498 SPECIAL PHYSIOLOGY. 



cesses of the sphenoid bones, and from the palate bones, and pass, the 

 external one horizontally backwards and outwards, to the inner sur- 

 face of the neck of the condyle of the lower jaw, and the internal one, 

 obliquely backwards and downwards, to the inner surface of the ascend- 

 ing part and angle of the jaw. The latter muscles, on each side, co- 

 operate with the temporals and masseters, in raising the jaw, and assist 

 a little in drawing the bone forwards; but the external pterygoids are 

 the muscles chiefly concerned in executing this latter movement, as in 

 protruding the chin. The backward movement is accomplished by aid 

 of the posterior fibres of the temporal, and by the internal pterygoids. 

 The external pterygoid of one side, causes the lateral motion of the 

 bone upon its opposite condyle, and the lateral movement of the chin 

 over to the other side. To accomplish the forward gliding movement 

 of the interarticular cartilage, and, at the same time, to withdraw the 

 two synovial membranes, situated above and below it, from the risk of 

 pressure, certain fibres of the external pterygoid muscle are fixed to 

 the anterior edge of the interarticular cartilage, and also to both syno- 

 vial membranes. The movements of the masticatory muscles accelerate 

 the flow of saliva and mucus into the mouth. 



The chief movement, employed in dividing or lacerating soft food, 

 is a direct ascent of the lower jaw, accomplished by the temporal, mas- 

 seter, and internal pterygoid muscles. In crushing harder food, or in 

 the bad practice of cracking nuts with the teeth, the same movement 

 occurs, the substance being placed far back between the molar teeth, 

 not only because these teeth are broader and stronger than the rest, 

 but because the muscular force is used with greater effect, the nearer 

 to the fulcrum it is exerted. The advantage of having the molar teeth 

 in the part of the jaw nearest to the fulcrum is obvious. A simple up- 

 ward movement of the lower jaw is insufficient for the purposes of 

 mastication ; but the necessary bruising and trituration of the food, are 

 accomplished by its backward and forward movements, and especially 

 by the lateral movement, combined with a slight backward and forward 

 action, which cause a rotatory or grinding motion of the lower teeth 

 upon the upper ones. 



Mastication is extremely important in the case of all solid, firm, or 

 fibrous food, as well as of that which is hard and dry, preparing it, by 

 comminution for the action of the digestive fluids; when it is hurriedly 

 or imperfectly performed, dyspepsia often ensues. 



In the act of mastication, the saliva plays an important mechan- 

 ical part, as, indeed, it also does in the movements of the tongue in 

 speech. Poured into the mouth at various points, especially from the 

 inner side of the cheeks near the molar teeth, it not only lubricates 

 the mucous membrane, thus facilitating the requisite and constant 

 motion of the food in the mouth, and moistens the teeth, so as to pre- 

 vent the adhesion of the food by the clogging of their grinding surfaces, 

 but, mixed with the food, it materially assists in softening it, and con- 

 verting it into a pulpy mass, fit to pass down through the membranous 

 gullet. In mastication, the food is also mixed with a small quantity 

 of air. It has been observed that in the mastication of dry food, such 

 -as crusts or biscuits, a larger quantity of saliva is, for a time, secreted 



