648 THE SECRETION OF 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 of 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 (WasseranziehungsverTnogen), 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 pressiu-e 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 hydremic plethora thus induced. We know that a condition of 
hydremic 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 hydremic 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 Lists little or no longer than the hydrsemie 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. Akad. d. Wissensch., Wien, 1SS3, Bd. Ixxxvii. ; I. Munk, 
Virchoxo's Archiv, 1886, Bd. cvii. S. 291 ; ibid., S. 187 ; and I. Munk and Senator, 
ibid., 1838, 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, 1891, vol. xvii. p. 30 ; Leatlies, ibid., 1895, vol. xix. p. 1. 
