The age, in beasts of burden,[243] is indicated by the teeth. In the horse they are forty in number. At thirty months it loses the two fore-teeth in either jaw, and in the following year the same number next to them, at the time that the eye-teeth[244] come. At the beginning of the fifth year the animal loses two teeth, which grow again in the sixth, and in the seventh it has all its teeth, those which have replaced the others, and those which have never been changed. If a horse is gelded[245] before it changes its teeth, it never sheds them. In a similar manner, also, the ass loses four of its teeth in the thirtieth month, and the others from six months to six months. If a she-ass happens not to have foaled before the last of these teeth are shed, it is sure to be barren.[246] Oxen change their teeth at two years old: with swine they are never changed.[247] When these several indications of age have been lost in horses and other beasts of burden, the age is ascertained by the projecting of the teeth, the greyness of the hair in the eyebrows, and the hollow pits that form around them; at this period the animal is supposed to be about sixteen[248] years old. In the human teeth there is a certain venom; for if they are placed uncovered before a mirror, they will tarnish its brightness, and they will kill young pigeons while yet unfledged. The other particulars relative to the teeth have been already[249] mentioned under the head of the generation of man. When teething first commences, the bodies of infants are subject to certain maladies. Those animals which have serrated teeth inflict the most dangerous bites.[250] CHAP. 65.—THE TONGUE; ANIMALS WHICH HAVE NO TONGUE. THE NOISE MADE BY FROGS. THE PALATE.