THE URINE. 385 



tity of alkaline earths in the urine depends on the composition of 

 the food. Nothing is known with positiveness in regard to the con- 

 stant and regular change in the elimination of these substances in 

 disease. 



The quantity of calcium and magnesium is determined accord- 

 ing to the ordinary well-known methods. 



Iron occurs in the urine only in small amounts, and, as it seems, not as a 

 salt, but as an organic combination part perhaps as pigment or chromogen 

 (KuNKEL, GIACOSA) and part in other forms. According to MAGNIER, the 

 quantity of iron in 1 litre of urine is 3-11 inilligrms. According to GOTTLIEB, 

 the elimination of iron by the healthy human urine amounts to 2.59 milligrms. 

 per day. Iron-salts introduced into the intestine do not pass into the urine at 

 all, or only in very small amounts. The quantity of silicic acid, according to 

 the ordinary statements, amounts to about 0.03 p. m. Traces of hydrogen per- 

 oxide also occur in the urine. 



The gases of the urine are carbon dioxide, nitrogen, and traces 

 of oxygen. The quantity of nitrogen is not quite 1 vol. per cent. 

 The carbon dioxide varies considerably. In acid urines it is hardly 

 one half as great as in neutral or alkaline urines. 



IV. The Amount and Quantitative Composition of Urine. 



A direct participation of the kidney substance in the formation 

 of the urinary constituents is proved at least for hippuric acid. It 

 is hardly to be doubted that the kidneys as well as the tissues gen- 

 erally have a certain part to play in the formation of other urinary 

 constituents, but their chief task consists in separating and remov- 

 ing urinary constituents dissolved in the blood which have been 

 taken up by it from other organs and tissues. 



It has been shown by the experiments of numerous investiga- 

 tors, HEIDEtfHAItf, V. WlTTICH, NlJSSBAUM, 1STEISSEE, USTIMO- 



WITSCH, J. MTOK, and others, that the elimination of water and 

 the remaining urinary constituents is not alone produced by simple 

 diffusion and filtration. It is generally conceded that the processes 

 of urinary secretion depend essentially upon a specific activity of 

 the cells of the epithelium of the urinary passages, besides which 

 also processes of filtration and diffusion take part. The process of 

 the secretion of urine in man and the higher animals is generally 

 considered to proceed chiefly as follows: The water together with 



