

DIGESTION 283 



work of the stomach, but it is normally begun in the mouth, 

 and it is of consequence that this preliminary stage should 

 be properly performed. 



I. The Mechanical Phenomena of Digestion. 



Mastication. It is among the mammalia that regular 

 mastication of the food first makes its appearance as an 

 important aid to digestion. The amphibian bolts its fly, the 

 bird its grain, and the fish its brother, without the ceremony 

 of chewing. In ruminating mammals we see mastication 

 carried to its highest point ; the teeth work all day long, 

 and most of them are specially adapted for grinding the 

 food. The carnivora spend but a short time in mastication ; 

 their teeth are in general adapted rather for tearing and 

 cutting than for grinding. Where the diet is partly animal 

 and partly vegetable, as in man, the teeth are fitted for all 

 kinds of work ; and the process of mastication is in general 

 neither so long as in the purely vegetable feeders, nor so 

 short as in the carnivora. 



In man there are two sets of teeth : the temporary or milk 

 teeth, and the permanent teeth. The milk-teeth are twenty 

 in number, and consist on each side of four incisors or 

 cutting-teeth, two canines or tearing-teeth, and four molars 

 or grinding-teeth. The central incisors emerge at the 

 seventh month from birth, the other incisors at the ninth 

 month, the canines at the eighteenth, and the molars at the 

 twelfth and twenty-fourth month respectively. Each tooth 

 in the lower jaw appears a little before the corresponding 

 one in the upper jaw. Each of the milk-teeth is in course 

 of time replaced by a permanent tooth, and in addition the 

 vacant portion of the gums behind the milk set is now filled 

 up Dy twelve teeth, six on each side, three above and three 

 below. These twelve are the permanent molars ; they raise 

 the number of the permanent teeth to thirty-two. The 

 permanent teeth which occupy the position of the milk 

 molars now receive the name of premolars. The first tooth 

 of the permanent set (the first true molar) appears at the 

 age of 6^ years ; the last molar, or wisdom tooth, does not 

 emerge till the seventeenth to the twenty-fifth year. 



