DIRECTION OF THE INCISORS. 



29 



destruction from use brings the table to the middle of the 

 original tooth ; when it assumes the round form, the cup of 

 central enamel has disappeared, and the dental star is now mid- 

 way between the anterior and posterior borders. (Fig. 10, (7.) 



Still further use brings the triangular form, and finally the 

 biangular; in these two. forms the dental star gradually becomes 

 larger, and, at times, when the papilla has not been entirely 

 replaced by the dentine, a cavity is found which, by tlie un- 

 informed, has been mistaken for the cup (Fig. 10, (7 and 7>), but 

 should not be, as there is no surrounding enamel. Figure 16 

 shows the forms at the various parts of the original tooth. 



The Direction of the Incisors. — 

 The direction of the incisors, or the posi- 

 tion which they hold in regard to the jaw, 

 is to be studied first in profile, considering 

 the relative angle which the incisors of 

 the upper and lower jaws have to each 

 other ; and second, in face, considering the 

 position which they hold in regard to the 

 median line. 



Direction of the Plane of Con- 

 tact OF the Two Jaws (Fig. 17). — In a 

 young horse the incisors meet and form 

 an arch which, if viewed in profile, rep- 

 resents tlie half of a circle; so that a 

 tangent drawn from the point of contact 

 of the two jaws is perpendicular to their 

 tables. But as the progressive wearing of the table brings 

 it nearer to tlie roots of the teeth, the half-circle changes to 

 the form of an ogive, which becomes more and more acute 

 as the surface of contact, which is displaced above and below 

 parallel to itself, extends gradually from the primitive diameter. 

 Consequently the tangents, a a\ b h\ c c', drawn from the new 

 points (a, &, c) of contact of the arches, are no longer perpen- 

 dicular to this point, but tend to become parallel. 



As the angle of incidence of the incisors increases in 



Fig. 16. 

 Scheme of sections of wedge- 

 shaped incisor. 



