THE SECRETION OF THE URINE 495 



Both are indiffusible through membranes, and to the physical 

 chemist the differences between them may appear superficial and 

 minute. But the kidney does not hesitate for an instant. A large 

 part of the egg-albumin is promptly excreted as a foreign substance ; 

 the serum-albumin passes on untouched. 



Not only does the kidney exercise a power of qualitative selec- 

 tion; it also takes cognizance of the quantitative composition of 

 the blood. So long as there is less sugar in the plasma than about 

 1-5 to 2 parts in 1,000, it is refused passage into the renal tubules. 

 But when this limit is passed, and the proportion of sugar in the 

 blood becomes excessive, the kidney begins to excrete sugar, and 

 continues to do so till the balance is redressed. 



The advocates of the theory of nitration through the glomeruli 

 under the influence of the difference of hydrostatic pressure in the 

 capillaries and in the lumen of the capsules have made their firmest 

 stand on the excretion of the inorganic constituents of the urine. 

 They have laid stress particularly on the fact that the hydraemic 

 plethora caused by intravenous injection of salts is accompanied 

 by diuresis. It is true that the direct introduction of water into 

 the blood, or its attraction from the lymph-spaces when the osmotic 

 pressure of the blood is increased by the injection of substances like 

 urea, sugar, and sodium chloride, may cause a condition of hydrcemic 

 plethora, and that this plethora may sometimes be associated with 

 an increase of pressure in the capillaries in general, and therefore 

 in the vessels of the Malpighian tuft. It may also be admitted that 

 such an increase of pressure might be accompanied by an increased 

 filtration 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, filtration 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 composi- 

 tion as the perfusion fluid (Sollmann). It would certainly appear 

 unlikely that the glomerular epithelium should make no use what- 

 ever for the furtherance of its task of the difference of hydrostatic 

 pressure on its two surfaces. It is in taking advantage of such 

 circumsta ces 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 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 



