612 THE EXTERIOR OF THE HORSE. 



will whenever the observation is made in the defective manner that we have 

 indicated. 



If, however, the section be made so as to divide the point of this cul-de-sac 

 into two moieties absolutely equal, its disposition will be different ; at' its summit 

 the layer of enamel does not present any thickening, and in certain cases it is 

 even somewhat thinner than anywhere else. 



These facts are demonstrated, besides, in the transverse sections (Fig. 281). 

 After the external cavity has disappeared, the central cement, which fills its 

 bottom, still persists in the form of a white spot bordered by a thin band of 

 enamel. This spot then diminishes in size, approaches the posterior border of 

 the dental table, and is finally effaced at the same time as the zone of enamel 

 which surrounds it. 



In relation to its hardness, the enamel is certainly the most remarkable layer 

 of the teeth. When the latter are still Enclosed by the gums in their alveoli, it 

 can be easily cut by a sharp instrument, and its histological elements, disposed 

 almost perpendicularly to the surface which it covers, disassociate themselves 

 readily. But as soon as this substance has been submitted to the contact of the 

 air it becomes hard, even to the extent of striking fire with steel. It is also 

 much more resistant to wear than the dentine, and shows itself constantly in 

 relief upon the surface of friction. 



The enamel is a tissue of epithelial origin formed by the superior papilla 

 of the primitive dental follicle. It is composed, microscopically, of an infinite 

 number of small hexagonal prisms (enamel prisms), solidly united and directed 

 obliquely in relation to the subjacent surfaces ; it is, by its deep surface, applied 

 upon the peripheral lacunar spaces of the dentine. 



It does not cover the extremity of the root of very old teeth ; but, as soon 

 as it has disappeared on their table, it is replaced by the cement in the man- 

 ner which we have already described. (See the longitudinal sections of Fig. 

 286.). 



Of a milky-white color more or less clear according to the lustre of the 

 tooth, but always very brilliant and slightly striated longitudinally, the enamel 

 preserves the same thickness when once it is formed, and does not repair its 



3d. The fundamental substance, the eburnated substance, the den- 

 tine or the ivory, / (Fig. 283), constitutes the greater portion of the tooth. 

 Produced by the inferior papilla of the dental follicle, and strongly depressed at 

 its superior extremity for the reception of the central enamel, it is everywhere 

 covered by the enamel. It forms the parietes of the pulp-cavity and surrounds 

 the point of the central enamel, at first projecting into its interior. 



Primitively not abundant, it does not delay to fill the cavity which it con- 

 tains in its interior by the addition of new layers which are deposited on the 

 surface of the old ones. This leads to a progressive atrophy of the pulp, which 

 extends gradually from the crown to the root. 



The more recent layers have a deeper coloration ; the tint of the obliterated 

 parts also allows, in transverse and longitudinal sections, the recognition of the 

 outlines of the internal cavity at the beginning. This cavity is obliterated at 

 first behind the central enamel, the distance which separates the latter from the 

 posterior face being less considerable than that which separates it from the an- 

 terior. Then the obliteration takes place in front. It is these layers of this new 



