The permanent teeth are now rapidly growing, and preparing to take the place of the milk-teeth, which are gradually absorbed till they disappear, or are pushed out to give place to the two permanent central incisors, which, at a year and a half, will generally present the appearance indicated in Fig. 23, which shows the internal structure of the lower jaw at this time, with the cells of the teeth, the two central ones protruding into the mouth, the two next pushing up, but not quite grown to the surface, with the third pair just perceptible. These changes require time; and at two years past the jaw will usually appear as in Fig. 24, where four of the permanent central incisors are seen. After this the other milk-teeth decrease rapidly, but are slow to disappear; and at three years old the third pair of permanent teeth are but formed, as in Fig. 25; and at four years the last pair of incisors will be up, as in Fig. 26; but the outside ones are not yet fully grown, and the beast can hardly be said to be full-mouthed till the age of five years. But before this age, or at the age of four years, the two inner pairs of permanent teeth are beginning to wear at the edges, as shown in Fig. 26, while at five years old the whole set becomes somewhat worn down at the top, and on the two centre ones a darker line appears in the middle, along a line of harder bone, as appears in Fig. 27. [Illustration: Fig. 25. Three years past.] [Illustration: Fig. 26. Four years past.] Now will come a year or two, and sometimes three, when the teeth do not so clearly indicate the exact age, and the judgment must be guided by the extent to which the dark middle lines are worn. This will depend somewhat upon the exposure and feeding of the animal; but at seven years these lines extend over all the teeth. At eight years another change begins, which cannot be mistaken. A kind of absorption begins with the two central incisors, slow, at first, but perceptible, and these two teeth become smaller than the rest, while the dark lines are worn into one in all but the corner teeth, till at ten years four of the central incisors have become smaller in size, with a smaller and fainter mark, as seen in Fig. 28. At eleven the six inner teeth are smaller than the corner ones; and at twelve all become smaller than they were, while the dark lines are nearly gone, except in the corner teeth, and the inner edge is worn to the gum. [Illustration: Fig. 27. Five years past.] [Illustration: Fig. 28. Ten years past.]