404 MANUAL OF PHYSIOLOGY. 



acid injected into the portal vein appears unchanged in the 

 urine. 



Oxalic acid (C 2 H 2 O 4 ) occurs often, but not constantly, in the 

 urine. It is generally united with lime. It is said to appear in 

 greater quantity, together with an excess of uric acid, after 

 meals, and therefore to be related to the production of the latter 

 in the body ; but it probably is chiefly derived from oxalates 

 being contained in some material taken with the food. 



COLORING MATTERS. 



It appears probable that the color of the urine depends on the 

 presence of small quantities of distinct substances which have 

 different origins in the body. Three such have been described, 

 and may be taken provisionally to represent our knowledge of 

 the subject : 



1. Urobilin, which is an outcome of the coloring matter of the 

 bile, and therefore a remote derivative of the coloring matter of 

 the blood, is frequently present in the urine. It is probably the 

 same as hydrobilirubin, some of which is occasionally absorbed 

 from the intestinal tract and eliminated by the kidneys. 



2. Urochrom is said to be the special pigment of the urine. It 

 oxidizes on exposure, forming a reddish substance that gives the 

 dark color to some urinary sediments ( Uroerythriri). 



3. A certain material (Indican) capable of producing Indigo 

 is commonly present in the urine of man, and in greater quantity 

 in that of some animals, particularly the horse. It is supposed 

 to be formed from the indol that arises from the putrefactive 

 changes consequent on the pancreatic digestion. The indol is 

 absorbed and unites with sulphuric acid to form Indican, which 

 is a yellow substance. Under certain conditions it can be con- 

 verted by oxidation into indigo blue. 



INORGANIC SALTS. 



The urine is the great outlet for all inorganic salts. The most 

 important of these are : 



Common salt (NaCl), of which a very variable, but always 

 considerable amount passes away in the urine. The average 



