MECHANICS OF MASTICATION ,315 



The tongue has a threefold duty to perform as a unit of this 

 transport system (a) working in conjunction with the lip- 

 sphincter orbicularis oris, and with the triangular and other 

 muscles it acts as a suction-plunger ; (/3) during deglutition it 

 functions as a force plunger and (y) it forms with the cheeks an 

 effective hopper during the mastication of food. 



(c) The lower jaw is a horse-shoe shaped lever of the third 

 order. The load is placed on the teeth, the fulcra are at the 

 ends of the horse-shoe, where they articulate with the fixed upper 

 jaw while the power is applied at a point on either side between 

 the teeth and the fulcrum. The jaw is pressed against the upper 

 jaw by the action of the temporal, masseter and internal pterygoid 

 muscles which act antagonistically to the mylo- and genio-hyoids, 

 to the platysma and to the anterior belly of the digastric muscle. 

 Nuts having a crushing point of about 400 kilograms may be 

 crushed by a direct thrust of the front teeth. The molars, lying 

 as they do nearer the fulcra and further from the application of 

 the power, may exert a direct pressure of about 550 kg. The 

 employment of such pressures is rarely necessary on account of 

 the previous treatment of the food (milling, cooking, etc.), and 

 of the influence of saliva. Soft bread, for instance, is merely 

 compressed by a pressure of 100 kgs. but, after moistening with 

 saliva, only a twentieth of this pressure is necessary to obtain 

 a clean bite through. 



The grinding operations of the molars (and of the incisors at 

 times) are a compound motion made up of a side-to-side and a 

 forwards-backwards motion. The former is produced by the 

 action of the external pterygoids working in conjunction with the 

 posterior fibres of the corresponding temporal muscle. The 

 latter movement may be ascribed to the forwards pull of the 

 external pterygoids and the backwards pull of the posterior fibres 

 of the temporals. 



This mill-like motion tears the food with a smaller exhibition 

 of pressure than direct crushing. Cooked meat which could be 

 crushed by the application of from 30 to 100 kg. pressure can be 

 torn by a grinding movement when the pressure is only 2 to 5 kg. 



In Chap. XVI., we saw how bone was formed in accordance 

 with the stresses and strains upon it. It is, therefore, interesting 

 to note that in proportion as the food is prepared by factory 

 milling and cooking, in proportion in fact, to the avoidance of the 

 stimuli to growth furnished by incident stresses and strains, so 

 the jaws of civilised men tend to become weak. In consequence 



