THE SECRETION OF URINE 1149 



but also by the absorption of dissolved constituents, and that the relative 

 permeability of the cells to the constituents is at any rate one factor in deter- 

 mining the substances absorbed. It is not, however, the only factor. The 

 function of the kidney is to preserve the normal constitution of the body 

 fluids by turning out those substances which are abnormal or present in too 

 great an amount. The behaviour of the tubule cells with regard to any given 

 substance will therefore depend to a certain extent on the previous nutritive 

 history of the body. 



If, for instance, in consequence of the administration of sodium chloride 

 in large quantities to the animal during the few days prece<jd(|g the experi- 

 ment, the body is overloaded with this salt, it becomes an abnormal con- 

 stituent and the kidney secretes a urine far richer in sodium chloride than is 

 the blood- plasma. Moreover, when diuresis is produced in such an animal 

 by the injection of equivalent quantities of sodium chloride and sodium 

 sulphate, there is no diminution of the NaCl in the urine towards the end of 

 the diuresis, but its percentage rises steadily as the rate of urinary flow 

 diminishes. On the other hand, a total deprivation of sodium chloride 

 extending over several days, although not altering to any large extent the 

 percentage amount of this salt in the blood-plasma, leads to a total dis- 

 appearance of the salt from the urine, the whole of the sodium chloride 

 present in the glomerular transudate being absorbed on its way through the 

 urinary tubules. 



It has been suggested that the effects of certain diuretics on the kidney, 

 such as caffeine, diuretine, or theocine, may be largely conditioned not so 

 much by their influence on the glomerular circulation as by a paralytic effect 

 on the absorptive functions of the tubules. According to Loewi, on injec- 

 tion of caffeine or diuretine, the increase of total amount of urine is not 

 accompanied by any diminution in the percentage amount of NaCl. Perhaps, 

 however, the strongest evidence in this direction is afforded by an experi- 

 ment of Pototzky. A rabbit had been fed on a diet almost totally devoid 

 of chlorides, and was therefore excreting a urine containing only -08 per 

 cent. NaCl. Under the influence of diuretine the urine was increased and 

 the concentration of the NaCl rose to 0-64 per cent. The same increase in 

 the percentage amount of sodium chloride in the urine has also been observed 

 after the injection of theocine, which has therefore been specially recom- 

 mended as a diuretic in cases of dropsy, where a diminution of the salt 

 content of the body is a valuable means for the diminution of the dropsical 

 fluid present in the tissue spaces. 



THE RENAL MECHANISM 



What conclusions can we draw from this mass of experimental data 

 as to the functions of the kidney as a whole, and as to the part played by 

 its various constituent elements in the secretion of urine ? ' The amazing 

 adaptability of its functions to the needs of the organism has been abund- 

 antly illustrated in the facts with which we have dealt. Its ordinary activity 

 is determined by the production, as a result of the normal processes of meta- 



