15. _The separate uses of the three component structures_, however, is not well shown in the incisors; for as the crusta petrosa is by a natural process removed, and the ivory is not of essential service in cutting the food, it might be supposed that the first was no more than a temporary covering to, and the last only a basement for, the enamel. When the mind, however, is directed to the observation of the molars or double teeth, the properties and uses of all become apparent. The sense of touch in the horse resides in the lips; with those organs he gathers together the food before he grasps it with the incisors. Delicate, however, as the animal's sense of touch is, the selection of the food is further aided by the sight and smell. The most fragrant, the softest, and the cleanest portions, therefore, are selected; but it would be too much to suppose, that no particle of dirt, sand, or grit, was ever taken into the mouth. That much is necessarily bitten, the incisors of those horses which pasture on sandy soils, afford sufficient proof, as such animals generally exhibit the anterior edges of those teeth considerably jagged or notched. In the best fields, the grass is never free from adherent impurities, and the manger, as well as the rack, is not always remarkable in that respect. All, however, that is gathered by the lips, or grasped by the incisors, passes to the molars to be comminuted and mixed with the saliva previous to being swallowed. The molars, in fact, are animal grindstones, and the different degrees of hardness which the three component substances display, by wearing unevenly, always keep the grinding surfaces irregular or sharp. The inequality of the grinding surfaces of the molars enables the horse to reduce the toughest fibre to a pulpy mass; but as many substances little less hard than the tooth itself must frequently be ground up with the food, the molars would, at first sight, appear to be subject to injury, especially as they have ten times the work of the incisors to perform, and the senses of touch, smell, and sight, cannot operate for their protection. Nature, however, has provided against the danger to which they appear to be exposed; for if, notwithstanding the guarding agency of the senses, the incisors are so often injured, the molars, blindly employed, and used when the full power of the horse's jaw is exerted, certainly needed some provision against fracture. The crusta petrosa gives all the security that could be desired; it encircles these teeth, dips into their fissures, and fills their infundibula, forming no small part of the substance of the molars. The subjoined wood-cut will better explain the manner in which the various substances are arranged. [Illustration: _Fig._ 1.] [Illustration: _Fig._ 2.]