THE KIDNEYS. SECRETION OF URINE. 609 



the acid which would otherwise show a predominance, is directed into another 

 channel ; and it is exaggerated in those states in which, either from the irri- 

 tating nature of the food, or from the irritable condition of the stomach, an 

 undue quantity of acid is poured out into that viscus ; so that, its reaction being 

 habitually acid, that of the urine becomes habitually alkaline. Such a state of 

 the urine must be carefully distinguished, as Dr. Bence Jones has pointed out, 1 

 from that in which the alkalescence is due to the presence of volatile, and not 

 to that of fixed alkali ; the difference being easily recognizable by the influence 

 of the liquid upon reddened litmus-paper, for the restoration of its blue color 

 is permanent in the latter case, but only transitory in the former. The alka- 

 lescence due to the presence of volatile alkali is due to the decomposition of 

 urea, whilst the urine is yet within the bladder, through the agency of morbid 

 secretions of that viscus ; and it disappears when this organ returns to its healthy 

 state. On the other hand, the alkalescence from fixed alkali proceeds from 

 disordered action of the stomach, which is usually connected with disorder of 

 the general system ; and it persists until this can be remedied. In both forms 

 of alkalescence, there is a precipitation of earthy phosphates ; but in the alka- 

 lescence from fixed alkali, the precipitate usually consists almost entirely of phos- 

 phate of lime ; whilst in that from volatile alkali, the amorphous sediment of 

 phosphate of lime is mingled with prismatic crystals of the phosphate of ammonia 

 and magnesia. These precipitates may be obtained from healthy urine, by 

 adding to it a solution of potash or of ammonia ; and the decomposition of such 

 urine, which begins to take place very soon after it leaves the body, gives rise 

 to the same precipitation, by the production of carbonate of ammonia at the 

 expense of its urea ( 52). 



645. Thus, then, we have seen that the Kidneys serve as the special instru- 

 ments for depurating the Blood of those highly-azotized compounds, which are 

 formed in the system by the decomposition of the materials of the albuminous 

 and gelatinous tissues, and also by that of the non-assimilated components of the 

 food. We have seen, also, that they serve for the removal of certain compounds 

 of which carbon is a principal ingredient; and these, although normally present 

 in but small amount, may undergo a marked increase in disease, especially when 

 the liver is insufficiently performing its functions, or the respiratory process is 

 obstructed. Further, we have been led to regard the Kidneys as the emunctory, 

 not only for the superfluous water of the blood, but also for those saline com- 

 pounds, which, having been introduced into the system, or generated within it, 

 in larger amount than is compatible with the normal constitution of the blood, 

 or than is required for the reparation of the solids of the body, or for the pro- 

 duction of its fluid secretions, are only fitted for elimination. And this state- 

 ment is to be extended from saline compounds to such other soluble matters 

 as are not removed by other channels. On this point a very elaborate series 

 of researches was made by Wb'hler, 3 who showed that of the soluble salts taken 

 into the circulation, those are most readily excreted which produce a determina- 

 tion of blood towards the kidneys, whereby an increased quantity of liquid is 

 filtered off through the outlet which they afford. And it is in this manner that 

 the system makes an effort to free itself (so to speak) from various foreign sub- 

 stances which have been introduced into it by absorption, and which would be 

 injurious if retained; the rate at which it does so being in a great degree depend- 

 ent upon the functional activity of the Kidneys ( 89, 207, 208). It does not 

 appear, however, that the excretion of the organic compounds which are formed 

 within the system is augmented by those " diuretic" medicines which, by deter- 

 mining an increased flow of blood to the Kidneys, cause a larger amount of liquid 



1 "Medical Times," Dec. 13, 1851. 



2 "Miiller's Elements of Physiology," translated by Baly, p. 589. 

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