THE rUOCESS OF TEETIIIXG. 2L'l 



motion about it. At tlie otlier and lower edge of tlie cavity, and on the 

 outside, G is placed — • not a curling ridge as in the tiger, but a mere 

 tu.bercle : 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 ? Not suddenly or abruptly ; but the 

 tubercle, of which we have already spoken as strengthening this portion 

 of the zygomatic arch, now discharging another ofi&ce, has a smooth and 

 gradual ascent to it, up which the lower jaw may chmb 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 comparative anatomy, by a mere in- 

 spection of the glenoid cavity, would 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, p. 199, is represented the masseter muscle, an exceedingly strong 

 one, constituting the cheek of the horse — arising from the superior maxillary 

 under the ridge continued from the zygomatic arch, and inserted into the 

 lower jaw, and particularly round the rough border at the angle of the jaw. 

 This acts ^vith the temporal muscle in closing the jaw, and in giving the 

 direct cutting or champing motion to it. 



Within the lower jaw, on either side, and occupying the whole of the 

 hollowed portion of them, and ojDposite to the masseters, are the pterygoid 

 muscles, going from the jaws to bones more in the centre of the channel, 

 likewise closing the mouth, and also, by their alternate action, giving that 

 grinding motion which has been described. 



The space between the branches of the lower jaw, called the channel, is 

 of considerable consequence. It can scarcely be too wdde ; for if it is too 

 narrow, the horse will never be able to bend his head freely and gracefully ; 

 he will be ahvays pulhng or boring upon the hand, nor can he possibly bo 

 well reined in. 



The jaws contain the teeth, which are the millstones employed in com- 

 minuting the food. The mouth of the horse at five years old contains forty 

 teeth, viz. six nippers or cutting teeth in front, above and below, a tush on 

 each side, and six molars, or grinding teeth, on each side, above and below. 

 They are contained in cavities in the upper and lower jaws, surrounded by 

 bony partitions, to which they are accurately fitted, 

 and by which they are firmly supported. For a little 

 way above these bony cavities, they are surrounded 

 by a firm substance called the gum, so dense, and 

 adhering so closely to the teeth and the jaws, as 

 not to be separated without very great difficulty — - 

 singularly compact, that it may not be wormded by 

 the hard or sharp particles of the food, and almost 

 devoid of feeling, for the same purpose. 



Seven or eight months before the foal is born, 

 the fferms or betjinnino-s of the teeth are visible in 

 the cavities of the jaws. The tooth grows, and 

 presses to the surface of the gum, and forces its way through it ; and, at 

 the time of birth, the first and second grinders have appeared, large com- 

 pared with the size of the jaw, and seemingly filling it. In the course of 

 seven or eight days the two central nippers are seen as here represented. 



