196 THE ANATOMY AND DISEASES OF THE NOSE AND MOUTH. 



which are employed in grinding down the hardest portion of the food. Nature 

 has, therefore, made an additional provision for their strength and endurance. 



This cut represents a grinder sawed across. It seems to be a most irregular 

 and intricate structure ; but the explanation of it is not difficult. The tooth is 

 formed 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 appear, and 

 then the jelly within the membrane begins to change 

 to bony matter, and a hard and beautiful crystal- 

 lization 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 the grinders of the horse, there are originally five membranous 

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

 degrees gives place to bony matter, which is thrown out by little vessels pene- 

 trating 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 there 

 would be five distinct bones or teeth. A third substance, however, 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 the sides, but not the top, 

 and the tooth is completed. By no other contrivance could we have the grind- 

 ing tooth capable, without injury and without wearing, to rub down the hay, 

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



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, and shelving inward, and thus the grinding motion is most advan- 

 tageously 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. 



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 above 

 and below in each jaw, which, with the alteration in the appearance of the nip- 

 pers that we have just described, will enable us to calculate 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 be much shorter and 

 fainter ; that in the two other pairs will have 

 undergone an evident change, and all the nippers 

 will be flat. 



At two yea^s this will be more manifest. The 

 accompanying cut deserves attention, as giving 

 an accurate representation of the nippers in the 

 lower jaw of a two-years-old colt. 



About this period a fifth grinder will appear, 

 and now, likewise, will commence another pro- 

 cess. The first teeth are adapted to the size 

 and wants of the young animal. They are sufficiently large to occupy and fill 

 the colt's jaws ; but when these bones have expanded with the increasing growth 

 of the animal, the teeth are separated too far from each other to be useful, and 



