viii THE EXCEETION OF UKINE 433 



secretion, i.e. act as diuretics (Segalas, 1823). Urea, which is 

 abundantly present in the blood, is one of the most potent diuretics, 

 just as we saw that bile is one of the best cholagogues. In experi- 

 ments with artificial circulation through the excised kidney, it is 

 necessary to add urea to the defibrinated blood in order to obtain 

 secretion of urine (I. Munk and Senator). 



According to Ustimowitsch (1870), Heidenhain (1874), and 

 Griitzner (1875), the injection of a few grammes of urea into a 

 rabbit increases the formation of urine under normal conditions, 

 and reinstates it when arrested by section of the cervical cord. 



The diuretic action of glucose when injected into the vein is 

 similar to that of urea. We have seen in Chapter V. that when- 

 ever the normal quantity of sugar in the blood exceeds certain 

 limits it escapes by the urine, carrying with it a large amount of 

 water, so that there is glycosuria associated with polyuria. In 

 diabetes mellitus this phenomenon may become very pronounced. 

 From this we may conclude that the kidney regulates not merely 

 the water content of the blood but also the amount of circulating 

 sugar, which (in excess of the normal) has a diuretic action like 

 urea, and probably acts by the same mechanism. 



The diuretic action of the different sugars varies ; while glucose, 

 maltose, and lactose increase diuresis, laevulose, on the contrary, 

 has little effect, as shown by the experiments of Albertoni (1881- 

 1891). The increase of diuresis with glucose, maltose, and lactose 

 is due in his opinion to increased velocity of circulation and 

 dilatation of the renal vessels, perhaps also to a specific exciting 

 action which these sugars exercise upon the secretory renal 

 epithelium. 



The diuretic action of caffeine is similar to that of urea. After 

 a brief delay or block of the urinary secretion, during which the 

 kidney shrinks, secretion begins again, and increases so as con- 

 siderably to exceed the normal value while the kidney expands. 

 Blood-pressure diminishes in the first stage ; in the second it rises, 

 and reaches or slightly exceeds the original level. 



The action of digitalis is more complex, because it reinforces 

 and retards the beats of the heart, with simultaneous increase of 

 the tone of the vessel, so that arterial pressure rises. Under 

 normal conditions, therefore, its diuretic action is unimportant ; 

 with impaired cardiac activity, on the other hand, it does act as a 

 diuretic by the improvement in the conditions of the renal circula- 

 tion, due to increase of arterial pressure. 



None of these facts militate seriously against the mechanical 

 theory, as regards the function of the glomerulus, in the above 

 sense. 



Since the diuretic action coincides with increase of arterial 

 pressure, and ceases on its return to the normal, Limbeck's theory, 

 according to which diuresis depends upon the power of diuretics 



VOL. II 2 F 



