412 EXCRETION. 



greatest at night, least in the forenoon, and between these extremes in the afternoon." 

 In estimating the degree of acidity of the urine, it is necessary to test the fluid as soon as 

 possible after it is discharged from the bladder ; for its acidity rapidly increases after 

 emission until ammoniacal decomposition sets in by the formation of organic acids, par- 

 ticularly the lactic. 



There has been considerable discussion and difference of opinion among physiological 

 chemists with regard to the cause of the acid reaction of the urine. At the moment of 

 its discharge from the bladder, it is distinctly and even strongly acid ; but it will not de- 

 compose the carbonates, like most acid solutions. The weight of chemical authority upon 

 this point is in favor of the view that there is no free acid in the urine when it is first 

 passed, although the lactic acid, the acid lactates, and, perhaps, some other of the organic 

 acids may be produced after emission, as the result of decomposition ; but nearly all 

 authors agree that it contains the acid phosphate of soda. The phosphates exist in the 

 fluids of the body in at least three different conditions. The basic phosphate of soda, for 

 example, possesses three atoms of the base and has an alkaline reaction. In contact 

 with carbonic acid, this salt may lose one atom of the base, forming the carbonate of 

 soda and what is called the neutral phosphate, the latter, however, having a feebly alka- 

 line reaction. In contact with uric acid, the neutral phosphate may lose still another 

 atom of base, forming the urate of soda and the acid phosphate ; and, according to most 

 authorities, it is in this form that it exists in the urine, and the presence of this salt is the 

 cause of its acidity. The acid phosphate of soda may or may not be associated, in the 

 human subject, with the acid phosphate of lime, which ordinarily gives the intensely acid 

 reaction to the urine of the carnivora. 



Composition of the Urine. 



Regarding the excrementitious constituents of the urine as a measure, to a certain 

 extent, of the general process of disassimilation, it is probably more important to recog- 

 nize the absolute quantity of these principles discharged in a definite time than to learn 

 simply their proportions in the urine ; and, in making out a table of the composition of 

 the urine, we shall give, as far as possible, the absolute quantity of its different constitu- 

 ents excreted in twenty-four hours. This latter point, however, will be more elaborately 

 considered in connection with the characters of the individual excrementitious principles 

 and their variations under physiological conditions. In compiling this table, we have 

 taken advantage of the elaborate bibliographical and experimental researches of Prof. 

 Robin, contained in his recent work upon the humors, 1 but we have ventured to make 

 some changes and corrections in his list of urinary constituents : 



1 EOBIN, Lefons swr lea humeurs, Paris, 1874. In the table given by Eobin (p. 762), there is evidently a very 

 serious error in one of the figures giving the proportion of water and an error in the proportion of oxygen. We have 

 omitted some of the constituents given by Robin, which are stated to be doubtful or accidental, or are noted as present 

 under pathological conditions. 



Although the table represents, very nearly, the latest and most reliable observations upon the relative and abso- 

 lute quantities of the urinary constituents, there are a few minor points that demand some explanation. For example, 

 Eobin estimates the proportion of hippurates at a little less than the proportion of urates, while many writers of high 

 authority speak of the hippurates as excreted in rather larger quantity ; but the investigations with regard to the 

 daily excretion of hippuric acid have not been so definite and satisfactory as those upon which the estimates of the ex- 

 cretion of uric acid are based. Kobin gives, also, the proportion of creatine as 1-4 to 2'6 parts per 1,000, and of crea- 

 tinine, 0-2 to 0'4 per 1,000 ; and most authors give in the urine a larger proportion of creatinine. This difference, how- 

 ever, is not important, for, as far as the process of excretion is concerned, these two substances may be regarded as a 

 single principle, creatine being readily converted into creatinine in the urine by simple decomposition. In our endeavor 

 to make this table as complete as possible, we have reduced the figures given by many authors to represent the amounts 

 of uric acid, phosphoric acid, sulphuric acid, chlorine, etc., to the quantity of the salts as they actually exist. This is 

 particularly important in a work on physiology, for chlorine and the various acids just enumerated are not proximate 

 constituents of the urine, except when combined with bases. It is simply a matter of convenience to estimate them 

 separately, and the proportions of salts are readily calculated from the combining equivalents of the different ele- 

 ments. 



