CHAPTER XV. 



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." 



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. 



Changes the food undergoes in the mouth ; mastication and 

 deglutition. When solid food is taken into the mouth it is cut 

 and ground by the teeth, being pushed between them again and 

 again by the muscular contractions of the cheeks and the move- 

 ments of the tongue until the whole is thoroughly crushed and 

 ground down. During this process of mastication the salivary 

 glands are excited to very active secretion, the saliva is poured 

 in large quantities into the mouth, and mixing with the food 

 moistens it and reduces it to a soft pulpy condition. A certain 

 amount of air caught in the bubbles of the saliva also becomes 

 entangled in the food. 



The food thus softened and moistened is collected from every 

 part of the mouth by the movements of the tongue, brought 

 together upon its upper surface, and then pressed backwards 



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