340 EXCRETION 



resemble saltpetre. These crystals are very soluble in water and in 

 alcohol but are insoluble in ether. In its behavior with reagents, urea 

 acts as a base, combining readily with certain acids, particularly nitric 

 and oxalic. It also forms combinations with certain salts, such as mer- 

 curic oxide, sodium chloride etc. It exists in the economy in a state of 

 watery solution, with perhaps a small portion modified by the presence 

 of sodium chloride. 



Origin of Urea. It is now admitted by physiologists that urea is not 

 formed in the kidneys but preexists in the blood. It finds its way into 

 the blood, in part directly from the tissues, and in part from the lymph, 

 which contains a greater proportion of urea than is found in the blood 

 itself. The principal seat of its formation, however, is the liver, although 

 it is produced in other organs in small quantity. The quantity of urea 

 in the blood is kept down by the eliminating action of the kidneys. In 

 certain cases of structural diseases of the liver, the excretion of urea is 

 much diminished, and it may, indeed, disappear from the urine. 



Proteids, in the metabolic processes that result in the change of the 

 albuminous constituents of the tissues into urea, are supposed to un- 

 dergo the following changes. The carbon molecules are oxidized into 

 carbon dioxide, nitrogen is split off as ammonia (NH 3 ), and the two 

 combine to form ammonium carbonate. In the liver, ammonium car- 

 bonate loses one molecule of water and is converted into ammonium car- 

 bamate. This loses a second molecule of water and is converted into 

 carbamid (urea). Carbamic acid in a free state is unknown ; but it 

 exists as ammonium carbamate in commercial ammonium carbonate. 



Assuming that urea is the most abundant and important of the ni- 

 trogenous excrementitious products which is fully justified by physio- 

 logical facts it is difficult to avoid the conclusion that this substance 

 represents, to a great extent, the katabolism of the nitrogenous parts of 

 the tissues and necessarily the physiological wear of the muscular sub- 

 stance. The fact that urea exists in very minute quantity in the muscles 

 some chemists state that it is absent probably is due to its constant 

 removal by the blood and lymph. 



Uric acid, creatin, creatinin, xanthin, hypoxanthin, leucin, tyrosin 

 and some other analogous substances are to be regarded as formations 

 antecedent to urea, urea being the final and perfect excrementitious 

 product of the liver. 



Influence of Ingesta on the Composition of the Urine and on the Elimi- 

 nation of Nitrogen. Water and other liquid ingesta usually increase 

 the proportion of water in the urine and diminish its specific gravity. 

 This is so marked after the ingestion of large quantities of liquids, that 

 the urine passed under these conditions is sometimes spoken of as the 



