350 EXCRETION 



of the other phosphatic salts, but the daily quantity excreted has not 

 been estimated. There exists in the urine a small quantity of the 

 ammonio-magnesian phosphate, but it never in health is in sufficient 

 quantity to form a crystalline deposit. The daily excretion of the phos- 

 phates is subject to great variations, but the average quantity of phos- 

 phoric acid excreted daily may be estimated at about fifty-six grains 

 (3.629 grams). 



The urine contains, in addition to the inorganic salts that have been 

 mentioned, a small quantity of silicic acid ; but so far as is known, this 

 has no physiological importance. 



Coloring Matter and Mucus. The peculiar color of the urine is due 

 mainly to the presence of a nitrogenous substance called urochrome. 

 Normal urine, however, contains a number of pigments. A substance 

 called urobilin, although its proportion is very small, is the pigment that 

 has been most carefully studied. This is supposed to result from a 

 transformation of a certain quantity of stercobilin that is taken up by 

 the blood from the intestinal tract and is separated from the blood by 

 the kidneys. After this, however, most of the urobilin is thought to be 

 converted into urochrome. The sum of definite knowledge in regard to 

 the urinary pigments is small ; and the physiological relations of these 

 substances are not understood and may be unimportant. It is sufficient 

 to state here that the color of normal urine is due to a mixture of color- 

 ing matters, the most important being urochrome. All these matters 

 are originally iron-free derivatives of hemoglobin. 



Normal urine always contains a small quantity of mucus, with more 

 or less epithelium from the urinary passages and a few leucocytes. 

 These form a faint cloud in the lower strata of healthy urine after a few 

 hours' repose. The properties of the different kinds of mucus have 

 already been considered. An important peculiarity, however, of the 

 mucus contained in normal urine is that it does not excite decomposi- 

 tion of urea and that the urine may remain for a long time in the bladder 

 without undergoing putrefactive changes. 



Gases of the Urine. In the process of separation of the urine from 

 the blood by the kidneys, certain gases in solution are removed. By 

 using the method employed by Magnus in estimating the gases of the 

 blood, Morin was able to extract about two and a half volumes 

 of gas from a hundred parts of urine. He ascertained, however, that 

 a certain quantity of gas remained in the urine and could not be 

 extracted by the ordinary process. This was about one-fifth of the 

 whole volume of gas. Adding this to the quantity of gas extracted, he 

 obtained the following proportions to one liter of urine, in cubic cen- 

 timeters (one part per thousand in volume) : 



