EXCRETION OF UEEA. 79 



of those materials which are in demand for the due mainten- 

 ance of the constitution of the blood and of the solids, and 

 may not be at all an absolute necessity. 



The supposed analogy above referred to between the supply 

 of nourishment from without, and its supply from the disin- 

 tegration of the living solids, is neither improved nor impaired, 

 whichever necessity is assumed that is, whether the necessity 

 for organic products as the food of animals be contingent or 

 absolute. 



Excretion of Urea. The quantity of urea excreted in a 

 given time along with the secretion of the kidneys, is the prin- 

 cipal criterion of the amount of the living animal solids which 

 in the same period has undergone complete disintegration. 

 Nevertheless, the exact estimate of the quantity of urea 

 thrown off by the kidneys in a given time, does not indicate 

 the absolute amount of disintegration in the solids within 

 that period, because there is reason to think that some part 

 of the nitrogenous principles of the food may undergo de- 

 composition without ever having become a part of the living 

 solids. But the distinction between the blood and the solids 

 may be put aside in the mean time ; for it is certain that 

 whatever is taken into the blood from the digestive organs 

 acting normally, is in a certain measure assimilated to the 

 living system for the period under consideration. Thus it is 

 correct to say that the quantity of urea thrown off by the 

 kidneys in a given time indicates the amount of disintegration 

 of the nitrogenous organic principles in the blood and the 

 living solids during that period. Now, it seems certain that 

 this disintegration is uniformly a complete or chemical death 

 that is, that the products thereof are no longer plastic sub- 

 stances, like fibrine, albumen, and caseine, but either water, 

 carbonic acid, ammonia, or substances capable of being changed 

 into urea. It is commonly assumed that plastic proximate 



