OF THE TEETH. 37 



dip down to line the funnel-like cavity in its centre. This 

 central hollow thus formed, and thus lined, becomes very- 

 dark within by incrustation ; and, therefore, as long as this 

 oval cavity remains, so long a striking mark continues : 

 but when, by constant attrition, the cavity has been worn 

 away, such horse is said to have ' lost his mark.' 



The duration of the mark, however, is in a great measure 

 determined by the thinness or thickness of the lining 

 membranes or outer coats : as in proportion as these are 

 of less substance, of course the cavity is deeper and endures 

 the longer. The wear of the teeth is also in some measure 

 influenced by the food ; and in cases where the enamel 

 and crusta petrosa are very thick, every mark or hole may 

 be lost by the sixth year, but then the rim of enamel, en- 

 closing a brown plug of the darker membrane, shows where 

 the mark should have been. The crusta petrosa and 

 enamel which, when the tooth is cut, are developed upon its 

 table or biting surface, soon by attrition become worn away, 

 and then the bone or ivory (which forms the great sub- 

 stance of the tooth) is laid bare between two layers of the 

 membranes, one covering the exterior, and the other lining 

 the small cavity in the centre of the table. The bone or 

 ivory being softer than the enamel, wears away faster, and 

 thus leaves two ridges on the table, biting or nipping 

 surface, one of which surrounds the central cavity, and 

 the other embraces the outer surface ; but both serve to 

 raise prominences for the double purpose of cropping the 

 food and holding it between the teeth when necessary ; and 

 by which also the teeth become a more terrible weapon of 

 offence. 



The horse, as is wtII known, has two sets of nipping and 

 grinding teeth, a temporary and a permanent set. On the 

 outset we shall treat only of what are called the nippers 

 among horsemen, and incisors by naturalists. At birth, or 

 sometimes about a week or ten days after birth, the foal 

 has two front nippers above and below, which seem to fill 

 up almost the whole of the mouth. The little jaw enlarges, 

 however, so rapidly, that about the third or fourth week 

 the middle nippers also find room to appear, and, with this 

 complement of temporary or milk teeth, this sportive and 

 interesting animal frolics about, until towards his ninth 



