DIGESTION. 225 



the biting and grinding movement of the lower range of teeth against the 

 upper. The simultaneous movements of the tongue am^heeks assist partly 

 by crushing the softer portions of the food against the hard palate, gums, 

 etc., and thus 'supplementing the action of the teeth, and partly by re- 

 turning the morsels of food to the action of the teeth, again and again, 



_ FIG. 165. Diagram of the Alimentary Canal. The small intestine of man is from about 3 to 4 

 tunes as long as the large intestine. 



as they are squeezed out from between them, until they have been suffi- 

 ciently chewed. 



The simple up and down, or biting movements of the lower jaw, are 

 performed by the temporal, masseter, and internal pterygoid muscles, the 

 action of which in closing the jaws alternates with that of the digastric 

 and other muscles passing from the os hyoides to the lower jaw, which 

 open them. The grinding or side to side movements of the lower jaw 

 are performed mainly by the external pterygoid muscles, the muscle of 

 one side acting alternately with the other. When both external ptery- 

 VOL. I. 15. 



