120 



THE DIGESTIVE SYSTEM 



to a great extent in some parts, but is just as truly a tube as 

 the simple straight alimentary canal in the worm's body. The 

 alimentary canal consists of the mouth cavit}^ with its accessory 

 organs : the teeth, tongue, glands, throat or pharynx, esophagus, 

 stomach, the small intestine, and large intestine. 



Mouth. The mouth cavity, nearly oval in shape, is lined with a 

 soft membrane which is kept moist by saliva secreted from glands. 

 The palate or roof of the mouth consists of a hard portion in front, 



formed bv a bone covered bv 

 mucous membrane, and of a soft 

 portion farther back containing 

 no bone. The hard palate forms 

 the partition between the mouth 

 and nose ; the soft palate arches 

 backward, and from the middle of 

 its lower border there hangs a 

 pointed portion of the soft palate 

 called the uvula (little grape). 



The tongue is a muscular organ, 

 and has on its upper surface many 

 small projections called pajnllae. 

 These papillae contain certain nerve cells that make it possible 

 for us to taste sweet, bitter, sour, and salt. When these special- 

 ized structures (taste-buds) are stimulated by the food, the secre- 

 tion of saliva is increased and gastric fluid in the stomach starts 

 to flow. The tongue helps mastication by pushing and rubbing 

 portions of the food against the roof of the mouth, and also guides 

 the food into position for chewing. The tongue assists in swallow- 

 ing by manipulating the food into small masses and pushing them 

 back to the throat cavity. 



Due to the complex movement of the lower jaw, up and down, 

 forward and back, and from side to side, the teeth shave, slice, 

 crush, and grind the food. The broad surfaces of the molars 



petroti^ 



There are three pairs of salivary glands 

 which empty into the mouth. The teeth 

 may become coated with tartar which is par- 

 tially a deposit from the saliva. 



