A CTION OF DIURE TICS. . 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 ischsemia abolishes the flow \'<>v a considerable period 
after the obstruction has l>een 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 ischaemia 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 ilow 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 to 
be sensitive to slight changes in the composition of the blood. As a 
matter of fact, we find that such is the case, and that the easiest way to 
excite the urinary flow is by altering the composition of the blood", 
through the administration of large quantities of water, or of certain 
drugs which are known as diuretics. Of these bodies the only ones we 
need discuss are the large class known as saline diuretics and the drugs 
caffein and digitalis. 
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 
