006 THE EXTERIOR OF THE HORSE. 



pincer, for example, a series of transverse sections three or four milli- 

 metres in thicknesss. We can, in this manner, obtain artificially the 

 successive forms of all the tables which are furnished l)y the natural 

 usage. The forms which are best defined may be arranged successively 

 in the following manner (Fig. 281) : 



1st. The dental table is at ^Vi=,i flattened from before to behind, — that 

 is to say, its transverse diameter is more extensive than the antero-pos- 

 terior (sections 1 and 2). 



2d. The dental table becomes oval; here there is less diiference 

 between the extent of the two diameters, although the transverse is 

 still greater than the antero-posterior (sections 3 to 5). 



3d. It takes a rounded form, and its two diameters are almost equal 

 (sections 6 and 7). 



4th. It becomes triangular, and is limited by three borders, an 

 anterior and two lateral. The summit of the triangle looks backward 

 (sections 8 to 11). 



5th. Finally, the wearing surface \s flattened from side to side (sec- 

 tions 12 to 16). This last form characterizes old age, and persists, 

 whatever may be the duration of the life of the animal. Girard desig- 

 nated it by the metaphoric expression, biangular. 



Those configurations of the dental table are much more regular in 

 the pincers than in the intermediates, and in the latter more so than in 

 the corners. They are almost identical in the incisors of the superior 

 jaw, but, in general, the latter are not examined in this relation. 



C. — Structure of the Incisors. 



The study of the structure of the incisors furnishes important indications 

 for the determination of the age. In conjunction with what we have already 

 said, it enables us to understand fully the peculiarities which the tooth presents, 

 according to the age, at its free extremity. 



We will choose, as the type for description, an inferior incisor ; the reader 

 can easily apply the details into which we shall now enter to the superior 

 incisors. 



The sac or follicle in which all the incisors are developed presents for con- 

 sideration two papillary prolongations or papillae, one superior, a, the other 

 inferior, b (Fig, 282), which penetrate each other mutually. The former, or 

 enamel germ, conical in form, is lodged in the external dental cavity ; the second, 

 or dentine germ, bifid, cup-shaped at its summit, and flattened autero-posteriorly, 

 fills the pul|) cavity in the interior. The one is endowed with the formation of 

 a substance called enamel ; the other, secreting the dentine or ivory, alone per- 

 sists, under the name of pulp, until a very advanced period of life, and it is 

 from this structure that the tooth receives its nourishment. In the very old 

 tooth it may become almost entirely absorbed. 



