220 



THE LOWER JAW. 



rounded process, is received into the glenoid (shallow) cavity of the 

 temporal bone, at the base of the zygomatic arch, and forms the joint on 

 which the lower jaw moves. This joint is easily seen in the cut at p. 140 ; 

 and being placed so near to the insertion of the muscle, or the centre of 

 motion, the temporal muscle must act with very considerable mechanical 

 disadvantage, and, consequently, must possess immense power. 



The joint is admii^ably contrived for the purpose which the animal re- 

 quires. 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 

 braised before it is fit for digestion. Simple champing, which is the 

 m.otion 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 grmding motion to bring 

 it into a pvilpy form. We first examine this cavity in the tig'er repre- 

 sented at B. At the root of the zygomatic process D, is a hollow with a ridge 

 along the greater part of the u.pper and inner side of it, standing to a 

 considerable height, and curling over the cavity. At the lower and op- 



posite 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 view 

 of this cavity in a different direction. The head of the lower jaw is re- 

 ceived into this hollow, and presses against these ridges, and is partially 

 surrounded by them, and forms with them a very strong joint where dis- 

 location 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 gynurid. Simple 

 bruising and champing Avould not sufficiently comminute it for the 

 piirposes 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 inastoid process ; 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 



