616 THE EXTERIOR OF THE HORSE. 



central enamel ; it has been called the dental star by Girard, who was 

 the first to observe it. We know that a second one exists close to the 

 posterior border of the dental table, but it is so little marked that it does 

 not concern us. 



The dental star is at first narrow and elongated transversely ; after- 

 wards its situation, dimensions, and form change in a notable degree, 

 in proportion to the age of the animal. It progressively gains the 

 central part of the dental table, diminishes in length transversely and 

 increases in thickness antero-posteriorly, and, finally, becomes tri- 

 angular and round. It never stands in relief above the surface of 

 friction, and can therefore never be confounded with the central enamel. 

 It is constituted by the dentine of more recent formation, which is 

 deposited by successive layers in the internal cavity of the tooth. 



4th. Save a few modifications, the preceding details are maintained 

 during many years ; then the central enamel disappears, and with it 

 the nucleus of cement which it enclosed. 



From this period until extreme old age nothing but the dental star 

 and the dentine are found upon the surface of friction. The periph- 

 eral enamel diminishes in thickness ; it is effaced very late, at first 

 behind, and afterwards in front; finally, there appears, upon many 

 teeth, the radical cementation of which we have already spoken when 

 on the subject of the structure. 



E. — Direction of the Incisors. 



The incisors must be viewed in two principal directions : the one, 

 relative to the plane of meeting of the arcades, is judged by inspecting 

 the jaws in profile ; the other, relative to the median plane, is appreci- 

 ated by viewing the latter from in front. Both furnish important data 

 for the determination of the age. 



1st. Direction in Relation to the Plane of Meeting of 

 the Jaws. — If the teeth of a young horse be exposed by removing 

 the bone with a chisel (Fig. 288), it will be found that they are greatly 

 convex upon the anterior face. It follows therefrom that, viewed in 

 profile, they project in the manner of a vertical arc, at first almost 

 regularly semicircular. At this moment each of the moieties of this 

 arc is therefore opposed to the other, like two semi-circumferences 

 that are placed in contact by their diameter in such a manner that 

 a tangent to the point of contact of the jaws will be perpendicular to 

 their plane of apposition. But as the table of friction, in consequence 

 of the progress of wear, approaches more and more the region of the 

 root, the aforesaid semicircle becomes altered and resembles an ogive 



