CHAPTER XXII 

 DIGESTION AND ABSORPTION 



Digestion. — ^We have learned that it is the function of 

 food to nourish the cells of the body. The cells can, how- 

 ever, absorb liquid food only, so the food requires further 

 treatment than mere cooking to render it of use to the cells. 



It is the function of the alimentary canal and its assisting 

 or digestive glands to dissolve thoroughly the food, to separate 

 the nutritious from the non-nutritious, and to treat it with 

 such reagents that it can be taken up by the cells. This is 

 digestion. 



The alimentary canal. — Digestion, as has been said, 

 takes place in the alimentary canal. Our study of the ali- 

 mentary canal of the frog, together with the use of the accom- 

 panying diagram (fig. 156), will aid us in understanding this 

 structure in the human body. It is a continuous tube from 

 the mouth to the anus. Part of the tube is twisted and 

 doubled back upon itself, as shown in the figure. Its 

 entire length, were it stretched out, is about thirty feet. 

 Its diameter varies at different points, being widest at the 

 stomach. There are outgrowths at different points called 

 glands that furnish the digestive juices for dissolving and 

 preparing the food. These glands pour their secretions 

 into the various parts of the aUmentary canal where they 

 become thoroughly mixed with the food. 



The alimentary canal is lined with a soft mucous membrane 

 like that within the mouth. Its secreting cells furnish 

 mucous for keeping the inner surface moist. 



The mouth or buccal cavity. — If we close the lips and 

 feel around with the tongue we find the mouth bounded on 



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