On coming to speak of the teeth generated in extra-uterine life, that is of the permanent teeth, Fallopius relates having observed that they have their origin in the following manner: A membranous follicle is formed inside the bone furnished with two apices, one posterior (that is to say, deeper down, more distant from the surface of the gums), to which is joined a small nerve, a small artery, and a small vein (_cui nervulus, et arteriola, et venula applicantur_); the other anterior (that is more superficial), which terminates in a filament or small string, like a tail. This string reaches right to the gum, passing through a very narrow aperture in the bone, by the side of the tooth which is to be substituted by the new one. Inside the follicle is formed a special white and tenacious substance, and from this the tooth itself, which at first is osseous only in the part nearest the surface, whilst the deeper part is still soft, that is, formed of the above-mentioned substance. Each tooth comes out traversing and widening the narrow aperture through which the “tail” of the follicle passes. The latter breaks, and the tooth comes out of the gum, bare and hard; and in process of time the formation of its deeper part is completed. The author says that his long and laborious researches into the development of the teeth were carried out with great accuracy, and he is, therefore, in a position to give as absolute certainties the facts exposed by him. Indeed, the observations of Fallopius were, for the most part, confirmed by subsequent research. As to the “tail” of the dental follicle, it is identical with the _iter dentis_ or _gubernaculum dentis_ of some authors. Fallopius described it as a simple string, but later on this prolongation of the dental follicle has been considered, at least by some, as the narrowest part or neck of the follicle itself, that is, as a channel through which the tooth passes, widening it, on its way out, and precisely for this reason it has been called _iter dentis_ (the way of the tooth) or _gubernaculum dentis_ (helm or guide of the tooth). BARTHOLOMEUS EUSTACHIUS, another great anatomist of the sixteenth century, occupied himself in the study of teeth with special interest, and wrote a very valuable monograph on this subject. He was a native of San Severino, Marche (Italy), and a contemporary of Vesalius, Ingrassia, Realdo Colombo, and Fallopius; he died in 1574, after having immortalized his name through many anatomical discoveries and writings of the highest value. [Illustration: Bartholomeus Eustachius] His book on the teeth, _Libellus de dentibus_, published at Venice in 1563, is the first treatise ever written on the anatomy of teeth, and represents a noteworthy progress in this branch of study.