608 



THE EXTERIOR OF THE HORSE. 



It is between these two papillary systems that the constituent parts, d, of the 

 tooth are deposited. They are represented at first by a thin lamella, hollowed 



internally and deeply depressed at 

 the level of its free extremity. 

 Later, the parietes of the dental 

 follicle, <?, are transformed into 

 the alveolar periosteum. This 

 much being said, let us view the 

 incisor at a more advanced period 

 of its development. 



A longitudinal section of a 

 tooth "still enclosed in its alveolus 

 shows us the relations which 

 exist between the two cavities 

 (Fig. 279). It is there seen that 

 the point of the enamel sheath 

 which surrounds the external 

 dental cavity approaches the pos- 

 terior face, but is not confounded 

 with it. Whence it follows that 

 the internal cavity is prolonged 

 around the periphery of this 

 sheath and leaves the latter abso- 

 lutely free. The case is different 

 in the aged tooth : the pulp cavity 

 at a later period becomes filled 

 up behind this projection and 

 remains only in front, as is seen 

 in Fig. 283. 



Three substances enter into 



the construction of an incisor tooth : one fundamental and two enveloping. They 

 differ in situation, thickness, position, hardness, and microscopical structure. 

 Let us rapidly examine them. 



Of the two enveloping substances, the most superficial has received the 

 name cement ; the other is the enamel. 



1st. The Cement, (7, (Fig. 283), forms a superficial coating directly applied 

 upon the enamel. It envelops the entire periphery of the tooth and is reflected 

 into the external dental cavity, at the bottom of which it forms a more or less thick 

 nucleus, according to the subjects and the teeth which are examined. There are 

 cases in which this layer is only 2 or 3 millimetres in thickness ; in others it reaches 

 10, 15, and even 20 millimetres. Generally, it is less thick in the superior incisors. 

 This fact has been determined, but it is not very long since that Messrs. Chau- 

 veau and Arloing, 1 in France, and Mr. Mayhew, in England, 2 have recognized its 

 practical importance concerning ' the determination of the age, to whose works 

 the reader is referred. It is perceived that the depth of the unfilled portion of 



FIG. 282. Schematic section of the dental follicle of 



an inferior incisor of a horse. 



a, superior papilla, enamel germ ; b, inferior pa- 

 pilla, dentine germ or dental pulp; c, parietes of 

 follicle; d, dentine. 



1 Chauveau et Arloing, Traite d'anatomie compared des aniinaux domestiques, 3e 6d., 

 Paris, 1879, p. 404. 



2 Mayhew, The Horse's Mouth, showing the Age by the Teeth, p. 32. 



