194 THE ANATOMY AND DISEASES OF THE NOSE AND MOUTH. 



abruptly ; but the tubercle, of which we have already spoken as strengthening this i 

 portion of the zygomatic arch, 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 desti- 

 nation, that a tolerable student in comparative anatomy, by a mere inspection 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 j 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. 172, is represented the masaeter 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 

 with the temporal muscle in closing the jaw, and in giving the direct cutting or 

 champing motion of it. 



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

 portion of them, and opposite to the masseters, are the pterygoid muscles, going 

 from the jaws to hones 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 may be a little too wide, and then the face will 

 have a clumsy appearance : but if it is too narrow, the horse will never be able 

 to bend his head freely and gracefully ; he will be always pulling or boring 

 upon the hand, nor can he possibly be well reined in. 



The jaws contain the teeth, which are the millstones employed in comminuting 

 the food. The mouth of the horse at five years old contains forty teeth, viz. six 

 nippers or cutting teeth in front, a tush on each side, and six molars, or grind- 

 ing teeth, 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 hard 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 wounded 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 germs or beginnings 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 compared 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. They like- 

 wise appear to be large, and to fill the front of 

 the mouth ; although they will afterwards be found 

 to be small, compared with the permanent teeth 

 that follow. In the course of the first month the 

 third grinder appears above and below, and, not long 

 after, and generally before six weeks have expired, 

 another incisor above and below will be seen on each side of the two first, 



