470 



The urine is natural in appearance when passed, but on exposure to 

 air gradually darkens from the surface downwards, until it ultimately 

 becomes a deep brown-black colour. Such a urine reduces Fehling's 

 solution, but does not give Nylander's test. With Millon's reagent 

 it gives a yellow precipitate; the addition of ferric chloride drop by 

 drop causes a passing blue colour. It does not ferment with yeast, 

 nor rotate the plane of polarized light. After poisoning with carbolic 

 acid, the urine may contain a similar body. Alkaptonuric urine may 

 be distinguished from one containing the pigment melanin by the 

 fact that the latter does not reduce Fehling's solution. 



Cystinuria. Cystin, di-amino-di-thiolactic acid is the sulphur- 

 holding product of protein decomposition. It is a condensation 

 derivative of molecules of cystein thio-amino-propionic acid. 

 Taurin (of the bile acid taurocholic acid) yields cystein on oxidation. 

 It does not normally occur in the urine. It is a rare anomaly, the 



FIG. 227. CYSTIN CRYSTALS, x 350. 



result of an inborn error of metabolism. It affects particularly the 

 male issue of the marriage of near relations, such as first cousins. 



Cystin only occurs in acid urine. Six-sided crystals of cystin 

 (Fig. 227) may form from the urine on the addition of acetic acid, 

 but usually the urine yields a deposit of these crystals. They are 

 soluble in alkali. If some of the deposit be placed iinder a cover- 

 glass, and strong hydrochloric acid added, delicate prisms are formed 

 as the acid reaches the crystals. These disappear if water be added. 

 Dissolved in caustic potash, cystin gives a violet colour when fresh 

 sodium nitroprusside is added and the solution warmed. An alkaline 

 solution of cystin gives a black precipitate of lead sulphide when boiled 

 with lead acetate solution. Stones composed of cystin have rarely 

 been found in the bladder. 



Urinary Deposits. The deposits of normal urine vary in character, 

 according as they are deposited from acid or alkaline urine. The 

 following occur in acid urine: 



