THE TEETH AND MASTICATION 



molars which thoroughly crush and grind it. Now this grinding 

 could not be carried on effectually if the lower jaw were capable 

 of a vertical motion only. But if we observe the motion of the 

 jaw during the grinding of the food, we shall find that it moves a 

 little to the right and left, and also forward and backward. The 

 broad and rough surfaces of the molar teeth are consequently 

 caused to slide over each other, thus thoroughly grinding the food 

 which may be between them. The carnivorous animals have not 

 broad molars like ours. They bite and tear the flesh on which 

 they feed, generally giving the jaw an up and down motion only. 

 The herbivorous animals, on the other hand, have very large and 

 perfect molars, and they give to the lower jaw a much greater 

 variety of motion than we do This may be readily observed in 

 the cow, sheep, goat, and horse. 



During mastication the food must be continually moved about 

 so that every portion of it may be brought between the molar 

 teeth. This is effected by the muscles of the tongue, cheeks, 

 and lips. 



Fig. 90. Enamel Fibres. Magnified 350 diameters. 



A, the sides, and B, the ends of the fibres. 



The action of the muscles concerned in mastication would 

 generally be regarded as a voluntary action ; but it is often partly 

 or wholly an involuntary act, since it not unfrequently continues 

 for some time while we give no thought whatever to it, as for 

 example, when we are eating and engaged in reading at the same 

 time. In this case the reading may absorb the attention to such 

 an extent that we become unconscious of the continued action of 

 the lower jaw. 



