“I was going to tell you about them,” his uncle replied. “If the horse’s teeth had perfectly smooth surfaces, without any roughness to act as a grater, is it not true that in pressing and rubbing, each against the opposite tooth, they would simply crush the fodder or hay as you would crush it between two smooth stones without changing it into fine powder? Millstones, if they were polished like marble tables, would flatten the grain without making flour of it; they must be rough on the surface in order to seize the wheat during the grinding of the upper stone on the stationary lower one and to make it into powder. When by long use the surface is worn smooth, the stones are of no service until they are dented again with the hammer. Well, the folds of a horse’s teeth may be likened to the roughness of a millstone: they project a little above the general surface of the tooth, making a sort of coarse file that tears to pieces blades of grass or hay when rubbed by the opposite tooth.” “I think I see a danger threatening the herbivorous animal,” put in Jules at this point. “Those projecting folds must soon be worn down by rubbing against one another, just the same as the roughness on the millstone. If smooth millstones can’t make flour without being roughened again, no more can the herbivorous animal’s worn teeth go on grinding.” “That is provided for, admirably provided for, my boy. Everything in the world is arranged so that it can do its work: a wisdom that nothing escapes watches over the smallest details; everything, even to a donkey’s jaw, shows this to be so. Listen, and judge for yourselves. “There are two different substances in a tooth: one very hard, a little like glass and called enamel; the other quicker to wear out, but very difficult to break, and known as ivory. These two substances are combined in different ways, according to the animal’s diet. In the horse, the sheep, the ox, the donkey, and many other herbivorous animals the ivory makes up the main part of the tooth, while the harder substance, the enamel, extends in winding sheets throughout the former, projecting a little above its surface in a fold which varies in form in the different kinds of animals. So, then, it is the enamel, a substance as hard as a pebble, that composes the folds in the herbivorous animal’s teeth. From the rubbing of the lower teeth against the upper the ivory wears away faster than the enamel, so that the folds of the latter embedded in the mass of the tooth have their cutting edges brought above the general level as fast as required, and thus the grinding surfaces are kept in constant repair. You see how it is: in the donkey’s food-mill, for instance, the millstones re-roughen themselves as fast as necessary for the chewing of a thistle; the machinery is self-repairing even while at work.” “What you tell us, Uncle, is wonderful,” commented Jules. “I never should have guessed that such an arrangement was necessary for chewing a thistle.”