COMPOSITION OF THE URINE. 415 



that it always preexists in the blood and is not formed in the kidneys. One of these is in 

 the disassirailation of the nitrogenized constituents of the tissues, and the other, in a trans- 

 formation in the blood of an excess of the nitrogenized elements of food. Urea, as we 

 have already seen, exists in considerable quantity in the lymph and chyle, and it is found, 

 also, in small proportion, in the blood. It has lately been detected in still smaller quan- 

 tity in the muscular tissue ; but chemists have thus far been unable to extract it from any 

 other of the solid tissues, under normal conditions, except the substance of the liver. The 

 fact that it exists in considerable quantity in the liver has led to the supposition that this 

 is the organ chiefly concerned in its production. With the small amount of positive infor- 

 mation that we have upon this point, the view that the liver produces urea, while the kid- 

 neys are the organs chiefly concerned in its elimination, must be regarded as purely hy- 

 pothetical. But, if it be true that urea is the result of the physiological wear of the nitro- 

 genized elements of the body, the liver would probably produce its share, in the ordinary 

 process of disassimilation. The fact that urea has not yet been detected in normal mus- 

 cular tissue is by no means a conclusive argument against its formation in this situation. 

 We have lately shown that, although the liver is constantly producing sugar, none can be 

 detected in its substance, for the reason that it is washed out as fast as it is formed, by 

 the current of blood. In the case of the muscles, it is by no means improbable that the 

 lymph, and perhaps the blood, wash out the urea constantly and keep these parts 

 free from its presence during normal conditions. In some late experiments by Meissner, - 

 in which the observations of Prevost and Dumas on the accumulation of urea in the 

 blood of nephrotornized animals were confirmed, urea was found in dogs and rabbits, after 

 removal of the kidneys, not only in the liver but in the muscles and brain. 



Although our experimental knowledge does not warrant the unreserved conclusion 

 that urea is produced primarily in the nitrogenized parts of the organism, particularly 

 the muscular tissue, this view is exceedingly probable ; and we must wait for farther 

 information on this subject, until physiological chemists are able to follow out more 

 closely the exact atomic changes that intervene between the functional operation of or- 

 ganized parts and the change of their substance into excrementitious matters. 



When we come to consider the influence of food upon the composition of the urine, it 

 will be seen that an excess of nitrogenized matter taken into the alimentary canal causes 

 a proportionate increase in the quantity of urea discharged. This fact has led to the 

 supposition that a part of the urea contained in the urine is the result of a direct trans- 

 formation in the blood of the nitrogenized alimentary principles. This view must be 

 regarded as purely hypothetical. We do not even know the nature of the process by 

 which the nitrogenized elements of the tissues are transformed into excrementitious mat- 

 ter, and we are still more ignorant of the essential characters of nutrition proper. When 

 more nitrogenized food is taken than is absolutely necessary, it is evident that the excess 

 must be discharged from the system. This is never discharged in the form in which 

 it enters, like an excess of chloride of sodium or other inorganic matter, but it is well 

 known that a series of complicated changes are necessary, even before organic matters 

 can be taken into the blood by absorption. There is no evidence of the direct transfor- 

 mation of these principles into urea before they have become part of the organized struct- 

 ures, except in a comparison of the proportions of nitrogen ingested and discharged ; and 

 this proves nothing with regard to the nature of the intermediate processes. At the 

 present time, the most rational 'supposition is, that the nitrogenized elements of food 

 nourish the corresponding constituents of the body, which are constantly undergoing 

 conversion into excrementitious matters. Observations which have appeared to demon- 

 strate the formation of urea directly from albuminoid substances have not been confirmed. 



There are certain arguments, based upon comparisons of the atomic constitution of 

 urea with the elements of uric acid, creatine, and creatinine, in favor of the view that 

 urea is the product of a higher degree of oxidation of the other excrementitious matters 

 above mentioned. It has been found, also, that urea may be formed artificially from uric 



