418 



THE DIGESTIVE APPARATUS IN MAMMALIA. 



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 longi- 

 tudinal groove on the anterior face, and particularly 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 



Fig. 230. 



Fig. 231. 



SECTION OF THE INCISOR TOOTH 

 OF A HOUSE, SHOWING THE 

 ARRANGEMENT OF ITS DIF- 

 FERENT SUBSTANCES. 



I, Dentine ; E, enamel ; c, ce- 

 ment. 



other hand we possess incisors unworn, or nearly 

 so, in which the cavity is almost entirely filled 

 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 ivear ; 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. 



All the characteristics just indicated belong to 

 the deciduous teeth (Fig. 232, 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 cvl-de-sac {wfun- 

 dibulum) is shallow ; and that they are not constantly pushed outwards from 

 their cavities, their growth ceasing when they begin to be used. When the 

 replacing teeth appear, they do so a little behind the temporary ones, the shedding 

 of which they cause by gradually destroying their roots, these at last becoming 

 only a long and very thin shell of dentine. 



The follicle in which the incisor teeth are developed shows only two papillae 



DENTITION OF THE INFERIOR JAW 

 OF THE HORSE, THE TEETH SEEN 

 ON THEIR TABLES. 



Consult Fig. 38 for the deiitition 

 of the upper jaw. 



