COMPOSITION OF URINE. 75 



During sleep the renal excretion is diminished, but increased in the 

 morning hours, and especially after the ingestion of hearty meals. 



The nervous system influences the secretion of urine. Irritation of the 

 medulla oblongata, a little above the origin of the pneumogastric and 

 auditory nerves, increases the quantity ; division of the renal nerves 

 destroys the nutrition of the kidney, and thus interferes with the elimina- 

 tion of the urine. Mental emotion, fear, anxiety, etc., increase the amount 

 secreted. 



2. Secretion. While it is established that the Malpighian corpuscles 

 permit the filtration of water and salts, it has also been shown that the 

 renal epithelial cells lining the convoluted tubes are the agencies by which 

 the solid matters, urea, creatin, etc., are removed from the blood, by a 

 process of true secretion, which is independent of blood pressure and 

 caused by the presence of these ingredients in the blood. 



Urea is the most important of the organic constituents of the urine. It 

 is a colorless, neutral substance, crystallizing in four-sided prisms, soluble in 

 boiling alcohol and water; when subjected to prolonged boiling it is decom- 

 posed,' with the production of ammonium carbonate. 



Urea is not formed in the kidneys, but pre-exists in the blood. 



The Amount of Urea excreted in 24 hours is estimated at about 500 

 grains ; it is increased during the waking hours, by an animal diet and by 

 prolonged muscular exertion ; diminished during sleep and by non-nitrogen- 

 ized food. 



Source. Urea results from an imperfect oxidation of the albuminous 

 principles of the food, and from a disintegration of the organic constituents 

 of the tissues. 



Uric acid, or lit hie acid, is a constant ingredient of the urine; the 

 amount excreted daily is about 8 grains; it is increased by nitrogenized, 

 decreased by non-nitrogenized food. It exists in the urine in a free state, 

 and as the urate of soda. It arises from the disassimilation of albuminous 

 compounds, and when secreted in excess is deposited in a crystalline form, 

 as a brown or " brick- red " sediment, with the sodium and ammonium 

 urates. 



Creatin is a colorless, transparent substance, crystallizing in prisms ; 

 found in blood, kidneys, and muscular tissue ; by boiling in acid solutions 

 it is transformed into 



Creatinin, which resembles creatin chemically. It is soluble in water 

 and alcohol, and crystallizes in colorless prisms. About 15 grains are 

 excreted daily. 



