564 DIGESTION. 



ticatory and salivary organs, by dividing and moistening the food, per- 

 mit the organs of gustation to enjoy the savour by successive applica- 

 tions. 



When the food is received into the month, if it be sufficiently soft, 

 it is commonly swallowed immediately ; unless the flavour is delicious? 

 when it is detained. If solid, and, especially, if of any size or density 7 

 it is divided into separate portions, or chewed, the action constituting 

 mastication. If the consistence of the substance be moderate, the 

 tongue, by being pressed strongly against the bony palate, is sufficient 

 to effect this division; bruising it, and at the same time, expressing its 

 fluid portions. If the consistence be greater, the action of the jaws 

 and teeth is required. For this purpose, the lower jaw is successively 

 depressed and elevated by the action of its depressors and levators ; 

 and the horizontal or grinding motion is produced at pleasure by the 

 action of the pterygoids. Whilst these muscles are acting, the tongue 

 and cheeks are incessantly moving, so as to convey the food between 

 the teeth, and insure its comminution. Mastication is chiefly effected 

 by the molares. There is advantage in using them, independently of 

 their form, in consequence of the arm of the resistance being much 

 shortened, as has already been shown. 



The teeth are well adapted for the service they have to perform. 

 The incisors, as their name imports, are used for cutting ; hence their 

 coronas come to an edge ; the canine teeth penetrate and lacerate, and 

 their coronas are acuminated; whilst the molares bruise and grind, and 

 their touching surfaces are tuberous. The first, having usually no great 

 effort to sustain, are placed at the extremity of the lever ; the latter, for 

 opposite reasons, are nearest the fulcrum. To preclude displacement 

 by the efforts they have occasionally to sustain, they are firmly fixed in 

 the alveoli or sockets; and, as the roots are conical, and the alveoli 

 accurately embrace them, the force, as in the case of the wedge, i& 

 transmitted in all directions, instead of bearing altogether upon the 

 jaw, which it would do, were the fangs cylindrical. The molar teeth, 

 having the greatest efforts to sustain, are furnished with several roots; 

 or with one that is extremely large. 



The gums add materially to the solidity of the junction of the teeth 

 with the jaws. They are themselves formed of highly resisting mate- 

 rials, so as to withstand the pressure of hard and irregular substances. 

 Whenever they become spongy, and fall away from the teeth, the latter 

 become loose ; and are frequently obliged to be extracted, in conse- 

 quence of the loose tooth acting as an extraneous body, and inflaming 

 the lining membrane of the alveolus. The arrangement of the jaw is 

 well adapted to the function; the lower jaw passing behind the upper 

 at its anterior part; but coming in close contact at the sides, where 

 mastication is chiefly effected. 



During the whole time that mastication is going on, the mouth is 

 closed ; anteriorly, by the lips and teeth, which prevent the food from 

 falling out of the cavity; and posteriorly by the velum palati, the 

 anterior surface of which is applied to the base of the tongue. At the 

 same time, the food is undergoing insalivation or admixture with the 

 various fluids poured into the mouth, and particularly with the saliva, 



