Neither tushes nor incisors are known to be exposed to other accidents; it is, however, different with the molar teeth. These teeth consist of three components; bone or ivory constitutes the chief bulk of the organ, and over that is spread a thin covering of inorganic enamel, the whole being invested with a fibrous coating of crusta petrosa. The enamel is the material on which the tooth depends for its cutting properties; the manner in which the edge is preserved deserves attention, for the brick-layer's trowel appears to have been suggested by it. A thin coat of hard but brittle enamel is held between the two other bulky and tough substances, just as a thin layer of steel is protected by coatings of yielding iron in the house-builder's instrument. The highly organized crusta petrosa is often injured; to understand this, we must first comprehend the vast power which urges the jaw of the horse. The motion resides entirely in the lower portion of the skull, which is moved by strong, very strong muscles, going direct from their attachments to their insertions. No force is lost by the arrangement, and no less a motor power was required to comminute the hays and oats on which the horse subsists. The machinery seems to be admirably adapted to its purposes; and to be so strongly framed as to defy all chance of injury. Man, however, has a mighty talent for evil; it does not always suit the convenience of the groom to sift the pebbles from the grain; corn and stones are hastily cast into the manger, and the poor horse, having no hands to select with, must masticate all alike. The reader can imagine the wrench which will ensue, when a flint suddenly checks the movement of the molar teeth. The crusta petrosa is bruised upon the large fang of the tooth. Disease is established, and sad toothache has soon to be endured. [Illustration: A HORSE WITH TOOTHACHE.] Then there are the effects of the powerful acids in much favor with most grooms and too many veterinary surgeons; moreover, there are the sulphates, which in every possible form enter into veterinary medicine; the nitrates, likewise, are much esteemed, and are given in enormous doses. All of these much affect the crystalline enamel of the molar tooth; a small hole is first formed; into this the food enters and there putrifies; caries and toothache are the result. [Illustration: A HORSE QUIDDING, OR ALLOWING THE FOOD TO FALL FROM ITS MOUTH SUBSEQUENT TO MASTICATION. ] A horse with toothache upon certain days sweats and labors at its work; saliva hangs in long bands from the under lip; the countenance is utterly dejected; the head is carried on one side or pressed against some solid substance, as a wall. The food is "quidded"--that is, it is partially masticated, when, from acute agony, the jaws relax, the teeth separate, the lips part, and the morsel falls from the mouth, more or less resembling what is termed "a quid of tobacco."