82 HUMAN PHYSIOLOGY. 



Uric acid is also a constant ingredient of the urine and is closely allied 

 to urea. It is a nitrogenized substance, carrying out of the body a large 

 quantity of nitrogen. The amount eliminated daily varies from 5 to 10 

 grains. Uric acid is a colorless crystal belonging to the rhombic system. 

 It is insoluble in water, and if eliminated in excessive amounts it is de- 

 po-ited as a " brick-red " sediment in the urine. It is doubtful if uric acid 

 exists in a free state, being combined for the most part with sodium and 

 potassium bases forming urates. It is to be regarded as one of the termi- 

 nal products of the disassimilation of albuminous compounds, and is prob- 

 ably produced in the liver. 



Hippuric acid is found very generally in urine, though it is present 

 only in small amounts. It is increased by a diet of asparagus, cranberries, 

 plums and by the administration of benzoic and cinnamic acids. It is 

 probably formed in the kidney. 



Kreatinin resembles the kreatin derived from muscles. It is a colorless 

 crystal, belonging to the rhombic system. Its origin is unknown, though it 

 is largely increased in amount by albuminous food. About 15 grains are 

 excreted daily. 



Xanthin, Sarkin, Oxaluric Acid and Allantoin are also constituents 

 of urine. They are nitrogenized compounds and are also terminal products 

 of albuminous compounds. 



Urobilin, the coloring matter of the urine, is a derivative of the bile pig- 

 ments. It is particularly abundant in febrile conditions, giving to the urine 

 its reddish- yellow color. 



Inorganic Constituents of Urine. Earthy Phosphate. Phos- 

 phoric acid in combination with magnesium and calcium is excreted daily 

 to the extent of from 15 to 30 grains. The phosphates are insoluble in 

 water, but are held in solution in the urine by its acid ingredients, alkalinity 

 of the urine being attended with a copious precipitation of the phosphates. 

 Mental work increases the amount of phosphoric acid excreted, a condition 

 caused by increased metabolism of the nervous tissue. 



Sulphuric acid in combination with sodium and potassium constitute the 

 sulphates, of which about 30 grains are excreted daily. Sulphuric acid 

 results largely from the decomposition of albuminous food and from 

 increased destruction of animal tissues. 



The Gases of the urine are carbonic acid and nitrogen. 



Mechanism of Urinary Secretion. As the kidney anatomically pre- 

 sents an apparatus for filtration (the Malpighian bodies), and an apparatus 



