114 THE FEEDING OF ANIMALS 



substance that activates the mother substance of tryp- 

 sin; in the second place, they aid in splitting the pep- 

 tones into simpler bodies, and, lastly, they convert cer- 

 tain sugars into the final form (dextrose) in which they 

 are absorbed into the blood circulation. 



168. Summary of changes in digestion. — If we con- 

 sider the digestion of the food compounds by classes, the 

 following is a summary of the ways in which they are 

 acted upon: pepsin, trypsin, and erepsin secreted by the 

 stomach, pancreas, and intestinal glands act on the pro- 

 teins; ptyalin in the saliva, amylopsin from the pancreas, 

 and lactase, maltase, and sucrase in intestinal secretions 

 act on the carbohydrates, and the fats are acted on 

 mainly by the lipase of thfe pancreatic juice. 



The bacteria are not surely known to have necessary 

 specific digestive functions, unless it be their solvent action 

 on the cellulose. 



It should be observed that the above is a presenta- 

 tion of the general scheme of digestion, and takes no 

 account of differences between the various species of 

 farm animals of which our knowledge is incomplete. 



ABSORPTION OF FOOD 



From the time the food enters the stomach, during 

 nearly its entire course along the alimentary canal, there 

 is a constant production of soluble compounds, which 

 progressively disappear into other channels, so that when 

 the anus is reached oijly a portion of the original dry 

 matter is found in the residue. In some way, not wholly 

 explainable in all its details, the digested food has been 

 absorbed and received into vessels through which it is 

 distributed to the various parts of the body. 



