198 EXCEETION. 



these parts free from its presence during normal conditions. 

 In some late experiments by Meissner, in which the observa- 

 tions of Prevost and Dumas on the accumulation of urea in 

 the blood of nephrotomized 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. 1 



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 further information on this subject, until phys- 

 iological chemists are able to follow out more closely the 

 exact atomic changes that intervene between the functional 

 operation of organized parts and the change of their sub- 

 stance 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 transformation 

 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 nitroge- 

 nized elements of the tissues are transformed into excremen- 

 titious matter, 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 same 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, 

 called catalytic, are necessary, even before organic matters 

 can be taken into the blood by absorption. There is 110 evi- 



1 MEISSNER, Bericht uber Versuche der Urdmie betreffend. Zeitschrift fur 

 rationelle Medicin, Leipzig u. Heidelberg, 1866, Dritte Reihe, Bd. xxvi., S. 232. 



