426 THE DIGESTIVE APPARATUS IN MAMMALIA. 



The free portion, flattened above and below, and thinnest and widest towards its anterior 

 extremity, presents two faces — an inferior or external, the other superior or internal , with 

 three borders, an anterior and two lateral. 



The external face, slightly convex, and milk-white in colour, is covered with fine, undulat- 

 ing, longitudinal striae, which disappear with age, and leave the surface beautifully polished 

 (Fig. 236, a). 



The internal face, flatter than the preceding, presents in its middle a slight conical 

 eminence, whose base widens and is terminated ne.tr the free extremity of the tooth, while its 

 sides are circumscribed towards each border by a well-defined groove (Fig. 236, a'). 



The two lateral borders (the internal slightly convex in its length, the external slightly 

 concave in the same direction) make the free portion appear as if thrown outwards. The 

 anterior border is sharp, and slightly convex from one side to the otlier ; it is the first part of 

 the tooth destroyed by wear. 



The root is rounded, slightly conical, and implanted in an alveolus of the same form ; in 

 youth, it shows at its extremity an opening communicating with an internal cavity analogous 

 to that in tlie teeth of Solipeds, and prolonged into the interior of the free portion (Fig. 236, b). 



In the virgin tooth, the enamel forms around the free portion a continuous layer, thinnest 

 on the internal surface, and extending very scantily over a part of the root. 



The dentine forms the remainder of the organ, and the (pulp) cavity, which is originally a 

 large space of the same form as tlie tooth, is filled, as the animal grows old, by new dentine, 

 which, as in the Horse, has a yellower tint than the primitive ivory. 



When the cavity is completely filled, the tooth ceases to grow, and is not pushed beyond 

 the alvenlu^f during wear, like the teeth of the Horse. 



The incisor tooth has scarcely arrived at its perfect development, before it begins to be 

 worn. Its horizontal position, and its coming in contact with the pad on the upper jaw, 

 exposes the anterior border and superior face to friction, and consequent wear from before to 

 behind. The wear, therefore, chiefly aff"ects this upper face, which really forms the table of 

 the tooth, and which Girard designates the avale. When use has worn away the conical 

 eminence and the grooves bordering it, the tooth is levelled. 



As wear goes on, there appears at first, and at the extremity of the tooth, a yellow band, 

 which is the dentine denuded of its enamel; and later, in this dentine a yellower transverse 

 band shows itself. With increase of wear, this contracts, then widens, and finislies by forming 

 a mark nearly square, and then round, which is nothing else tlian the recently formed dentine 

 that fills the pulp cavity of the tooth. It is a veritable dental star, analogous to that in the 

 Horse's tooth, and varying in form according to the incisor in which it appears. 



In proportion as the teeth are used, they seem to separate from one another, although they 

 still remain in tlie same phices. This is because these teeth, in youth, only touched each other 

 by their extremities, and as they became worn they decreased in width, and were necessarily 

 separated to an extent varying with their degree of wear. 



Finally, when the tooth has reached its last stage of wear, there only remains the root, the 

 upper poi-tion of which, becoming apparent by the retreat of the gum, stands as a yellow 

 stump— very distant from those which form with it the remains of the incisive arch. 



The first incisors (or milk-Ueth) of the Ox, like those of the Horse, are all deciduous, and 

 differ from those which replace them by their smaller volume, less width, the transparency of 

 their enamel, and their being mere curved outwanls. Their roots are much shorter, nnd are 

 destroyed by the succeeding teeth. The two temporary centrals are always separated l)y a 

 marked interval, depending on the thickness of the fibro-cartilage in the maxillary symphysis 

 during youth. 



Molars. — As in Solipeds, the molars are six in each side of the jaws, but they are much 

 smaller, and form a much shorter arch. Their reciprocal volume is far from being as uniform 

 as in the Horse, but goes on augmenting from the first to the sixth ; find to such a degree, 

 that the space occupied by the three anterior molars is only about one-half of that required 

 for the three posterior ones ; the last molar alone occupies nearly four times as much space, 

 lengthwise, as the first. 



Their wearing surface, constructed on the same system as that of the Horse's molars, 

 present eminences a little more acute. 



The arrangement of their three constituents is in principle the same as in the latter animal. 



As in the Horse, the three front molars are deciduous. 



The teeth of the Sheep and Goat are, like those of the Ox, thirty-two in number, divided 

 into eiglit incisors and twenty-four molars, to which are sometimes added supplementary 

 molars. 



The incisors of the smaller Ruminants are not disposed like a key-board, as in the Ox, but 



