618 



THE EXTERIOR OF THE HORSE. 



M.I. 



FIG. 289. Incisors exposed by their ante- 

 rior face to show their relative inclination 

 towards the median line. 



2d. Direction in Relation to 

 the Median Plane. In the young 

 animal (Fig. 289), in consequence of 

 the greater extent of the arcades 

 formed by the surfaces of friction 

 elongated transversely, and also on 

 account of the lateral flattening of the 

 roots, the incisors all appear to be 

 convergent at the level of their roots. 

 They, however, do not remain so. 

 With the progress of age the crown 

 of the tooth is little by little worn 

 off, at the same time that the maxil- 

 lary bones push them out farther and 

 farther from their alveolar cavities. 

 Under the combined influence of 

 these two causes the roots, which at 

 first were almost contiguous, are 

 gradually separated from each other, 

 and draw towards the entrance of 

 the alveolus, while the tables dimin- 

 ish in width transversely. 



The evolution of these facts has 

 for its result a modification of the 

 primitive direction of the incisors : 

 their axis is slowly separated from 

 the median line, so much so that at 

 a given moment it becomes alto- 

 gether parallel to it (Fig. 284). 



Finally, the phenomenon of the 

 gradual expulsion of the teeth from 

 their alveoli continuing to manifest 

 itself in concert with the transverse 

 narrowing of the arcade, there comes 

 a time when the teeth are found con- 

 verging by their free extremity and 

 diverging by their opposite extrem- 

 ity, that is to say, inversely dis- 

 posed in relation to the axis of the 

 jaws. In this case, the intervals 

 separating the pincers from the in- 



