One can judge the age of many animals by their teeth. As the animal grows older, the teeth become darker in color, except in the case of the horse, whose teeth grow whiter with age. The last molars are cut by men and women about the twentieth year; but in some cases, and especially with women, they have been known to come forth—not without pain—very much later, even so late as at eighty years of age. The man has more teeth than the woman; this peculiarity is also to be found in the female of some animals (such as sheep, goats, and pigs). Individuals provided with many teeth generally live the longest, those instead who have fewer teeth (or simply far apart) are generally shorter lived. The teeth are generated by the nourishment distributed in the jawbone; they are, in consequence, of the same nature as bones. Their surface, however, is very much harder than that of the bones. The teeth, contrarily to all other bones, grow throughout life, so as to provide for their wearing away through mastication; and for this reason they lengthen when the antagonizing teeth are wanting.[98] The teeth differ from all the other bones, therein that they are generated after the body has been already constituted; they are, therefore, secondary formations; and precisely for this reason are able to be shed and to be renewed. Some of the veins of the head, says Aristotle, terminate with very slender branches inside the teeth.[99] The dental system of the monkey is altogether similar to that of man. The molar teeth exist in viviparous quadrupeds as well as in man; in the oviparous quadrupeds and in fish they are wanting. They serve to grind food, a function in which the lateral movements of the inferior jaw have, in many animals, a large share. For this reason, in animals who have no molars, these lateral movements do not exist. In birds, the beak takes the place of the lips and teeth; the substance of which it is formed is similar to that of the horn or the nails. In those animals which, instead of having all the teeth sharp, are furnished with incisors, canines, and molars, these three species of teeth are disposed in the same order as in man. The setting on edge of the teeth may be produced not only by eating acid things, but also simply by seeing them eaten. This sensation may be made to cease by the use of purslane and salt. In the book entitled _Problems_, many of which have reference to medical matters, one is to be found to the following effect: “Why do figs, when they are soft and sweet, produce damage to the teeth?” Perhaps, answers Aristotle, because the viscous softness of the fig causes small particles of its pulp to adhere to the gums and insinuate themselves into the dental interstices, where they very easily become the cause of putrefactive processes. But, he adds, it may also be that harm is produced to the teeth by masticating the small hard grains of this fruit.