THE TEETH. 139 



ular and intricate machine ; but the explana- 

 tion of it is not difficult. The tooth is <ornied 

 and prepared in cavities within the jaw-bones. 

 A delicate membranous bag, containing a jelly- 

 like substance, is found, in the unborn animal, 

 in a little cell within the jaw-bone. It assumes, 

 by degrees, the form of the tooth that is to 

 ajjpear, and then the jelly within the nriembrane 

 begins to change to bony matter ; and a hard 

 and beautiful crystallization is formed on the membrane without, and so 

 we have the cutting tooth covered by its enamel. In the formation, how- 

 ever, of each of tiie grinders of the horse, there are originally five of these 

 membranous bags in the upper jaw, and four in the lower, filled with jelly. 

 This jelly, by degrees, gives place to bony matter, which is thrown out by 

 little vessels penetrating into it, and is represented by the darker portions 

 of the cut with central black spots. The crystallization of enamel can be 

 traced around each, and so tliere would be five distinct bones or teeth. 

 A third substance, iiowever, is now secreted, (which is represented by 

 the white spaces.) and is a powerful cement, uniting all these distinct 

 bones into one body, and making one tooth of the five : this being done, 

 another coat of enamel spreads over tiie sides, but not the top, and the 

 tooth is completed. By no other contrivance could we have the grinding 

 tootii, capable, without injury and without wearing, to rub down the hay, 

 and oats, and beans, which constitute the stable food of horses. Tlie teeth 

 of the animal who lives on flesh, and the upper part of whose teeth alone 

 are covered with this enamel, and even the nipping teeth of the horse, with 

 the simple well of enamel running a certain way down their centre, would 

 soon be rubbed down and destroyed. It is necessary to have columns of 

 enamel penetrating through the whole substance of the tooth. There is 

 another advantage: the bony matter, and cement by which the different shells 

 are united, and which occupy the spaces between the columns of enamel, 

 soon begin to wear away, while the enamel remains; and thence results 

 the irregular surface of the grinding teeth, being that kind of surface which 

 it is necessary for them to possess, in order to effect the purpose for which 

 they were intended. 



The grinders in the lower jaw, having originally but four of these bags 

 or shells, are smaller, and narrower, and more regular, than the upper ones. 

 They are not placed horizontally in either jaw ; but in the lower, the 

 higher side is within, and shelving gradually outward ; in the upper jaw 

 the higher side is without, shelving inward, and thus tiie grinding motion is 

 most advantageously performed. There is also an evident difference in 

 the appearance and structure of each of the grinders, so that a careful 

 observer could tell to which jaw every one belonged, and what situation it 

 occupied ; but we should depart from the object of our work if we entered 

 into these minutiae. 



At the completion of the first year, a fourth grinder usually comes up, 

 and the yearling has then, or soon afterwards, six nippers, and four grinders 

 aiiove and below in each jaw, which, with the alteration in the appearance 

 of the nippers we have just described, will enable us to calculate prettv 

 nearly the age of the foal, subject to some variations, arising from the 

 period of weaning and the nature of the food. 



At the age of one year and a half, the mark in the central nippers will 

 ^>o much shorter and fainter; that in the two other pairs will have under- 

 gone an evident change, and all the nippers will be flat. 



