THE LOWER JAW. 



143 



so lear to the insertion nf the muscle, 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 liipping 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 



F 



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 jav/ 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 

 iiold 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 n'^x^ want it. 



As before mentioned, the food of the horse must be ci^round. Simple bruising and 

 champing would not suificiently comminute it for the purposes of digestion. We 

 then observe the dilferent 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, arid 

 therefore called the ma^tai'l process; und — 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 1 evidently to limit this lat?ml or circuUir 

 motion — to permit it as far as the necessities of the animal require it, and then to 

 arrest it. How is this done ? Not suddenly or abruptly ; but the tuljerc^le, of which 

 we have already spoken as strengthening this portion of the zygomatic arch, now 

 discharging another office, has a smooth and gradual ascent to it, up which the lowei 

 jaw may climb to a certain extent, and then, by degrees, be stopped. We speak not 

 now of the moveable cartilaije which is placed in this cavity, and between the bones, 

 to rendei the motion easier and freer. It is found in this joint in every quadruped ; 

 and it is found wherever inotions are rapid and of long continuance. 



