ORIGIN OF UREA. 377 



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

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

 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 universally admitted by physiologists that 

 urea is not formed in the kidneys but pre-exists 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 quantity of urea in the blood is kept down by the eliminating action of 

 the kidneys. Although a great part of the lymph is probably derived from 

 the blood, it is not probable that the blood gives to the lymph all of the urea 

 contained in the latter fluid ; and it must be assumed that a part of the urea 

 of the lymph passes from the tissues into the lymph-spaces and canals, 

 although a certain quantity may be produced by the lymphatic glands. 



As an outcome of many contradictory experiments and opinions on the 

 subject, it must now be considered as proved that the liver produces urea in 

 large quantity. If defibrinated blood be passed several times through a per- 

 fectly fresh liver, it gains urea. This observation, which was first made by 

 Cyon, in 1870, has been repeatedly confirmed. In certain cases of struct- 

 ural disease of the liver, the excretion of urea is much diminished, and this 

 substance may disappear from the urine. A number of cases illustrating 

 this fact has been reported by Brouardel. 



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

 ized excrementitious products which is fully justified by physiological facts 

 it is difficult to avoid the conclusion that this substance represents, to a great 

 extent, the disassimilation of the nitrogenized parts of the tissues, and neces- 

 sarily the physiological wear of the. muscular substance. The fact that urea 

 exists in very minute quantity in the muscles and some chemists state that 

 it is absent is probably due to its constant removal by the blood and lymph. 



Uric acid, creatine, creatinine, xanthine, hypoxanthine, leucine, tyrosine 

 and some other analogous substances are to be regarded as formations ante- 

 cedent to urea, urea being the final and perfect excrementitious product. 



It is convenient, in this connection, to consider the principal conditions 

 which influence the formation and elimination of area, or in order to com- 

 pare this substance with certain constituents of food, the elimination of 

 excrementitious nitrogen from the body. 



Influence of Ingesta upon the Composition of the Urine and upon the 

 Elimination 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 by physiologists as the 

 urina potus ; but when an excess of water has been taken for purposes of ex- 

 periment, the diet being carefully regulated, the absolute quantity of solid 

 matters excreted is considerably increased. This is particularly marked as 

 regards urea, but it is noticeable in the sulphates and phosphates, though not 



