138 DIGESTION. 



sarily arrested only during deglutition ; for the mere act of 

 suction has never any thing to do with the condition of the 

 thoracic walls. The mechanism of drinking from a vessel is 

 essentially the same. The vessel is inclined so that the lips 

 are kept covered with the liquid, and are closed around the 

 edge. By a gentle sucking action the liquid is then intro- 

 duced. This is the ordinary mechanism of drinking; but 

 sometimes the head is thrown back and the liquid is poured 

 into the mouth, as in " tossing off" the contents of a small 

 vessel, as a wine-glass. 1 



In drinking from a spoon, or in taking hot liquids, 

 or when it is desired to introduce but a small quantity at a 

 time, the liquid is drawn into the mouth with an act of 

 inspiration. In this process the lips are not covered by the 

 liquid, as in ordinary drinking, and it enters the mouth with 

 a more or less audible sound. 



Mastication. 



In the human subject, mechanical division of food in the 

 mouth is neither so completely and laboriously effected as in 

 the herbivora, particularly the ruminants, nor is the process 

 so rapid and imperfect as in the carnivora. In order that 

 digestion may take place in a perfectly natural manner, it is 

 necessary that the food, as it is received into the stomach, 

 should be so far comminuted and incorporated with the flu- 

 ids of the mouth as to be readily acted upon by the gastric 

 juice ; otherwise stomach-digestion is prolonged and difficult. 

 Non-observance of this physiological law is a frequent cause 

 of what is generally called dyspepsia. In animals that do 

 not masticate, as some which live exclusively on flesh, the 

 process of stomach-digestion is much more prolonged than 



1 Any one can easily convince himself that the ordinary mechanism of drink- 

 ing is by suction, by simply analyzing his own movements during this act. The 

 vessel is inclined so as simply to cover the lips, and not sufficiently to pour the 

 liquid into the mouth ; and the fact that a suction force is exerted by the mouth 

 becomes very evident when the attention is directed to it. 



