CHAPTER XV. 

 THE DIGESTIVE SYSTEM. 



DIGESTION IN THE MOUTH. 



The Object of Food. The tissues are worn out by their 

 oxidation. They are built up again by the blood, and the 

 blood is renewed by the food. 



The Digestive Tube. All food must be reduced to the 

 liquid condition, if it is not already liquid. The chief organ 

 in this work of liquefying the food is the digestive ttibe, or 

 "alimentary canal." As the food passes through the 

 digestive tube it is ground and liquids are poured upon it. 

 Thus it is reduced to a liquid that can be absorbed and 

 taken into the blood. 



The Work of the Digestive Tube. To take a special 

 instance, a muscle is in part worn out by the oxidation 

 during its activity ; to replace the loss suppose we take a 

 piece of steak. We cannot substitute this directly in the 

 place of the worn-out tissue. In digesting the steak we 

 must tear it to pieces, and reduce it to a liquid form by the 

 action of the teeth and by the various liquids from the 

 glands along the digestive tube. The beefsteak, as such, 

 must be thoroughly destroyed ; in the liquid produced by 

 the digestion of the beef there is no trace whatever of the 

 structure of the beef. But the blood, taking this material, 

 builds muscle which can hardly be distinguished from the 

 original beef. 



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