But what is the use of so many arguments? exclaimed Eustachius. To remove even the slightest doubt and to put an end to any controversy on such a point, only one fact is sufficient, which is revealed to us by anatomical dissection, and that is, that the teeth which appear about the seventh year are not only not united to those which fall at the same period, but cannot even be in contact with them, owing to the presence of a thin osseous partition. In the following chapter[291] Eustachius speaks of the central cavity of the teeth and of the substance contained in it. In young teeth, he says, the dental cavity is very large, in proportion to the size of the tooth. According to some anatomists, the central cavity of a tooth is coated by a very soft and thin membrane, formed by a tissue of very small vessels and nerves; and besides, this cavity is filled with marrow, like hollow bones. The observations of the author, however, do not agree with these statements. The dental cavity does not contain any fatty substance analogous to the marrow of bones. As to the above-mentioned membrane, Eustachius doubts its existence. The large hollow existing in children’s teeth contains, he says, a mucous substance, somewhat hard, and very smooth at its surface—almost like a cuticle—but which has rather the appearance of a concretion than of a membranous tissue. At any rate, adds Eustachius, if the substance alluded to is made to dry up in the shade, it acquires an appearance not unlike that of a membrane. It is certain, however, that at an early age the substance contained in the dental cavity does not adhere to the walls of the latter after the manner of a periosteum, but is found in simple contact with the same, and can, therefore, be separated from them with the greatest ease. As years pass by, the dental cavity becomes narrower and narrower, because the substance contained inside the tooth gradually becomes ossified at the surface, adhering to the dental scale previously formed, in the very same manner as the internal or woody part of a tree adheres to the bark. Of the two hard substances which make up a tooth, the outer one is white, tense, and dense, like marble, the underlying one, instead, is somewhat dark, rough, and less compact. To observe accurately the above-mentioned facts, the author advises searching for them, first, in the molar teeth of the ox or the ram, and then in human teeth, and likewise, first in children or in recently born animals, and then in adults.