332 FOOD AND DIGESTION 



compounds some excretory or waste compounds such as 

 cholesterin and lecithin. They are supposed to be decom- 

 position products of the nerve tissue and are eliminated 

 from the blood stream through the liver. The bile has no 

 direct solvent action on any of the food constituents, but 

 its action decidedly increases the power of the pancreatic 

 enzymes and it serves as the principal solvent for the fatty 

 acids in the formation of soaps. 



In addition to the normal enzvme secretions of the bodv 

 which have a solvent action on food constituents, there are 

 ordinarily present in animals large numbers of bacteria, 

 principally in the intestines. Their presence is not necessary 

 for the decomposition and solution of food constituents, 

 but it is probable that their fermentative action is in some 

 cases of benefit in digesting food. 



Food material which has been acted upon by the various 

 chemical agents is rendered soluble and ready for ab- 

 sorption. Not all of it, however, can be dissolved. Par- 

 ticularly do crude fiber and cellulose remain unattached 

 except in the case of some of the domestic animals, more 

 particularly the ruminants, where these food constituents 

 are partly digested, due probably to the activity of bacteria. 

 The undigested portion of the food is discharged into the 

 large intestine for final elimination. 



238. Absorption of Food Constituents. — The absorption of 

 the various constituents of the food is limited almost wholly 

 to the small intestine, little if any being absorbed from the 

 mouth or stomach into the circulatory system. The interior 

 of the small intestine is covered with minute conical projec- 

 tions called villi (Fig. 81), through which all the dissolved 

 material is absorbed. They serve the same purpose in the 

 animal that the root hairs do in the plant, but they differ 

 in that they are not each one a single cell but a large number 

 of cells containing blood capillaries and lymph vessels, which 

 carry the absorbed material into the general circulatory 

 system. 



The carbohydrates in the form of dextrose, levulose, and 

 possibly galactose are absorbed through the outer cells of the 

 villi into the capillaries, which finally unite into the portal 



