CHAPTER V 



DIGESTION AND ABSORPTION OF THE NUTRIENTS 



I. General Survey of the Digestive System 



HO. Necessity of digestion. — In Chapter III we dis- 

 cussed the composition, uses, and the preparation of foods. 

 We learned in our study of plant biology (P. B., Ch. IV) 

 that certain of the food substances will readily pass through 

 the walls of plant cells, while others will not. Hence, the 

 latter, to become available for use in other cells, must be 

 changed to soluble form, and this change we called digestion. 

 We shall now discuss similar changes that take place in 

 foods within our bodies; for before the different food sub- 

 stances can reach the cells of the brain, the muscles, or the 

 bones where they are needed, they must be changed from 

 a solid or semifluid condition into liquids that can pass 

 through the walls of the cells that lie between the interior 

 of the food canal and the blood. These necessary changes 

 are accomplished within our bodies in the alimentary canal, 

 a complicated tube nearly thirty feet in length. 



111. Parts of the alimentary canal. — The alimentary 

 canal (Fig. 26), as in the other vertebrates studied, begins 

 with the mouth opening; it enlarges to form the mouth cavity, 

 and this in turn communicates behind with a somewhat 

 smaller throat cavity. Below the throat is the gullet, which 

 conducts the food into an enlarged pouch, the stomach. Most 

 of the lower half of the trunk is filled with the much coiled 



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