40 A COURSE ON ZOOLOGY. 



Solid substances would be absolutely tasteless were they 

 not moistened and partially dissolved by the saliva. The 

 odor or the sight of food will cause a flow of saliva, the 

 expression " to make the mouth water" being universally 

 understood. 



At the moment of deglutition the tongue gathers the 

 food into a sort of ball and carries it into the pharynx, 

 whose walls then contract and force the bolus into the 

 oesophagus. 



As has already been seen, the pharynx has several 

 openings : the entrance of little portions of food into the 

 nasal passages would not have serious consequences ; but 

 the smallest particle entering the respiratory tube at 

 once causes a convulsive cough that continues until the 

 foreign body is expelled. Happily this accident is rare, 

 because the opening of the air-passage is protected in a 

 very thorough manner by a membranous trap-door, called 

 the epiglottis, which closes at the instant of swallowing. 



The progress of the food in the oesophagus is caused 

 by the contractions of the muscular coat ; the weight of 

 the food has nothing to do with it. The alimentary mass 

 hardly enters the stomach before the fibres of the mus- 

 cular tissue of that organ take up the contractions of 

 the oesophagus, and begin to knead the mass, carrying it 

 at the same time from one end to the other. Towards 

 the end of this work the movements tend to direct the 

 food to the pyloric opening, but at the beginning the 

 direction is just the contrary, that is, towards the large 

 curvature and the cardiac orifice. 



In the stomach the food comes in contact with the 

 gastric juice. This is a very acid liquid, secreted by the 

 mucous membrane of the stomach, and owing the greater 

 part of its digestive power to a substance whose action is 



