THE PROCESS OF TEETIIIXG. 



2-23 



or they appear -within three or four days after the birth. Before the ex- 

 piration of a month they are succeeded by a third, more backward. The 

 crowns of the grinders are entirely covered by enamel on the top and sides, 

 but attrition soon Avears it away from the top, and there remains a com- 

 pound surface of alternate layers of crusta petrosa, enamel and ivory, 

 which are employed in grinding down the hardest portions of the food. 

 Natui'e has therefore made an additional provision for their streng-tli and 

 endurance. 



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

 irregular and intricate structure ; but the explanation 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 appeal", and then the jelly within ^''c-, 

 the membrane begins to change to bony matter, t=;s: 

 and a hard and beautiful crystallisation is formed 

 on the membrane without, and so we have the cutting tooth covered by 

 its enamel. In the formation, however, of each of these grinders of the 

 horse, there are oi'iginally 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 penetrating into it, and is 

 represented by the darker poi'tions of the cut with central black spots. 

 The crystallisation of enamel can be traced round each, and there would 

 be five distinct bones or teeth. A thii'd substance, however, is now 

 secreted (which is represented by the white spaces), and is a powerful 

 cement, uniting all these distinct boiies 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 con- 

 trivance could we have the grinding tooth capable, without injury and 

 without wearing, to rub down the hay, and oats, and beans, which consti- 

 tute 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 jaAv ; 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 advantageously performed. There is also an evident dif- 

 ference in the appearance and structure of 

 each of the grinders, so that a careful ob- 

 server could tell to which jaw every one 

 belonged, and what situation it occupied. 



At the completion of the first year, a llf^^^/ -i^ ^1 



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 nippers that we 

 have just described, will enable us to cal- 

 culate 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. 



