354 THE DIGESTIVE APPABATUS IN MAMMALIA 



been worn, this frictional surface assumes a particular aspect, which will ha 

 indicated with most advantage to the student by examining the structure of 

 the molar. 



" The root, if examined a short time after the eruption of the free 

 portion, looks only like the shaft of the latter, without any appear- 

 ance of 'fangs, and has internally a wide cavity. It is not until the tooth 

 begins to be pushed from the alveolus and its crown to become worn, that 

 its fangs are formed ; these are at first hollow, and afterwards filled, as well 

 as the cavity of the tooth, by the formation of a new quantity of dentine. 

 From this time the fangs cease to grow ; but the tooth, constantly projected 

 beyond the alveolar cavity, allows the walls which inclose it to contract ; so 

 that, in extreme old age, it happens that the shaft, completely worn away, 

 instead of the tooth, leaves several stumps formed by the fangs. 



" The molars of the two jaws exhibit a variety of roots. In the molars 

 terminating the arcades, either above or below, or at the extremities of these, 

 there are three ; while the intermediate molars have four fangs in the upper 

 jaw, and only two in the lower. 



" The molars are separated from each other by theix imbedded portion, 

 particularly at the two extremities of the arcade ; an arrangement which 

 strengthens them by throwing the strain put upon the terminal teeth towards 

 the middle of the line." 



The structure of the molars resembles that of the incisors, though it is 

 much more complicated. The internal cavity is extremely diverticulated, 

 and enveloped by the dentine. The ejiamel is applied in a layer over it, and is 

 doubled in its external culs-de-sac exactly as in the incisors. There is also 

 on the table of the tooth which has been worn, an external covering of 

 enamel, and two circles, or rather two irregular polygons, of central enamel 

 circumscribing the two cavities. In the superior molars, these bands of 

 enamel represent a Gothic B, having a small appendix on the loop nearest 

 the entrance to the mouth. This figure is modified in the teelh of the 

 lower jaw, the enamel of the infundibuli being continuous, on the inner side, 

 with the external enamel. The cement is extremely abundant, and in the 

 upper molars its total quantity nearly equals that of the dentine ; it 

 accumulates in the culs-de-sac and on the external 

 '^' ■ covering of enamel, where it partially fills up the 



ilutings on the faces of the crown. Prolonged 

 steeping of a molar tooth in hydrochloric acid 

 easily permits the isolation of these elements. 



Owing to the arrangement above described, the 

 section of an adult molar tooth, naturally repre- 

 sented by the surface of friction (Fig. 161), 

 exhibits, outwardly, a layer of cement ; next, the 

 external enamel ; between this and the central 

 _ enamel, the dentine, always yellower, and sometimes 



TRANSVERSE SECTION OF A *^^®° ^^^°^ ^^. ^^^ middle; lastly, the enamel 

 horse's upper molar bands of the infundibuli, and the crusta petrosa 

 TOOTH. filling them. As these enamel bands are much 



A, External cement; B, Ex- harder than the other substances, they are worn 



DrTn1ei-nr:nLmd'?'=:'t: "^^ '}?^^^ ^""^ '^^"^ °"* '"^ ''^^'^^ °^ ^^^'^^ 



tei-nal crusta petrosa. ' ^"-^ table of the tooth has also, for this reason, the 

 appearance of a veritable mill-stone, and is admi- 

 rably disposed for the trituration of those fibrous substances on which the 

 animal usually feeds. 



