444 A MANUAL OF PHYSIOLOGY 



constantly present in minute traces in normal urine. Certain drugs 

 e.g., sulphonal cause an increase in its amount. It can be sepa- 

 rated from urine by the addition of sodium or potassium hydroxide, 

 which precipitates the earthy phosphates. The haematoporphyrin 

 is carried down with the precipitate, and may be dissolved out with 

 chloroform. The chloroform is then evaporated and the residue 

 dissolved in alcohol acidified with hydrochloric acid. The alcoholic 

 solution is filtered, and examined with the spectroscope. In 

 paroxysmal haemoglobinuria, meth&moglobin, mixed with some oxy- 

 haemoglobin, is found in the urine in large amount ; and it is worthy 

 of note that it is not formed in the urine after secretion, but is already 

 present as such when it reaches the bladder. 



In the rare condition termed alkaptonuria a body, alkapton, now 

 known to be identical with homogentisinic acid (C ( .H 3 . (OH) 2 CH 2 .COOH) , 

 a dioxyphenylacetic acid, is present. The urine becomes dark brown 

 on the addition of an alkali, or simply on exposure to air. It gives 

 Fehling's test for sugar. The substance has relations to the aromatic 

 amino-acids tyrosin and phenyl-alanin, and when either of these is 

 given to a person suffering from alkaptonuria, the amount of alkapton 

 excreted is increased. We may suppose, therefore, that in this con- 

 dition the normal decomposition of these products of proteolysis is 

 interfered with. 



Ferments. The urine contains traces of proteolytic and amylo- 

 lytic ferments (Fig. 168). These may be easily separated from it by 

 putting a little fibrin, which has the power of fixing (adsorbing) 

 enzymes, into the urine. 



Of the inorganic constituents of urine the most important 

 and most easily estimated are the chlorine, phosphoric acid, and 

 sulphuric acid. 



Chlorine. Much the greater part of the chlorine is united with 

 sodium, a smaller amount with potassium. The chlorides of the 

 urine are undoubtedly to a great extent derived directly from the 

 chlorides of the food, and have not the same metabolic significance 

 as the organic, and even as some of the other inorganic constituents. 

 But it is noteworthy that in certain diseased conditions the chlorine 

 may disappear entirely from the urine, or be greatly diminished 

 e.g., in pneumonia, and in general in cases in which much material 

 tends to pass out from the blood in the form of eifusions (p. 477). 



Phosphoric Acid. The phosphoric acid of the urine is chiefly 

 derived from the phosphates of the food, but must partly come from 

 the waste products of tissues rich in phosphorus-containing sub- 

 stances, such as lecithin and nuclein. The phosphoric acid is united 

 partly with alkalies, especially as acid sodium phosphate, and partly 

 with earthy bases, as phosphates of calcium and magnesium. The 

 earthy phosphates are precipitated by the addition of an alkali to 

 urine, or in the alkaline fermentation. In some pathological urines 

 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 (Practical Exercises, p. 486) . A small amount 

 of phosphorus may appear in the urine in a less oxidized form than 

 phosphoric acid. 



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 proteins. About nine-tenths of the 



