In this little book—divided into thirty chapters, forming in all ninety-five pages—the author treats with great accuracy and in an admirable manner all that concerns the anatomy, physiology, and development of the teeth. Eustachius not only treasured up what ancient authors had written on this subject, but he himself made very long and patient researches and observations on men and animals, on living individuals as well as on corpses, and not only on adult subjects, but also on children of every age, on stillborn children and on abortive fetuses. The macroscopic anatomy of the teeth was brought by him to a high degree of perfection. Very wonderful, among other things, is the accuracy with which he studied and specified in several synoptical tables the number of the roots of molar teeth, and all the variations occurring not only in their number, but also in their form, length, etc. In Chapter IV, speaking of the means by which teeth are held in their sockets, Eustachius mentions in quite explicit terms the ligaments of the teeth. He begins by saying that the perfect correspondence between the dental roots and the alveoli, both in shape and in size, is one of the elements which contribute to the firmness of the teeth, since the alveolus, being exactly applied, on all sides, to the root or roots of the tooth, causes the latter, by this simple fact, to be fixed in a determined position. Also, the nerves inserted in each single tooth contribute, as was already the opinion of Galen, to the stability of these organs. “There exist besides”—Eustachius continues—“very strong ligaments, principally attached to the roots, by which these latter are tightly connected with the alveoli” (_adsunt præterea vincula fortissima radicibus præcipue adherentia, quibus præsepiolis arctissime colligantur_). Lastly, says the author, the gums, too, embracing the teeth at their exit from the alveoli, contribute to their firmness. And here Eustachius notes that in the joining of the gums to the teeth there is great analogy to that of the skin with the finger nails; a very proper observation, which makes us almost suppose that the perspicacious mind of Eustachius may have guessed the kindred nature of nails and teeth. In Chapter XV are related the researches made by the author to ascertain at what period the development of the teeth begins. Here is a passage of this chapter, almost literally translated: