CHAPTER III. 

 DIGESTION. 



Necessity and General Nature of Digestion: Digestion in the 



Mouth. 



The never-ceasing process of combustion that goes on in the 

 animal body, as well as the constant wear and tear of the tissues, 

 makes it necessary that the supply of fuel and of building mate- 

 rial be frequently renewed. For this purpose food is taken. This 

 food is composed of fats and carbohydrates, which are mainly 

 fuel materials, of inorganic salts and water, which are neces- 

 sary to repair the worn tissues and of proteins which are both 

 fuel and repair materials, and are therefore the most important 

 of the organic foodstuffs. The blood transports the foodstuffs 

 from the digestive canal to the tissues. In the digestive canal the 

 foodstuffs are digested by hydrolyzing enzymes (see p. 36), 

 which are furnished partly in the secretions of the digestive 

 glands and partly from the numerous micro-organisms that 

 swarm in the intestinal contents. The enzymes, as we have seen, 

 are very discriminative in their action, for not only is the enzyme 

 for protein without action on a fat or carbohydrate, but each of 

 the different stages in protein break-down requires its own pe- 

 culiar enzyme. It becomes necessary therefore that the enzymes 

 be mixed with the food in proper sequence, and to render this 

 possible the digestive canal is found to be divided into special 

 compartments, such as the mouth, the stomach, the small intes- 

 tines, etc., each provided with its own assortment of enzymes 

 and with some mechanism by which it can pass on the food to 

 the next stage when it has been sufficiently digested. 



Such correlation between the different stages of digestion 

 necessitates the existence, in the different levels of the gastro- 

 intestinal tract, of mechanisms which are specially developed to 



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