300 A MANUAL OF PHYSIOLOGY 



made protein. Mechanical division of the food is an important 

 aid to the chemical action of the digestive juices. We shall see 

 that this mechanical division forms a great part of the 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 masti- 

 cation 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 by 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 6J years ; the last molar, or wisdom-tooth, does not emerge 

 till the seventeenth to the twenty-fifth year. 



In mastication the lower jaw is moved up and down, so as to 

 alternately separate and approximate the two rows of teeth. It 

 has also a certain amount of movement from side to side, and 

 from front to back. The masseter, temporal and internal ptery- 

 goid muscles raise, and the digastric, with the assistance of the 



