SECRETION. 447 



preponderant and perhaps altogether the chief excretion in birds, 

 reptiles, and insects. 



Uric acid, or lithic acid, is colorless, inodorous, and insipid; it 

 usually crystallizes in whetstone crystals, which have for a funda- 

 mental type the vertical rhombic prism. It is insoluble in alcohol 

 and ether, only very slightly soluble in water. The rhombic crystals 

 are characteristic of uric acid. 



If HC1 be added to urine, there will be deposited on the bottom 

 of the vessel after several hours a deposit resembling Cayenne 

 pepper. Uric acid occurs in the urine as acid sodium urate. The 

 HC1 decomposes the urates, setting free the acid, which does not 

 crystallize at once, by reason of the presence of phosphates. Accord- 



X 



Uric Acid. 



Fig. 170. Uric Acid, Effect of on Intestinal Peristalsis. 



ing to Liebig, it is especially by the phosphates that the acid is dis- 

 solved, under the form of urate. 



Uric acid is dibasic, so that there are two classes of urates : the 

 normal urates and the acid urates. The amorphous urates are quad- 

 riurates; acid urates are crystalline. 



Uric acid is trioxy-purin. The purin bases are hypoxanthin, 

 xanthin, adenin, guanin, and uric acid. All these bodies are derived 

 from a substance called purin. 



The elimination of nitrogen in the urine can be augmented by 

 the food. Thus, nuclein (of which the thymus contains a large 

 amount), coffee, cocoa, and meat (veal and ham especially), and beer 

 are rich in purins. The bodies poor in purins are milk, potatoes, 

 white bread, rice, eggs, salads and cabbage. 



FORMATION OF THE URIC ACID. It is a result in part of the 

 breaking up of the nuclein of cells, forming xanthin, which by xanthin- 

 oxidase is changed into uric acid. 



