358 



THE DIGESTIVE APPASATUS IN MAMMALIA. 



J Free 

 border 

 border 



ox's INCISOR TOOTH. 



portion, externtil face, outer 

 ; a', If)idj internal face, outer 

 ; b. Root ; u. Neck ; /, Anterior 



border ; g, g' Inner border. 



stituting the root, and separated from one another by a very marked constriction — the. 

 neck. This arrangement gives to the tooth the form of a shovel, the root representing 

 the handle (fig. 163). 



"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 tliree borders, an anterior and two lateral. 



'■ Tlie external face, slightly convex, and milk-white in colour, is covered with fine, 



undulating, longitudinal strise, which disappear 

 with age, and leave the surface beautifully 

 polished (fig. 163, a). 



" The internal face, flatter than the pre- 

 ceding, presents in its middle a slight conical 

 eminence, whose base widens anil is terminated 

 near the free extremity of the tooth, while its 

 sides are circiunscribed towards each border by 

 a well-defined groove (fig. 163, 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 con- 

 vex from one side to the other ; 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 analo- 

 gous to that in the teeth of Solipeds, and pro- 

 longed into the interior of the free portion 

 (flg. 163, 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 composes 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 



Tbeyond the alveolus 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 tace to friction, and consequent wear 

 -from before to behind. The wear, therefore, chiefly aft'ects this upper face, which really 

 . forms the table of the tooth, and which Girard designated 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 fiist, and at the extremity of the tooth, a yellow 

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

 transverse baud shows itself. With increase of wear, this contracts, then widens, and 

 finishej by forming a mark nearly square, and then round, which is nothing else than 

 ithe recently-formed dentine tliat 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 the same places. This is because tliese teeth, in youth, 

 only touched each other by their extremities, and as they became worn they decreased in 

 width, and necessarily became 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 portion of which, becoming apparent by tlie retreat of the gum, stands as 

 a yellow stump, very distant from those which form with it the remains of the incisive 

 arcade. 



"Theifii-st incisors (or milk-teeth) of the Ox, like those of the Horse, are all deciduous, 

 and ditfer from those which replace them by their smaller volume, less width the 

 transparency of tlieir enamel, and their being more curved outwards. Their roots are 

 much shorter, and are destroyed by the succeeding teeth. The two temporary pincers 

 are always separated by a marked interval, depending on the thickness of the fibro- 

 cartUage in ihe maxillary symphysis during youth." 



