388 EXCRETION BY THE SKIN AND KIDNEYS. 



variations, but the average quantity of phosphoric acid excreted daily may 

 be estimated at about fifty-six grains (3*629 grammes). 



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

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

 no physiological importance. 



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

 the presence of a nitrogenized substance called urochrome. This is also called 

 urohgematine, uroxanthine and purpurine. There is no accurate account of 

 its composition, and all that is known is that it contains carbon, oxygen, 

 hydrogen and nitrogen, and probably iron. Although its exact chemical 

 composition is not absolutely determined, its elements are supposed to be 

 nearly the same as those of the coloring matter of the blood, the proportion 

 of oxygen being much greater. These facts point to the probability of the 

 formation of urochrome from haemaglobine. 



The quantity of coloring matter in the normal urine is very small. It is 

 subject to considerable variation in disease, and almost always it is fixed by 

 deposits and calculi of uric acid or the urates, giving them their peculiar 

 color. This substance first makes its appearance in the urine and is probably 

 formed in the kidneys. So little is known of its physiological or pathologi- 

 cal relations to the organism, that it does not seem necessary to follow out 

 all of the chemical details of its behavior in the presence of different re- 

 agents. 



The 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 seem to excite decomposition of the urea, 

 and that the urine may remain for a long time in the bladder without under- 

 going putrefactive changes. 



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

 blood by the kidneys, a certain proportion of the gases in solution in the 

 circulating fluid is also removed. For a long time, indeed, it has been known 

 that the normal human urine contained different gases ; but observations on 

 this subject have been made by Morin (1864), in which the proportions of 

 the free gases in solution have been accurately estimated. 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 litre 

 of urine, in cubic centimetres (one part per thousand in volume) : 



Oxygen 0-824 



Nitrogen 9-589 



Carbon dioxide . . . 19-620 



