KIDNEYS. 171 



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

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

 berries, 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 fifteen grains are excreted daily. 



Xanthin, hypo-xanthin, and guanin are also constituents of 

 urine. They are nitrogenized compounds and are also terminal 

 products of nucleic acid. 



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

 bile pigments. It is particularly abundant in febrile conditions, giv- 

 ing 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 fifteen to thirty 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 meta- 

 bolism of the nervous tissue. 



Sulphuric acid in combination with sodium and potassium con- 

 stitutes the sulphates, of which about thirty grains are excreted 

 daily. Sulphuric acid results largely from the decomposition of 

 albuminous food and from increased destruction of animal tissues. 



The gases of urine are carbonic acid and nitrogen. 



Mechanism of Urinary Secretion. As the kidney anatomically 

 presents an apparatus for filtration (the Malpighian bodies) and an 

 apparatus for secretion (the epithelial cells of the urinary tubules), 

 it might be inferred that the elimination of the constituents of the 

 urine is accomplished by the twofold process of filtration and 

 secretion; that the water and highly diffusible inorganic salts 

 simply pass by diffusion through the walls of the blood-vessels of 

 the glomerulus into the capsule of Miiller, while the urea and re- 

 maining organic constituents are removed by true secretory action 

 of the renal epithelium. Modern experimentation supports this view 

 of renal action. 



