136 



THE HORSE. 



lower and opposite edg-e of the cavity, but in the outside, is a similar ridge, 

 E, hkewise 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 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 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 slight 

 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 horse. 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 temporal 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? 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 — but how? not suddenly or 

 abruptly ; but this tubercle, of which we have already spoken as strengthen- 

 ing this portion of the zygomatic arch, yet, now discharging another oflice, 

 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 Avhether 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 con- 

 stant use of this admirably constructed mill. 



At g-, p^ge 119, is represented ilie massder muscle, an exceedingly 



