152 DIGESTION. 



food is taken into the mouth, the tongue prevents its escape 

 from between the teeth ; and, by its constant movements, 

 rolls the alimentary bolus over and over, and passes it at 

 times from one side to the other, so that it may undergo 

 thorough trituration. Aside from functions of the tongue as 

 an organ of taste, its surface is endowed with peculiar sensi- 

 bility as regards the consistence, size, and form of different 

 articles ; 1 and this property is undoubtedly important in de- 

 termining when mastication is completed; although the 

 thoroughness with which mastication is accomplished is very 

 much influenced by habit. 



Tonic contraction of the orbicularis oris is necessary to 

 keep the fluids in the mouth during repose ; and this muscle 

 is sometimes brought into action when the mouth is very 

 full, to assist in keeping the food between the teeth. This 

 latter function, however, is mainly performed by the buc- 

 cinator j the action of which is to press the food between the 

 teeth and keep it in place during mastication ; assisting, from 

 time to time, in turning the alimentary bolus so as to subject 

 new portions to trituration. 



The process of mastication is regulated to a very great 

 extent by the exquisite sensibility of the teeth to the impres- 

 sions of hard and soft substances. It is only necessary to call 

 attention to the ease and certainty with which we recognize the 

 presence and the consistence, even of the smallest substance 

 between the teeth, in order to appreciate the advantages of 

 this tactile sense in mastication. It is in this way, mainly, 

 that we are notified when the process of mastication is com- 

 pleted ; and it is this sense which admonishes us instantly of 

 the presence of bodies too hard for mastication, which, if al- 

 lowed to remain in the mouth, might seriously injure the 

 teeth. Attention was called to these interesting facts by the 

 late Dr. Graves, of Dublin, who says : " In truth, the teeth 



1 Every one is aware how readily a hair, or any minute substance, which 

 would hardly be felt even by the ends of the fingers, is detected in the mouth ; 

 and how annoying the sensation is until the offending substance is removed. 



