PROTEIN METABOLISM 83 



verted into molecules of a higher order, fit to serve in 

 building up and maintaining alive the body of the 

 animal." "How admirably simple after we have 

 acquired a knowledge of this relation between plants 

 and animals, appears to us the process of formation 

 of the animal body, the origin of its blood and organs ! 

 The vegetable substances, which serve for the produc- 

 tion of blood, contain already the chief constituent of 

 blood ready formed, with all its elements." "The true 

 starting point for all the tissues is, consequently, 

 albumen; all nitrogenized articles of food, whether 

 derived from animal or from the vegetable kingdom, 

 are converted into albumen before they can take part 

 in the process of nutrition." 



According to Liebig digestion is merely a process 

 whereby food becomes changed to a soluble condition 

 capable of absorption without transformation of its 

 identity. This soluble albumen is built up into organ- 

 ized tissue previous to its degradation (an idea later 

 adopted by Pfluger). Thus we read: "There can be 

 no greater contradiction, with regard to the nutritive 

 process, than to suppose that the nitrogen of the food 

 can pass into the urine as urea, without having pre- 

 viously become part of an organized tissue; for albu- 

 men, the only constituent of blood which, from its 

 amount, ought to be taken into consideration, suffers 

 not the slightest change in passing through the liver or 

 kidneys ; we find it in every part of the body with the 

 same appearance and the same properties." 



Liebig divided all foods into two groups, the nitro- 



