35(r 



THE DIGESTIVE AFPAMATUS IN MAMMALIA. 



158. 



two borders, of whicii the internal is always thicker than the external; 

 lastly, the surface of friction (table). The latter does not exist in the tooth, 

 which has not been used; but in its stead is found two sharp margins 

 circumscribing a cavity named the external dental cavity (or better, in- 

 fundibulum). This cavity terminates by a conical cul-de-sac, which 



descends more or less deeply into the sub- 

 stance of the tooth. The margins are dis- 

 tinguished into anterior and posterior ; the 

 last, less elevated than the first, is cut by one 

 or more notches which are always deepest in 

 the corner teeth. It is by the wear of these 

 margins that the surface of friction is formed, 

 and ill the centre of which the infundibulum 

 persists during a certain period of time 

 (Fig. 159, 2).^ 



The fang is perforated by a single aper- 

 ture, through which the pulp of the tooth 

 penetrates into the internal cavity (Fig. 

 159, 3, c). 



In the composition of the incisor teeth is 

 found the three fundamental substances of 

 the dental organ. The dentine (Fig. 159, 3, 6) 

 envelops, as has been shown, the pulp cavity. 

 That which is deposited in this cavity after 

 the complete evolution of the tooth, to replace 

 the atrophied pulp, has always a yellower 

 tint than the dentine of the first forma- 

 tion ; it forms on the table of the tooth the 

 mark designated by Girard the dental star 

 (Fig. 159, 4, c). The enamel covers the 

 dentine, not only on its free portion, but 

 also on the roots of the incisors ; it is not 

 prolonged, however, to their extremities. It 

 is doubled into the external dental cavity, 

 lining it throughout (Fig. 159, 3, a); and 

 when the surface of friction is established, 

 there can be perceived a ring of enamel 

 surrounding that surface, and an internal ring 

 circumscribing the infundibulum : the first 

 circle forms what is called the encircling 

 enamel; the second, the central enamel (Fig. 

 159, 4, a. h). 



In the virgin tooth, the latter is con- 

 tinuous with the external enamel, and passes 



THE DENTITION OF THE INEEEIOR ^.^^j. ^^^ ^^^g^^^ ^J^i^]^ cirCUmScribeS the 

 JAW OF THE HORSE, THE TEETH . j ii • j. ti i mi j 



SEEN ON THEiE TABLES. entrance to the miundibulum. The cement 



Consult figure 21 for the dentition of '^ applied over the enamel like a protecting 

 the upper jaw. varnish ; but it does not exhibit the same 



thickness everywhere : on the salient portions 

 it is extremely thin, and does not even exist when the tooth has been 

 submitted for some time to the friction arising from the contact of 

 the aliment, the lips, and the tongue. It is more abundant in depressed 

 situations, as in the longitudinal groove on the anterior face, and particularly 



