The commonest irregularity of the molar teeth is that in which the outer edge of the upper molars, and the inner margin of the lower ones, become ragged and protrude beyond the corresponding margins above and below respectively (fig. 122).
Fig. 122. - Undulating Irregularities of Molars.
Fig. 123. - Marginal Irregularities of Teeth.
a, Right side of upper jaw, showing (1) excavation of the bone and abnormal wear of the inner surface of the teeth.
B, Lower jaw, showing corresponding teeth similarly worn on the outer side as in fig. c, and excavation of the jaw (1) by the downward projection of the upper molars.
In quite young horses the tables of the molars are practically level, but in course of time they gradually take the direction of inclined planes. The reason why the edges are not kept down by wear is said to be that the upper jaw is wider than the lower one. It has been observed that individuals acquire a habit of grinding their food with a movement almost exclusively from right to left, and irregular wear of the teeth naturally follows as a consequence. When, from the presence of a decayed or malformed tooth, the motion of the jaw is limited and altered from its normal direction, irregularity is often observed to result at the edges.
Fig. 124. - Tooth-Rasp.
The lower molars invariably wear away faster than the upper, but it will be remembered that they are smaller to begin with; those in the middle of the jaw are sometimes worn quite low, while those at either end appear to have escaped a fair amount of work and remain prominent, the surface altogether presenting an undulating character.
Fig. 125. - Tooth-Shears.
We cannot alter the animal's manner of eating, but we can reduce the sharp edges of his teeth from time to time, and remove parts that project unduly. The former operation is accomplished with an instrument known as a tooth-rasp (fig. 124), and requires a considerable expenditure of energy on the part of the operator to perform it effectually. The latter may require the use of tooth-shears. Rasping may often be done without a gag, by simply drawing out the tongue on the opposite side to that upon which the rasp is to be applied. Many horses submit to the proceeding with less opposition, when secured in this simple manner, than they would do if twitch and gag were called into requisition. When the tooth-shears (fig. 125) are to be used, the animal will require to be under more thorough restraint, and is most favourably placed for the operation when cast.