The existence of the central chamber of the teeth appears to have been unknown to Galen, as he does not allude to it in the least. Vesalius was the first to put this most important anatomical fact in evidence. He expresses an opinion that the central cavity facilitates the nutrition of the tooth. He says, besides, that when a hole is produced in a tooth by reason of acrid corrosive humors, the corrosion, when once the internal cavity is reached, spreads rapidly and deeply in the tooth, owing to the existence of the said cavity, and sometimes reaches even the end of the root. In the chapter in which Vesalius treats of the anatomy of the teeth (Chapter XI, p. 40), two very well-drawn figures are found, one of which represents a section of a lower molar, showing the pulp cavity and its prolongation into the two root canals. The other represents the upper and lower teeth of the right side, in their reciprocal positions, and shows very clearly their general shape, the length of their roots, and the number of these. The changes which take place in the alveolus, after the extraction of a tooth have not escaped the notice of Vesalius. He says that after an extraction the walls of the alveolus approach one another, and the cavity is gradually obliterated. Aristotle had affirmed that men have a greater number of teeth than women. Vesalius declares this opinion absolutely false—although, after Aristotle, it has been repeated by many other ancient writers—and says that anyone can convince himself that the assertion of Aristotle is contrary to the truth, as it is possible for everybody to count his own teeth. In spite of this, we find the above-mentioned error even in writers subsequent to Vesalius; for example, in Heurnius (professor at Leyden toward the end of the sixteenth century), who expresses an opinion that rarely do women have thirty-two teeth, like men.