DIGESTION IN THE MOUTH 127 



the plant proteins, for example, are valuable foods, although 

 they differ from our body proteins in the relative amounts and 

 in the arrangement of the different amino acids they contain. 

 If the varied substances in our food all could get into the blood 

 stream, and into the cells, the problems of the cells would be 

 greatly complicated, for they would be obliged to deal with a 

 great variety of proteins, carbohydrates and fats. Nature has 

 solved this problem by arranging for the breaking up of the 

 various foodstuffs into their simple building stones in the di- 

 gestive tract. It is in the form of these simple building stones, a 

 limited uniform list of which is obtained in general from all 

 the food substances, that the food materials reach the cells. An 

 exception is the case of the fats, the building stones of which 

 are rebuilt for the most part into neutral fat before entering the 

 blood. The primary function of digestion thus is to break up 

 the diverse constituents of the foods into a fairly uniform mix- 

 ture of simple, diffusible compounds which enter the blood and 

 are presented to the cells for their use. The chief factors in this 

 breaking down process are the enzymes present in the digestive 

 juices. The enzymes and the conditions affecting their activities 

 already have been discussed. (See chapter on carbohydrates.) 



Preparation of Food. CHEWING. Proper cooking is of great 

 importance in preparing food, as was pointed out in the chap- 

 ter on foods. After proper cooking comes proper chewing. The 

 food should be thoroughly chewed to break it up into small 

 fragments so that the digestive enzymes may have better access 

 to its constituents. 



SALIVA. Secretion, Amount. In the process of chewing, the 

 food not only is broken up, but it becomes mixed with the first 

 of the digestive juices, the saliva. The saliva is secreted by 

 three sets of glands, the parotid, submaxillary and sublingual, 

 and also by small glands in the mucous membrane. The ma- 

 terials secreted by the different glands vary, as do also the 

 nature of the stimuli which will cause a secretion. Each set of 

 glands is controlled by nerves from the brain direct, and from 

 the sympathetic system. A flow of saliva may be produced by 



