EXCRETION 463 



pighian tuft. It may also be admitted that such an increase 

 of pressure might be accompanied by an increased nitration of 

 water and salts into Bowman's capsule. Even in the excised 

 kidney, after the vital activity of its cells may be presumed to 

 have ceased, nitration of the most varied solutions occurs when 

 the organ is perfused with them through the renal artery. The 

 liquid which escapes from the ureter always has the same com- 

 position as the perfusion fluid (Sollmann). It would certainly 

 appear unlikely that the glomerular epithelium should make no 

 use whatever for the furtherance of its task of the difference of 

 hydrostatic pressure on its two surfaces. It is in taking advan- 

 tage of such circumstances for the promotion of its specific work 

 up to the point at which they cease to favour it that a great part 

 of the true 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 hydro- 

 statics. 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 filtration 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 same 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 sub- 

 stances 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 considerable 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 solu- 

 tions of various salts are injected, those not normally present in 

 the blood produce a greater diuresis thanjiormal constituents. 

 Sodium chloride, which is present in normal plasma in greater 



