METABOLISM. 47 
Merasotic Propucts in Freces.—As already stated in Chapter 
I, the feces contain, in addition to undigested residues of the food, 
certain materials derived from the body of the animal. This fact 
was early recognized as true of both carnivora* and herbivora.t 
Of more recent investigations may be noted especially those of 
Miller, { Rieder,§ and Tsuboi || on carnivora, those of Prausnitz§ and 
his associates on man, and those of Kellner,** Stutzer,}t Pfeiffer, {t 
and Jordan §§ on herbivora. 
These “metabolic products” appear to consist of unresorbed 
or altered residues of the digestive fluids and of mucus and other 
materials excreted or otherwise thrown off by the walls of the intes- 
tines. Their production goes on even when the digestive tract is 
void of food, producing the so-called fasting feces which constitute 
a true excretory product. The consumption of highly digestible 
food—e.g., lean meat—does not seem to materially increase their 
amount, but when food containing indigestible matter is eaten it is 
believed that they increase in quantity. 
It is presumed that these substances are largely nitrogenous in 
character, and it is known at any rate that not inconsiderable 
amounts of nitrogen may leave the body by this channel. In other 
words, these nitrogenous substances, derived from the proteids 
of the body, instead of undergoing complete conversion into the 
ordinary crystalline products have their katabolism interrupted 
as it were at an intermediate stage. 
Many attempts have been made to determine the amount of 
these metabolic products, or of their nitrogen, in the feces, but 
without much success, and it may fairly be said that at present 
we have no method which can be depended upon to distinguish 
sharply between the nitrogen of undigested-food residues and that 
of metabolic products. 
* Bischoff and Voit, Die Ernahrung des Fleischfressers, p. 291. 
+ Henneberg, Beitriige, etc., 1864, p. 7. 
tZeit. f. Biol., 20, 327. 
§ Ibid., 20, 378. 
|| Zbid., 35, 68. 
q Ibid., 35, 287; 39, 277; 42, 377. 
** Landw. Vers. Stat., 24, 434; Bied. Centralbl., 9, 763. 
++ Zeit. physiol. Chem., 9, 211. 
tt Jour. f. Landw., 31, 221; 33, 149; Zeit. physiol. Chem., 10, 561. 
§§ Maine Expt. Station Rep., 1888, p. 196. 
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