THE PROCESS OF TEETHING. 



145 



mists. Wlioever has observed a horse in the act of browsing, and the twitch of the 

 head which accompanies the separation of each portion of grass, will perceive that 

 It is nipped or torn rather than cut olF. 



These teeth are covered with a polished and exceedingly hard substance, called the 

 enamel. It spreads over that portion of the teeth which appears above the gum, and 

 not only so, but as they are to be so much employed in nipping the grass, and gath- 

 ering up the animal's food, and in such employment even this hard substance must 

 be gradually worn away, a portion of it, as it passes over the upper surface of the 

 teeth, is bent inward, and sunk into the body of the teeth, and forms a little pit in 

 them. The inside and bottom of this pit being blackened by the food, constitutes the 

 mark of the teeth, by the gradual disappearance of which, in consequence of the wearing 

 down of the edge, we are enabled, for several years, to judge of the age of the animal. 

 The colt's nipping teeth are rounded in front, somewhat hollow towards the mouth, 

 and present at first a cutting surface, with the outer edge rising in a slanting direction 

 above the inner edge. This, however, soons begins to wear down until both surfaces 

 are level, and the mark, which was originally loag and narrow, becomes shorter, and 

 Avider, and fainter. At six raonths the four nippers are beginning to wear to a level. 

 The annexed cut will convey some idea of the appearance of the teeth at twelve 

 months. The four middle teeth are almost level, and the corner ones becoming so. 

 The mark in the two middle teeth is wide and faint ; in 

 the two next teeth it is darker, and longer, and narrower ; 

 and in the corner teeth it is darkest, and longest, and nar- 

 rowest. 



The back teeth, or grinders, will not guide us far in 

 ascertaining the age of the animal, for we cannot easily 

 inspect them ; but there are some interesting particulars 

 connected with them. The foal is born with two grinders 

 in each jaw, above and below; or they appear within 

 three or four days after the birth. Before the expiration 

 of a month they are succeeded by a third, more back- 

 ward. The crowns of the grinders are entirely covered 

 with enamel on the top and sides, but attrition soon wears 

 it away from the top, and there remains a compound sur- 

 face of alternate layers of crusted petraser, enamel, and ivory, which are employed in 

 grinding down the hardest portion of the food. Nature has, therefore, made an ad- 

 ditional provision for their strength and endurance. 



This cut represents a crrinder sawed across. It seems to 

 be a most irregular and intricate structure ; but the expla- 

 nation of it is not difficult. The tooth is formed and pre- 

 pared in cavities within the jaw-bones. A delicate mem- 

 branous 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 rnattter, 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, however, 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 ves- 

 sels penetrating into it, and is represented by the darker portions of the cut with cen- 

 tral black spots. The crystallization of enamel can be tmctvl 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 grinding tooth capable, with- 

 out injury and without wearing, to rub down the hay, and oats, and beans, which 

 "1 nstitute the stable-food of horses. 



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

 13 T 



