EXCRETION OF MATTER FROM THE BODY 421 



from phosphates taken in the food, and partly from the 

 nucleins of the food and tissues and from the bones. 



(a) Normally the phosphorus is fully oxidised to P 9 

 which is linked to alkalies and earths, and excreted in the urine! 

 The most important phosphate is the phosphate of soda, 

 NaH 2 P0 4 , which is the chief factor in causing the acidity of the 

 urine. About one quarter of the phosphoric acid is linked to 

 calcium and magnesium, and it is these earthy phosphates 

 which precipitate when the urine becomes alkaline. When the 

 urine becomes ammoniacal, triple phosphate is formed (p. 416). 



(&) It is probable that a small quantity of the phosphorus 

 is excreted in organic compounds, such as glycero-phosphates ; 

 but so far these have not been fully investigated. 



IV. Chlorine-containing Bodies 



Chloride of sodium is the chief salt of the urine. It is 

 entirely derived from the salt taken in the food, and its 

 amount varies with the amount ingested. In the horse it is 

 present in very small quantities. 



In starvation, and still more in fever, the tissues of the body 

 have an extraordinary power of holding on to the chlorine, 

 and the chlorides may almost disappear from the urine. 



V. Bases of the Urine 



Sodium, potassium, calcium, and magnesium occur in the 

 urine in amounts varying with the amounts taken in the food. 

 On a flesh diet and in starvation potassium is in excess of the 

 others. Calcium and magnesium are present in much smaller 

 quantities. In herbivora potassium is the chief salt, and in 

 the horse calcium is also abundant. 



VI. Pigments 



A brown hygroscopic substance, which gives no bands in the 

 spectrum, may be extracted from urine. This has been termed 

 uroehrome. By rediicing this, another pigment, urobilin, is 

 produced, which gives definite bands, and which is frequently 

 present in the urine. It is probably identical with the hydro- 

 bilirubin which has been prepared from the bile pigments, and 

 it contains C. H. 0. and N. 



