392 A MANUAL OF PHYSIOLOGY 



they come down when the carbon dioxide is driven off by heating ; a 

 precipitate of this sort differs from heat-coagulated albumin in being 

 readily soluble in acids (p. 422). 



Sulphuric Acid. This is only to a slight extent derived from 

 ready-formed sulphates in the food. The greater part of it is formed 

 by oxidation of the sulphur of proteids. About nine-tenths of the 

 sulphuric acid of normal urine are united to alkalies ; the other tenth 

 is combined, in the form of ethereal sulphates, with aromatic bodies 

 derived from the putrefaction of proteids in the intestine. Such are 

 potassium -phenyl- sulphate (C 6 H 5 KSO 4 ), potassium -kresyl-sulphate 

 (C 7 H 7 KSO 4 ), potassium-indoxyl-sulphate (C 8 H 6 NKSO 4 ), potassium- 

 skatoxyl-sulphate (C 9 H 8 NKSO 4 ), and two double sulphates of potas- 

 sium and pyrocatechin. Most of those aromatic compounds are 

 present in greater amount in the urine of the horse than in the normal 

 urine of man ; but in disease the quantity in the latter may be much 

 increased; and to a certain extent it must be looked upon as an 

 index of the intensity of putrefactive processes in the intestine and of 

 absorption from it. Munk made the curious observation that in the 

 urine of a starving dog the phenol-forming substances are absent, 

 while in the urine of a starving man they are present in abnormally 

 large amount. The indigo-forming substances (' indican '), on the 

 other hand, are in hunger excreted in considerable quantity by the 

 loer, and not at all by man (p. 423). 



Phenol and kresol can easily be obtained from horse's urine by 

 mixing it with strong hydrochloric acid, and distilling. These aromatic 

 bodies pass over in the distillate. Pyrocatechin remains behind, 

 and can be extracted by ether ; it gives a green colour with ferric 

 chloride, which becomes violet on the addition of sodium carbonate. 



A small amount of phosphorus and of sulphur may appear in the 

 urine in less oxidized forms than phosphoric and sulphuric acids. 

 Such sulphur compounds are potassium sulphocyanide, which is 

 probably, in part at least, derived from that of the saliva ; and ethyl 

 sulphide, a substance with a penetrating odour, which appears to be 

 a constant constituent of dogs' urine (Abel). 



Thiosulphuric acid (H 2 S 2 O 3 ) occurs almost constantly in cat's 

 urine, often in dog's. It is not free, but combined with bases. 



Carbonates of sodium, ammonium, calcium and magnesium occur 

 in alkaline urine. Their source is the carbonates and the vegetable 

 organic acids of the food. In acid urine a certain amount of carbon 

 dioxide is present, although not firmly united with bases, so that 

 most of it can be pumped out. 



The Urine in Disease. Although, strictly speaking, a truly 

 pathological urine has no place in physiology, the line which 

 separates the urine of health from that of disease is often 

 narrow, sometimes invisible ; while the study of abnormal 

 constituents is not only of great importance in practical 

 medicine, but throws light upon the physiological processes 



