within wide limits, should cause in the capillaries of the kidney 

 an increase of pressure exactly proportional to the increase in the 

 elimination of water in the urine, lasting for the sarne time and 

 disappearing at the moment when the normal composition of the 

 blood is restored ? Nor is it easier to explain on any mechanical 

 hypothesis how it is that in a starving animal the quantity of 

 inorganic substances eliminated in the urine drops almost to zero, 

 while the proportional amount in the blood and tissues is little, if 

 at all, affected. In a rabbit rendered poor in sodium chloride by 

 feeding it with salt-free food, the injection of a solution of sodium 

 chloride isotonic with the blood produces no diuresis for a con- 

 siderable time, but, on the contrary, a diminished flow of urine, 

 while a similar solution injected into the veins of a rabbit previously 

 fed with salted food causes an immediate and considerable diuresis. 

 When small quantities of isotonic solutions of various salts are 

 injected, those not normally present in the blood produce a greater 

 diuresis than normal constituents. Sodium chloride, which is 

 present in normal plasma in greater amount than any other salt, 

 causes the smallest diuresis of all (Haake and Spiro). 



Such facts suggest that the secreting cells of the kidney are stimu- 

 lated or inhibited by the contact of blood or lymph in which the 

 normal constituents are present in too great or in too small amount, 

 and that the intensity of the action is proportional to the degree of 

 deficiency or excess. The greater the velocity of the circulation 

 in the kidney, the more effective will be the stimulation produced" 

 by any given substance present in excess, and therefore the greater 

 the total amount of it eliminated in a given time. For in making 

 the round of the renal circulation the concentration of the sub- 

 stance in any given portion of blood will fall less, and therefore the 

 average stimulation exerted by it during the round will be greater 

 the faster the blood flows. It is quite in agreement with this that 

 when plethora is occasioned by transfusion of blood there is little 



