A HISTORY OF METABOLISM 51 



"It is a fundamental fact, so far without a contradictory experiment, 

 that the sulphur- and nitrogen-containing constituents of plants have the 

 same chemical composition as the principal components of the blood. We 

 know of no nitrogen-containing material of a composition different from 

 fibrin, albumin and casein which is able to sustain life. 



"The animal organism is certainly able to construct its membranes and 

 cells, nerves and brain, the organic materials of ribs, cartilages and bones 

 out of. the. constituents of its own blood, but these constituents must be 

 already constructed in proper form or the production of blood and life 

 itself is brought to an end. 



"Looking at the matter from this standpoint, it is easily understood 

 why gelatin is not a builder of blood or a supporter of life, for its com- 

 position is different from that of the fibrin and albumin of the blood." 



Concerning the ultimate disposal of the products of metabolism, Liebig 

 writes : 



The kidneys, skin and lungs cannot be the only ways through which products 

 of the metabolism are eliminated from the body. The intestinal canal functions 

 also as an organ of excretion and its relation to the respiration process must 

 not be misunderstood. 



If the quantity of oxygen absorbed in a given unit of time is that which 

 is exactly necessary to convert the products of metabolism present during the 

 same period into carbonic acid, urea and water, then the intestinal canal will 

 contain only indigestible substances. 



... In general it must be assumed that all of the nitrogen- and sulphur- 

 containing constituents of the food of man are completely digestible, are brought 

 into solution and absorbed into the circulating blood, for a property belonging 

 to some parts must belong to all. In such cases it is undoubtedly true that the 

 discovery of nitrogen-containing materials in the feces signifies that they can 

 only be the products of the metabolism of the intestinal canal itself or products 

 which have escaped normal metabolism and have been excreted from the blood 

 by the intestinal wall. 



Just before the publication of Liebig's great work Dumas, in glowing 

 language, pictured similar interpretations without giving Liebig credit 

 for the ideas. He utilized a formula similar to that given by Liebig 

 without stating its derivation. Thus, in 1842, Dumas and Cahours pre- 

 sented the following penetrating conception: 



The food of an ordinary maintenance ration contains 16 to 21 gm. nitrogen. 

 This nitrogen is almost entirely recoverable in the urine in the form of urea. 

 Ignoring the intermediary phases, protein breaks up as follows: 



C 48 H 37 N 12 15 + 100 O = C a H 12 N 12 6 urea 



^42 ^84 carbon dioxid 



H oc O , water 



C 48 H 37 N 12 115 



The only object in giving this formula is to enable one to calculate the heat of 

 combustion of protein. Allowing for the daily production of urea from protein, 



