334 HAND-BOOK OF PHYSIOLOGY. 



generally occurs in urine in combination with bases, forming urates, 

 and never free unless under abnormal conditions. A deposit of urates 

 may occur when the urine is concentrated or extremely acid, or when, 

 as during febrile disorders, the conversion of uric acid into urea is incom- 

 pletely performed. 



Properties. Crystallizes in many forms, of which the most common 

 are smooth, transparent, rhomboid plates, diamond-shaped plates, hexa- 

 gonal tables, etc. Very insoluble in water, and absolutely so in alcohol 

 and ether. Dried with strong nitric acid in a water-bath, a compound 

 is formed called alloxan, which gives a beautiful violet red with ammo- 

 nium hydrate (murexide), and a blue color with potassium hydrate. It 

 is easily precipitated from its solutions by the addition of a free acid. It 

 forms both acid and neutral salts with bases. The most soluble urate is 

 lithium urate. 



Composition. Very uncertain; has been however recently produced 

 artificially, but it is not easily decomposed; it may be regarded as diureide 

 of tartronic acid. The chief product of its decomposition is urea. 



Guanin, C B H 6 N 5 0, has been found in the human liver, spleen, and 

 faeces, but does not occur as a constant product. 



XantMn, C 5 H 4 N 4 2 , has been obtained from the liver, spleen, 

 thymus, muscle, and the blood. It is found in normal urine, and is a 

 constituent of certain rare urinary calculi. 



Hypoxanthin, 5 H 4 N 4 0, or sarkin, is found in juice of flesh, in the 

 spleen, thymus, and thyroid. 



Allantoin, C 4 H 6 N 4 3 , found in the allantoic fluid of the foetus, and 

 in the urine of animals for a short period after their birth. It is one of 

 the oxidation products of uric acid, which on oxidation gives urea. 



In addition to the amides and probably related to them, are certain 

 coloring and excrement itious matters, which are also most likely distinct 

 decomposition compounds. 



PIGMENTS, ETC. 



BiliruUn, C 9 H 9 N0 2 , is the best known of the bile pigments. It is best 

 made by extracting inspissated bile or gall stones with water (which 

 dissolves the salts, etc.), then with alcohol, which takes out cholesterin, 

 fatty, and biliary acids. Hydrochloric acid is then added, which decom- 

 poses the lime salt of bilirubin and removes the lime. After extracting 

 with alcohol and ether, the residue is dried and finally extracted with 

 chloroform. It crystallizes of a bluish-red color. It is allied in compo- 

 sition to haematin. 



Biliverdin, C 8 H 9 N0 5 , is made by passing a current of air through 

 an alkaline solution of bilirubin, and by precipitation with hydrochloric 

 acid. It is a green pigment. 



Bilifustin, C 9 H n N0 3 , is made by treating gall stones with ether, then 

 with dilute acid, and extracting with absolute alcohol. It is a non-crys- 

 tallizable brown pigment. 



