THE LOWER JAW. 



193 



or the centre of motion, the temporal muscle must act with very considerable 

 mechanical disadvantage, and, consequently, must possess immense power. 



This joint is admirably contrived for the purpose which the animal requires. 

 It will admit freely and perfectly of the simple motion of a hinge, and that is 

 the action of the jaw in nipping the herbage and seizing the corn. But the 

 grass, and more particularly the corn, must be crushed and bruised before it is 

 fit for digestion. Simple champing, which is the motion of the human lower 

 jaw, and that of most beasts of prey, would very imperfectly break down the 

 corn. It must be put into a mill ; it must be actually ground. 



It is put into the mill, and as perfect a one as imagination can conceive. 



The following cuts represent the glenoid cavity, in a carnivorous or flesh- 

 eating, and herbivorous or grass- eating, animal, viz. the tiger and the horse : the 

 one requiring a simple hinge-like motion of the lower jaw to tear and crush the 

 food ; the other, a lateral or grinding motion to bring it into a pulpy form. We first 

 examine this cavity in the tiger represented at B. At the root of the zygomatic 

 process D, is a hollow with a ridge along the greater part of the upper and inner 

 side of it, standing to a considerable height, and curling over the cavity. At the 



lower and opposite edge of the cavity, but on the outside, is a similar ridge, E, 

 likewise rising abruptly and curling over. At C is another and more perfect 

 viewof this cavity in a different direction. The head of the lower jaw is received 

 into this hollow, and presses against these ridges, and is partially surrounded by 

 them, and forms with them a very strong joint where dislocation is scarcely 

 possible, and the hinge-like or cranching motion is admitted to its fullest extent ; 

 permitting the animal violently to seize his prey, to hold it firmly, and to crush 

 it to pieces ; but from the extent and curling form of the ridges, forbidding, 

 except to a very slight degree, all lateral and grinding motion, and this, because 

 the animal does not want it. 



As before mentioned, the food of the horse must be ground. Simple 

 bruising and champing would not sufficiently comminute it for the pur- 

 poses of digestion. We then observe the different construction of the 

 parts to effect this. A gives the glenoid cavity of the horse. First, there 

 is the upper ridge assuming a rounded form, F, and therefore called the 

 tnastoid process; and a peculiarity in the horse the mastoid process of 

 the squamous portion of the temporal bone : sufficiently strong to support the 

 pressure and action of the lower jaw when cropping the food or seizing an 

 enemy, but not encircling the^head of that bone, and reaching only a little way 

 along the side of the cavity, where it terminates, having its edges rounded off 

 so as to admit, and to be evidently destined for, a circular motion about it. At 

 the other and lower edge of the cavity, and on the outside, G is placed not a 

 curling ridge as in the tiger, but a mere tubercle : and for what reason ? evidently 

 to limit this lateral or circular motion to permit it as far as the necessities of 

 the animal require it, and then to arrest it. How is this done I Not suddenly or 



