142 SANGUIFICATION. LiECT. VII. 



before and after its passage through the kidneys, the liver, 

 and the various tissues ? These are among the numerous 

 questions which should be resolved by very accurate expe- 

 riments, and by researches agreeing in their results, before 

 prosecuting our investigations on the phenomena of nutri- 

 tion and secretion. 



Source of Evolved Products. As we have seen, the ali- 

 ments pass into the blood after having undergone various 

 modifications by the act of digestion. Among these, many 

 are identical with the organic elements of the animal tis- 

 sues : this is the case with the neutral azotised substances. 

 So, also, with the fatty matters of the aliments, which are 

 found in the adipose tissues scarcely, if at all, altered. It 

 would be both unreasonable and absurd to assume that the 

 urea, carbonic acid, and water, which are the definite pro- 

 ducts of transformations going on during nutrition, are de- 

 rived from those organic elements of the blood, which were 

 introduced by the aliments. We must suppose that these 

 products are the results of transformations which the mate- 

 rials of our tissues have undergone, and which are replaced 

 by new organic elements supplied by the food. And, in 

 fact, the production of urea takes place in animals nou- 

 rished for a long time with sugar, starch, and gum, just as 

 it had done before this kind of food was used. The same 

 thing is observed in animals that have died from inanition. 



For the sake of accuracy I shall quote, on the subject of 

 these transformations, some illustrations taken from Liebig's 

 "Animal Chemistry , or Organic Chemistry in its Applica- 

 tions to Physiology and Pathology." 



A serpent, kept for some time without food, and then fed 

 on a goat, a rabbit, or a bird, expelled from the body, ap- 

 parently unchanged, the hair, hoofs, horns, feathers, or 

 bones of the devoured animal ; exhaled carbonic acid and 

 water ; and evacuated by the urinary passages urate of am- 

 monia. 



