

MASTICATION. 215 



movements are combined, as may be proved by placing the 

 finger on the temporo-maxillary articulation : the rotation 

 of the condyle in the cavity, and its forward projection take 

 place at the same time ; so that it is difficult and even im- 

 possible to decide exactly on a fixed axis around which all 

 the movements of the jaw are made. 



In all cases the lower jaw acts as a lever of which the 

 fixed point is behind, in the upright branch of the bone; the 

 point of application of the power, which is represented prin- 

 cipally by the masseter and temporal muscles, is in the front 

 edge of this upright branch ; the resistance may be found in 

 different points: if an aliment is to be divided, the resistance 

 lies on the level of the incisors, and in this case the lever 

 belongs to the third kind, and the arm of the power is very 

 short in comparison with the arm of resistance (see p. 103, 

 mechanism of the muscles). When the food requires to be 

 ground, the resistance is applied at the level of the molars, 

 and its lever arm becomes shorter, thus giving the advantage 

 to the action of the power, the lever arm of which keeps its 

 original length. Even in the case of a resistance opposed to 

 these latter molars, the fibres of the masseter may be found 

 anterior to the resistance ; and the maxillary lever then be- 

 comes a lever of the second kind, that which is most favor- 

 able to the action of the power (inter resisting lever, page 

 102). 



There is also a side movement in the lower jaw, which is 

 restricted in man, but of great extent in the ruminants. 

 It is due to the contraction of the external pterygoid 

 muscle which, by drawing one of the condyles forward, 

 brings it out of the glenoid cavity, while the jaw pivots on 

 the other condyle. 



We gee thus, that in man mastication is a compound 

 action, resembling both that of the carnivora and the 

 herbivora (ruminants), on account of the compound nature 

 of his food : the carnivora, which only tear their prey, make 

 no upward and downward or sideway movement ; thus 

 their condyle turns only on its transverse axis. In the 

 ruminants the sideway movements are very decided, and for 

 this purpose the condyle is flat and movable in all directions. 

 Another type of condyle is that of the rodents, the antero- 

 posterior diameter of which is of great extent, a glenoid 

 cavity being hollowed out in the same direction. In man, 

 the form of the condyle is intermediate between all these, 

 while the masticatory movements are more varied, and 



