392 THE TEETH OF THE HOESE 



near the upper surface to the bottom of the tooth, causing the opening at 

 the base of the fang to represent a triangular figure. Consequently, 

 sections commencing at the upper part of the tooth, and carried down to 

 the bottom of it, would represent the forms which are shown on the right- 

 hand side of the drawing. The wearing of the teeth from the upper 

 surface downwards is compensated by the growth of the tooth upwards 

 from its cavity, and the changes in the form of the table as the animal 

 advances in life are necessarily a gradual approach to the triangular form. 



Besides the changes in the form of the table which the tooth undergoes 

 during the course of its natural wear from constant attrition, there are also 

 changes relating to the "mark", which is the name in common use to 

 indicate the cavity in the centre of the tooth, which becomes dark or black 

 in colour from the action of the food on the bony structure. The cavity is 

 formed by the inversion of the three structures of the tooth, the crusta, 

 enamel, and ivory. 



The hollow cone extends about half-way down the incisor tooth, and 

 consequently, when the wear reaches to a certain point, the cavity is 

 obliterated, or, in the horseman's language, the mark is worn out. 



One result of the inversion of the tooth structures to form a hollow 

 cone in the interior of the incisor is a peculiar arrangement of lines on 

 the worn surface. This condition is shown in fig. 601. An outer line of 

 white enamel is seen, inside which is the broader line formed by the bulk 

 of -the tooth the ivory ; then the inner line of enamel which belongs to 

 the inverted cone, with the lining of crusta, which is originally on the out- 

 side of the tooth, but in the inverted structure is necessarily on the inside. 

 This structure is quickly darkened by contact with food. The darkening is 

 also distinctly seen on the crusta on the outside of the tooth, being espe- 

 cially marked in the grooves. The prominent parts of the surface become 

 w r hite in consequence of the darkened crusta being rubbed off by the move- 

 ment of the animal's lips. It will be obvious that the table of the tooth 

 exhibits the following features: 



1. An outer ring of enamel. The outer covering of crusta is at the 

 edge of the tooth worn away. 



2. A broad line of ivory, in the centre of which is a faint line, showing 

 the junction of the inverted cone with the outer shell of the tooth. 



3. A ring of enamel called central enamel, with the line of dark crusta 

 inside it. 



It is necessary to note here that the changes in form of the central 

 enamel afford important evidence of the age after seven years. 



Tricks. Horses at the middle period of life are more valuable than 

 when very young or very old. It is not remarkable, therefore, that certain 



