THE ACT OF SWALLOWING 415 



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 lower 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 kg. 

 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 kg., 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. XVII. 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, 

 faces are elongated and narrow instead of short and roimd like 

 those of primitive men. 



II. The act of swallowing. The food, after being chewed, is 

 collected on the surface of the tongue by the action of the bucci- 

 nator and other voluntary nuiscles. The jaws are closed and the 

 tip and sides of the tongue are pressed against the hard palate and 



