322 DIGESTION 



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 masti- 

 cation; 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 6 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 pterygoid muscles 

 raise, and the digastric, with the assistance of the mylo- and genio- 

 hyoid, depresses, the lower jaw, but its downward movement is 

 mainly a passive one. The external pterygoids pull it forward 

 when both contract, forward and to one side when only one con- 

 tracts. The lower fibres of the temporal muscle retract the jaw. 

 The buccinator and orbicularis oris muscles prevent the food from 

 passing between the teeth and the cheeks and lips. The tongue 

 keeps the food in motion, works it up with the saliva, and finally 

 gathers it into a bolus ready for deglutition. 



Deglutition. This act consists of a voluntary and an involun- 

 tary stage. Just before the beginning of the voluntary stage 

 mastication is suspended, and a slight contraction of the dia- 

 phragm generally takes place. The anterior part of the tongue 

 is suddenly elevated and pressed against the hard palate, and the 

 elevation travels back from the tip towards the root, as the mylo- 



