CHAPTER XVI. 



ALIMENTATION CONCLUDED: DIGESTION; CHANGES THE FOOD 

 UNDERGOES IN THE MOUTH, STOMACH, SMALL AND LARGE 

 INTESTINE; SUMMARY OF DIGESTION; ABSORPTION. 



Digestion. — Digestion is the process by means of which the 

 food we take into our mouths is transformed into a condition 

 of solution or emulsion suitable for absorption into the blood. 

 This transformation is rapid or gradual according to the nature 

 of the food-stuffs the digestive solvents are called upon to dis- 

 solve. We all know practically, for instance, that it takes much 

 longer to digest a piece of beefsteak than a cup of bouillon, and 

 that when we wish to save the digestive powers as much as pos- 

 sible we place a person upon "liquid diet." 



The digestion of the various food-stuffs depends entirely on 

 the action of a class of substances known as enzymes or fer- 

 ments. Although the exact composition and method of action 

 of enzymes is not understood, it may be said that an enzyme is 

 a substance a small amount of which, under certain conditions, 

 can by its presence convert certain other substances into still 

 other substances without itself being destroyed, or weakened 

 in any way. Thus, a small amount of the enzyme, pepsin, can 

 in an acid solution convert proteids into another class of sub- 

 stances known as peptones, without diminution in the quantity 

 or strength of the pepsin used. The enzymes are usually the 

 products of living organisms, and are not found in inorganic 

 matter. 



Remembering that the three solid food-stuffs are proteids, 

 fats, and carbohydrates, we will proceed to describe how each 

 of these is transformed into a soluble condition in its course 

 through the alimentary canal. 



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