EXCRETION OF WASTE PRODUCTS 189 



neutral sulphur, consists of compounds which can yield 

 sulphuric acid only after complete combustion, but not after 

 mere hydrolysis. 



The neutral sulphur is derived from the cystin of the protein, 

 and is related to the taurine of the bile and thiocyanate of 

 the saliva. 



Phosphorus. The phosphorus of urine is mainly in the form 

 of inorganic phosphates derived from the phosphates of food, 

 nucleoproteins, phosphoproteins and other phosphorus- 

 containing substances. 



PIGMENTS OF URINE 



Normal urine is yellow in colour, but it becomes darker 

 after being passed or on heating with acid ; thus it contains 

 chromogens in addition to pigments. 



Urochrome. The chief pigment of the urine is a yellow 

 pigment called urochrome. The yellow colour of the blood 

 and tissues of fowls has been shown to be derived from 

 xanthophyll of the food,* and it may be that the yellow 

 colour of urine may be traced to the same source. 



Urobilin is present in small quantities as its chromogen 

 urobilinogen. Under pathological conditions it is greatly 

 increased in amount. 



Hcematoporphyrin is present in small quantities, and its 

 amount may be greatly increased in diseased conditions. 



These two pigments are derived from haemoglobin. Haema- 

 toporphyrin is the iron free derivative of haematin. Haema- 

 toporphyrin is the source of the bile pigments which become 

 reduced in the intestine to stercobilin and stercobilin is 

 identical with urobilin. 



Uroerythrin is another pigment. It frequently colours uric 

 acid deposits a brick-dust colour. 



The excretion of waste products is concerned with the 

 physico-chemical means by which waste products are removed 

 and with the chemical nature of the substances excreted. 



The nature of the waste products is related to the problems 

 of metabolism and they vary in different species of animals. 

 As this subject has such important medical applications a 

 vast literature exists. In this chapter only an outline of the 

 subject has been possible. 



GENERAL REFERENCES 

 See books on urinalysis and allied subjects. 

 A. R. CUSHNY : The Secretion of Urine, 1917. Longmans, Green & Co. 



* L. S. Palmer and H. L. Kempster, Journ. Biol. Chem., 1919, vol. 

 39, P- 331- 



