TEE MOUTH. 



351 



at the bottom of the infundibulum. Nevertheless, the quantity accumulated 

 in this cul-de-sac is not always the same ; we have seen it sometimes almost 

 null, and on the other hand we possess incisors unworn, or nearly so, in 

 which the cavity is almost entirely obstructed by the crusta petrosa. We 

 are not aware that, up to the present time, any account has been taken of 

 these differences when calculating the progress of wear ; but it may be 

 imagined that they ought to influence in a sensible manner the period at 

 which effacement of the external dental cavity takes place. 



Fig. 159. 



INCISOR TEETH OP THE HORSE. DETAILS OF STRUCTURE. 



1, A tooth in which is indicated the general shape of a permanent incisor, and the 

 particular forms successively assumed by the dental table in consequence of 

 friction, and the continued pushing outwards of these teeth ; 2, A virgin tooth, 

 anterior and posterior faces ; 3, Longitudinal section of a virgin tooth, intended 

 to show the internal conformation and structure. Not to complicate the figure, 

 the external cement, and that amassed in the infundibulum, has not been 

 exhibited. 4, Transverse section for the same purpose ; a, Encircling enamel ; 

 by Central enamel ; c, Dental star ; d, Dentine ; 5, Deciduous tooth. 



All the characteristics just indicated belong to the deciduous teeth 

 (Fig. 159, 5), except that they are smaller than the permanent; that they 

 are of a shining milky-white colour, due to the thinness or absence of the 

 crusta petrosa ; that they show at the point of union between the free 

 portion and the root, a constriction named the neck ; that their crown is finely 

 striated, and not cannular, on the anterior face ; that the external cul-de-saG 



