CHAPTER XXVIII 

 MOVEMENTS OF THE ALIMENTARY CANAL 



Mastication serves to break the food into fine particles and by 

 mixing it intimately with saliva to reduce it to a semi-liquid state. 

 It consists primarily of cutting and grinding the food between 

 the upper and lower teeth, a process which is performed by move- 

 ments of the lower jaw. The articulation of the lower jaw with 

 the skull and its equipment of muscles permit both up and down 

 cutting movements and sidewise grinding movements. The ac- 

 tual chewing process involves, in addition, motions of the lips, 

 cheeks, and tongue in holding the food in position for the teeth 

 to act upon it. The whole process is carried on by skeletal muscles 

 and is, therefore, under the control of the will. 



It ought not to be necessary to emphasize the importance of 

 thorough mastication of the food. Salivary digestion depends 

 wholly, of course, upon the bringing of saliva into contact with 

 the starch particles, and it can easily be shown experimentally 

 that gastric digestion is several times more rapid when the ma- 

 terial exposed to the action of gastric juice is finely divided than 

 when it is in large masses. 



The interesting fact has recently been brought out that the 

 more the process of masticating each mouthful is prolonged the 

 less food is required to satisfy the appetite. Since many people 

 doubtless eat too much there is here a suggestion as to a way of 

 reducing the amount taken without serious sacrifice of appetite. 



Hygiene of the Mouth. The mouth cavity is almost never free 

 from micro-organisms. The alkaline reaction of saliva is favor- 

 able to their growth, and they scarcely ever lack for food. The 

 irregularly shaped teeth, packed closely along the jaw, have be- 

 tween them spaces where material that is being chewed readily 

 lodges, and where it stays unless special care is taken to remove 

 it. Such lodged food-masses shortly harbor flourishing colonies 

 of bacteria. These in connection with their growth and multi- 

 plication produce substances which attack the protective enamel 



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