194: DIGESTION MASTICATION, INSALIVATION, DEGLUTITION. 



Action of the Muscles which elevate the, Lower Jaw and move it laterally 

 and antero-posteriorly. The temporal, masseter and internal pterygoid 

 muscles are chiefly concerned in the simple act of closing the jaws. Their 

 anatomy alone gives a sufficiently clear idea of their mode of action ; and 

 their great power is explained by the number of their fibres, by the attach- 

 ments of many of these fibres to the strong aponeuroses by which they are 

 covered, and by the fact that the distance from their origin to their insertion 

 is very short. 



The attachments of the internal and external pterygoids are such that by 

 their alternate action on either side, the jaw may be moved laterally, as their 

 points of origin are situated in front of and internal to the temporo-maxil- 

 lary articulation. The articulation of the lower jaw is of such a kind that 

 in its lateral movements the condyles themselves can not be sufficiently dis- 

 placed from side to side ; but with the condyle on one side fixed or moved , 

 slightly backward, the other may be brought forward against the articular 

 eminence, producing a movement of rotation. 



The above explanation of the lateral movements of the jaw presupposes 

 the possibility of movements in an antero-posterior direction. Movements in 

 a forward direction, so as to make the lower teeth project beyond the upper, 

 are effected by the pterygoids, the oblique fibres of the masseter and the an- 

 terior fibres of the temporal. By the combined action of the posterior fibres 

 of the temporal, the digastric, mylo-hyoid and genio-hyoid, the jaw is brought 

 back to its position. By the same action it may also be drawn back slightly 

 from its normal position while at rest. 



Action of the Tongue, Lips and Cheeks, in Mastication. Experiments on 

 living animals and phenomena observed in cases of lesions of the nervous 

 system in the human subject have shown the importance of the tongue and 

 cheeks in mastication. Section of the facial nerves is a common physiologi- 

 cal experiment. Operations of this kind, and cases of facial palsy, which are 

 not uncommon in the human subject, show that when the cheek is paralyzed^ 

 the food accumulates between it and the teeth, producing great incon- 

 venience. 



The varied and complex movements of the tongue during mastication 

 are not easily described. After solid food is taken into the mouth, the 

 tongue prevents its escape from between the teeth, and by its constant 

 movements, rolls the alimentary bolus over and over and passes it at 

 times from one side to the other, so that the food may undergo thorough 

 trituration. Aside from the uses of the tongue as an organ of taste, its sur- 

 face is endowed with peculiar sensibility as regards the consistence, size and 

 form of different articles ; and this is undoubtedly important in determining 

 when mastication is completed, although the thoroughness with which mas- 

 tication is accomplished is much influenced by habit. 



Tonic contraction of the orbicularis oris is necessary to keep the fluids 

 within the mouth during repose ; and this muscle is sometimes brought into 

 action when the mouth is very full, to assist in keeping the food between 

 the teeth. This latter office, however, is performed mainly by the buccina- 



