648 THE SECRETION OP URINE. 



artificial circulation, before any secretion could be obtained. 1 On 

 inquiring into the mode of action of these bodies, we find that their 

 injection is followed by a slight rise of blood pressure accompanied with 

 a marked expansion of the kidney, and this expansion lasts throughout 

 the period of increased urinary now. These effects are observed even 

 after all the renal nerves have been severed, so far as is practically 

 possible, and it has therefore been concluded that the changes in volume 

 of the kidney must be due to the substances acting either upon some 

 peripheral vasomotor mechanism, or even more directly upon the blood 

 vessels themselves. Since the increased secretion of urine is cotermin- 

 ous with the increased blood flow through the kidney, it is natural to 

 place these two events in the relation f effect and cause. To this con- 

 clusion it has been objected that one may frequently observe an absolute 

 standstill of secretion, with a high aortic blood pressure changed into a 

 copious secretion by the injection of one of these bodies, although the 

 blood pressure has been practically unaltered. Heidenhain and others 

 with him, therefore, look upon the action of these bodies as secretomotor. 

 Against the specific secretomotor action, either of urea or of the salines, 

 the following arguments may be brought forward. V. Limbeck 2 has 

 shown that the power of these bodies to induce urinary secretion on 

 injection into the blood stream is proportional to their power of attracting 

 water ( Wasseranziehungsvermogeri), and is thus a function of their mole- 

 cular weights. Now it has been proved 3 that the result of injecting these 

 bodies into the blood is to cause an active flow of water from the tissues 

 into the blood, which therefore becomes diluted to an extent varying with 

 the osmotic pressure of the substances injected. The final effect, there- 

 fore, is the same as if a solution of the substance isotonic with or normal 

 to the blood had been injected into the circulation, and a condition 

 of hydraBinic plethora thus induced. We know that a condition of 

 hydraemic plethora is associated with dilatation (especially of the visceral 

 vessels), general rise of capillary and venous pressures, and increased 

 rapidity of blood flow. The fact that the diuretic action of these bodies 

 is proportional to their osmotic pressures, implies that it is also propor- 

 tional to the hydrsemic plethora produced by the injection ; and it seems 

 probable, therefore, that the plethora is the chief agent in causing, first, 

 the vascular changes in the kidney, and secondly, the diuresis. If these 

 bodies acted as specific stimulants of the kidney, we should expect the 

 increased flow of urine to continue until all the substance injected had 

 been excreted. Such, however, is not the case. The diuresis comes to 

 an end when only a small amount of the injected substance has been 

 excreted, and lasts little or no longer than the hydraemic plethora which 

 accompanies it. 



Of the other diuretics, the action of two, caffein and digitalis, has been 

 very fully investigated. If half a grain of caffein be injected into a vein, 

 the kidney after a few seconds diminishes in volume, and the flow of urine 

 is lessened or entirely arrested. This contraction soon passes off, and is 

 followed by a rapid expansion, which is more considerable and lasts much 

 longer than the preceding contraction. Simultaneously with the beginning 



1 M. Abeles, Sitzungsb. d. k. AJcad. d. Wissensch., Wien, 1883, Bd. Ixxxvii. ; I. Munk, 

 Virchovfs ArcTiiv, 1886, Bd. cvii. S. 291 ; ibid., S. 187 ; and I. Munk and Senator, 

 ibid., 18S8, Bd. cxiv. S. 1. 



2 Arch. f. exper. Path. u. PharmakoL, Leipzig, 1889, Bd. xxv. S. 69. 



3 V. Brasol, Arch. f. PhysioL, Leipzig, 1884, S. 211 ; Starling, Journ. PhysioL, Cam- 

 bridge and London, 1894, vol. xvii. p. 30 ; Leathes, ibid., 1895, vol. xix. p. 1. 



