138 



THE HORSE. 



ihe cavity, but in 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 diiferent direction. The head of the lower jaw is received into this hoi- 

 low, and presses against these ridges, and is partially surrounded by them, 

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

 possiliie, and admitting the hinge-like or cranching motion 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 sliglit degree, all lateral and grinding 

 motion, and this because the animal does not want it. 



The food of the horse must be ground — simple bruising and champing 

 would not reduce it sufficiently small for the purposes of digestion. Then 

 observe the different construction to effect this. A gives the glenoid cavity 

 of the norse. First, there is the upper ridge assuming a rounded form, F, 

 and therefore called the mastoid process ; and, a peculiarity in the horse, 

 the mastoid process of the squamous portion of the ♦emporal bone : suffi- 

 ciently 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 evi- 

 dently destined for a circular motion about it. Then, 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? evi- 

 dently 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 : but how ? not 

 suddenly or abruptly ; but this tubercle, of which we have already spoken 

 as strengthening this portion of the zygomatic arch, yet now discharging 

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

 jaw may climb to a certain extent, and then, by degrees be stopped. We 

 speak not now of the moveable cartilage which is placed in this cavity, and 

 between the bones, to render the motion easier and freer. It is found in 

 this joint in every quadruped ; and it is found wherever motions are rapid 

 and of long continuance. 



So great is the conformity between the structure of the animal and his 

 destination, that a tolerable student in natural history, by the mere inspec- 

 tion of the glenoid cavity, will at once determine whether the animal to 

 which it belonged was carnivorous, and wanted no lateral motion of the 

 jaw; or omnivorous, living occasionally on all kinds of food, and requiring 

 some degree of grinding motion ; or herbivorous, and needing the constant 

 use of this admirably-constructed mill. 



At g, page 119, is represented the masscler muscle, an exceedingly 



