120 THE CRYSTALLINE LENS. increased size of the jaw-bones, the formation of new teeth becomes necessary. The permanent teeth begin to appear about the sixth or seventh year, but -their crowns are formed at a very early period. Of the twenty temporary or milk-teeth, eight only are molares; the permanent teeth are thirty-two in number, and of these twenty are molares, or twelve true molares, and eight bicuspides. The dental substance of the first great molaris of the second set begins to be deposited towards the end of pregnancy. The sockets of the new teeth are gradually separated from those of the old, but the two cavities always communicate by a considerable opening through which the common portion of the external layers of the sacs passes. The change of the teeth commences aboiit the sixth or seventh year. The first great molaris is the first of the second set which appears, then follow the incisors and canine teeth, the penulti¬ mate molaris not till the thirteenth or fourteenth year, and the last molar between the sixteenth and twentieth year. ’ The roots of the milk-teeth are absorbed before they fall out. It has been frequently asserted that if the teeth of an animal, after being drawn, are re-inserted into their sockets, they will again take firm root. This appears to me very questionable. If a real organic union takes place in these experiments, it must be by the lacerated vessels of the pulp of the tooth uniting again with the vessels at the bottom of the socket. It is an interesting point which ought to be more accurately determined. A sure way of deciding the question would be, to feed animals, in which the teeth had been recently trans¬ planted, with madder. If any vascular connection had been formed, the innermost layer of the tooth towards the pulp would be coloured red. The teeth not being organised, fissures in them, of course, can¬ not be closed by the reproduction of new dental substance; they can, at most, be filled with crusta petrosa, or tartar from the salts of the saliva. In serpents new poison-fangs are being constantly repro¬ duced. The new teeth of the crocodile press forwards into the conical cavities of the old teeth, the posterior wall of which becomes absorbed. 3. The crystalline lens.—It would appear that, in certain cases where the lens has been removed, it is reproduced by the capsule— its matrix. Leroy d’EtioIe has observed this.* In the first case, thirteen days had elapsed since the extraction of the lens when the eye was examined; in the second case, thirty-three days; in the third, thirty-nine days; in the fourth, thirty-one days; in the fifth, forty-six days; and in the sixth, one hundred and sixty-five days. The ex¬ periments were made on rabbits, cats, and dogs. The contents of the capsule were either a crumbly mass, as in the second case; or a small lenticular body, as in most of the other cases; but in the sixth case a full-sized lens was found.t * Magendie’s.Journ. de Physiol. 1827, 30. f See Mayer, Graefe in Walther’s Journ. xvii. 1. Vrolik, ibid, xviii. 4. W. Soemmering, Boebachtungen über die organ. Veränderungen im Auge, nach Staaroperationen. Frankfurt, 1828. *