DIGESTION AND RESORPTION 1 09 



stands in the way of the supposition that the digested fats are 

 completely split up into glycerol and fatty acids in the process 

 of digestion and synthesized again in the epithelial cells, although, 

 on the other hand, of course, it does not prove that such is the 

 case. 



153. The feces. As the contents of the digestive tract move 

 forward through the small and large intestines they become 

 progressively more and more impoverished in digestible material 

 and also, in the lower portion of the large intestine, are deprived 

 of part of their water, so that there accumulates in the rectum 

 a more or less solid residue which is voided at intervals as the 

 feces. 



The feces are to be regarded as both an excretory product 

 (198) and a feed residue. 



154. The feces as an excretory product. The fact that the 

 feces are an excretory product is most obvious in the carnivora, 

 whose normal feed consists of substances almost wholly diges- 

 tible, but it is evident also in man. On a pure meat diet, for 

 example, feces continue to be produced in which undigested 

 feed residues are either absent entirely or present in minimal 

 amounts only. Even a fasting animal continues to produce 

 feces, while an empty loop of the intestine, separated from the 

 remainder of the digestive tract, soon fills up with fecal-like 

 material. 



The excretory ingredients of the feces include unresorbed 

 digestive juices and their decomposition products, intestinal 

 mucus, worn-out epithelial cells and cell fragments, leucocytes 

 and excretions of the intestinal mucosa. Especially notable 

 among the latter are salts of calcium and of iron and in herbivora 

 the phosphates of calcium and magnesium. The feces also 

 include a not inconsiderable proportion of intestinal micro- 

 organisms. 



155. The feces as a feed residue. The ordinary mixed 

 diet of man, and to a much more marked degree the ordinary 

 feed of herbivorous animals, contains relatively considerable 

 amounts of materials which are either indigestible or which for 

 one reason or another escape digestion and therefore reappear 

 in the feces. Among these, some, like lignin, cutin, the waxes, 

 chlorophyl and other non-fatty ingredients of the " ether 

 extract," and the insoluble ash ingredients, may be regarded 



