CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 685 



407. The Acidity of Urine. The healthy urine of man is 

 acid, owing to the presence of acid sodium phosphate, the absence 

 of free acid being shewn by the fact that sodium hyposulphite 

 gives no precipitate. The amount of acidity is about equivalent 

 to 2 grms. of oxalic acid in twenty-four hours, but the degree of 

 acidity at any one time varies much during the day, being in an 

 inverse ratio to the amount of acid secreted by the stomach ; thus 

 it decreases after food is taken, and increases again as gastric 

 digestion comes to an end. It varies with the nature of the food ; 

 with a vegetable diet the excess of alkalis in the food, being 

 secreted by the urine, leads to alkalinity, or at least to diminished 

 acidity, whereas this effect is wanting with an animal diet, in 

 which the alkalis are less abundant, earthy bases preponderating. 

 Hence the urine of carnivora is generally very acid, while that of 

 herbivora is alkaline. The latter, when fasting, are for the time 

 being carnivorous, living entirely on their own bodies, and hence 

 their urine becomes under these circumstances acid. 



The natural acidity increases for some time after the urine has 

 been discharged, owing to the formation of fresh acid, apparently 

 by some kind of fermentation. This increase of acid frequently 

 causes a precipitation of urates, which the previous acidity, even 

 after the cooling of the urine, had been insufficient to throw down. 

 After a while however the acid reaction gives way to alkalinity. 

 This is caused by a conversion of the urea into ammonium 

 carbonate through the agency of a specific 'organized' ferment. 

 This ferment as a general rule does not make its appearance 

 except in urine exposed to the air; it is only in unhealthy 

 conditions that the fermentation takes place within the bladder, 

 and in such cases is due either to micro-organisms introduced 

 into the bladder from without, during the use of instruments 

 for instance, or to the action of an unorganized ferment, secreted 

 apparently by the walls of the bladder. 



408. Abnormal Constituents of Urine. The structural ele- 

 ments found in the urine under various circumstances are blood, 

 pus and mucus corpuscles, epithelium from the bladder and 

 kidney, and spermatozoa. To these may be added the so-called 

 ' casts,' which are either ' epithelial casts,' that is to say cylinders 

 of more or less altered epithelial cells shed from the tubules, or 

 structureless ' fibrinous ' casts, which are cylinders of peculiar 

 material moulded in the lumina of the tubules ; the exact nature 

 of this material is at present a matter of doubt ; it is not always 

 the same and appears never to be fibrin. 



The most common and important abnormal constituents of 

 urine are albinrdn, giving rise to albuminuria, and sugar, giving 

 rise to glycosuria or diabetes. The soluble proteids generally 

 spoken of as ' albumin ' in the urine differ in different cases. The 

 exact determination of their nature is a matter of some difficulty, 

 since, as we have seen, we have in differentiating the various 



