A CT10N OF DIURETICS. 647 



subsequent restoration of the blood flow, the amount of lymph transuded 

 is greater than before the ligature. In the kidney the reverse is the 

 case. A temporary ischoemia abolishes the flow for a considerable period 

 after the obstruction has been relieved. This fact shows that, for the 

 normal production of urine, the integrity of the living cells between the 

 blood and Bowman's capsule is necessary ; but I do not think that it 

 can be looked upon as definitely proving the active co-operation of these 

 cells in the process. In the kidney we have two layers of cells, the 

 vascular endothelium and the glomerular epithelium, intervening between 

 the blood and urinary tubule, and we have no evidence to guide us as to 

 the effects of temporary ischsemia on the glomerular epithelium. We 

 know that in a certain sense it becomes more permeable, inasmuch as 

 the urine which is first secreted after the restoration of the circulation 

 contains albumin, which may be traced on its way through the glome- 

 rular epithelium into the capsule. But this fact in itself might tend to 

 impede the flow of water through the glomerular membranes. 



Ligature of renal vein. In the case of lymph formation, a rise of 

 venous pressure tends to increase the amount of lymph produced. In 

 the kidney, ligature of the renal vein stops the flow of urine at once, 

 although it must send up the pressure in the glomerular capillaries to a 

 height approaching that of the renal artery. Now, in this case there are 

 three factors which might be concerned in causing the cessation of 

 secretion : the blood flow through the kidney is checked ; the cells of 

 the glomerular epithelium are asphyxiated ; and the engorgement of the 

 renal veins causes the interlobular veins to swell up and press on the 

 adjoining collecting tubules. Heidenhain lays most stress on the first 

 factor, and, relying mainly on this experimental result, concludes that 

 the chief agent in exciting glomerular activity is not the blood pressure 

 in the glomerular capillaries, but the rapidity of the flow through the 

 capillaries. On the other hand, Ludwig has shown that the effect of the 

 swelling of the interlobular veins is to obstruct the urinary tubules ; and 

 he looks upon the cessation of flow as entirely due to this mechanical 

 obstruction. It is impossible at present to decide which of these 

 explanations is correct, or indeed whether all of them may not be 

 involved. 



Action of diuretics. Since the main office of the kidney is to assist 

 in maintaining the normal constitution of the blood by freeing it from the 

 waste products of tissue metabolism, we should expect it to react and tq 

 be sensitive to slight changes in the composition of the blood. As a 

 jai fact, we find that such ia the caflft, and t.hat t^e, ftam'pntr wa y fr> - 



thfT"rmnfl.ry flnw'ja^jT^^ltprmg- t.hft p.nmpnaif f ]'flp nf tho KlnnH, - 

 te Ifl-ygft rpa.ntit.ies ' 



fh'nrfttiflfii n f these bodies the only ones we 



need discuss are the large class known as saline diuretics and the drugs 

 caffein and digitalis. 4^ m 2t+&^si*y <&*<* ^-^ 



Saline diuretics include practically all crystalloid substances, which 

 can be injected into the blood in considerable quantities. As examples, 

 we may cite urea, dextrose, sodium chloride, potassium nitrate, sodium 

 acetate, etc. If these bodies be injected into the blood, a very copious 

 secretion of urine is soon evoked, even if, previously to the injection, 

 the secretion had been at a standstill. In experiments on the excised 

 kidney, it has in most cases been found necessary to add urea or some 

 other substance of this group to the defibrinated blood used for the 



