402 MANUAL OF PHYSIOLOGY. 



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 

 quantity excreted per diem may be said to be about half an ounce 

 (15 grammes). It depends greatly on the quantity taken with the 

 food, and falls during starvation, but does not completely disap- 

 pear. It is said that if absolutely no sodium chloride be taken 

 with the food the quantity excreted diminishes greatly, and that 

 albumin appears in the urine about the third day. The amount 

 of salt eliminated follows, with striking accuracy, the changes 

 that take place at different times and under different circum- 

 stances, in the quantity of urea excreted. These facts seem to 

 indicate that there is some relationship between the secretion of 

 the two bodies, or that sodium chloride participates in the 

 chemical changes of the nitrogenous tissues. In many diseases 

 there occur variations in the quantity of common salt in the urine, 

 which can hardly be explained by the change in, or absence of 

 food. 



Phosphates. About 60 grains (3 to 4 grammes) of phosphoric 

 acid is excreted daily in the urine, being combined with alkalies 

 to form salts, viz., potassium, sodium, calcium, and magnesium 

 phosphates. 



Sulphates. Nearly 40 grains (2 to 3 grammes) of sulphuric 

 acid, as sulphates of alkalies, are daily got rid of in the urine. 

 The acid comes partly from the food, but chiefly from the oxida- 

 tion of the sulphur contained in the proteids of the tissues. 



A considerable quantity of potassium, sodium, calcium, and 

 magnesium, combined as already mentioned, or with chlorine, is 

 contained in the urine. 



Small traces of iron are also always present in the urine. 



