CHAPTER X. 

 THE DIGESTIVE SYSTEM. 



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

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

 the blood is renewed by the food. 



All food must be reduced to the liquid condition, if it 

 is not already liquid. 



The Digestive Tube. The chief organ in this work 

 of liquefying the food is the digestive tube, or " alimentary 

 canal," as it is called. As the food passes through the 

 digestive tube it is subjected to various mechanical and 

 chemical processes which liquefy it and bring it into such 

 a condition that it can be absorbed through the mucous 

 lining of the digestive tube and passed 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 all 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. In short, the muscle, 

 as such, must be thoroughly destroyed ; in the liquid pro- 

 duced by the digestion of the beef there is no trace what- 

 ever of the structure of the beef. But the blood, taking 

 this material, builds muscle which can hardly, if at all, be 

 distinguished from the original beef. 



