194 OBJECT OF DIGESTION. 



THE DIGESTIVE SYSTEM. 



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 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 by 

 the mucous lining of the digestive tube and passed into the 

 blood. 



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 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, if at all, be distinguished 

 from the original beef. 



If the food taken be all ready to build tissue, for example, 

 certain forms of sugar, liquid, soluble, and of the proper 

 chemical composition, it will not need to go through these 

 changes. 



In order to understand the process of digestion, let us 

 first turn our attention to the anatomy of the organs of diges- 

 tion. 



