CHAPTER XIX 

 DIGESTIVE SYSTEM IN MAN 



115. The human food tube. We receive our food and drink 

 into our mouths. The mouth is the beginning of a long tube 

 inside of which all the digestion takes place. This tube is 

 called the food tube or the alimentary canal or the digestive 

 tract. This tract consists of several fairly distinct regions ; 

 in an adult it is about ten or eleven yards long. It manages to 

 keep inside the much shorter body by being coiled and twisted 

 in parts (see Fig. 28, y, k). 



116. Mouth digestion. Since what we eat is important to us 

 on account of its proteins, fats, and carbohydrates, we may con- 

 sider the digestive processes in relation to these substances. 



After the food enters the mouth, it is crushed and ground 

 by the teeth. During the process of chewing, however, some- 

 thing else happens. The taste of the food, the movement of 

 the jaws, and the rubbing of the food against the inside of the 

 mouth stimulate the action of the saliva glands (see Fig. 29) 

 so that a quantity of saliva is poured into the mouth and this 

 becomes mixed with the food. The more the food is chewed, 

 the smaller are the particles into which it is broken, and the 

 more thoroughly is the saliva mixed with the particles. As we 

 have already learned (p. 78), the action of the saliva upon the 

 starch in the food changes it into sugar. The other materials 

 in the food are probably not changed, except that salts and 

 sugars are dissolved by the water, of which the saliva contains 

 over 99 per cent. As the amount of ferment is very small, 

 the effectiveness of saliva as a digester of starch depends upon 

 the ferment's reaching every particle of starch, and upon its 

 having sufficient time to bring about the change. 



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