DOMENICO GAGLIARDI, professor of anatomy and of medicine at Rome, published an excellent work on the anatomy of the bones,[360] in which he occupies himself not only with the structure of bones, properly so called, but also with that of the teeth. He considers the enamel to be formed by parallel and contiguous fibers, coated, so to speak, by a concreted juice, _sui generis_, which acquires a much greater consistence than that of the bones. Gagliardi says that by rubbing teeth hard together, or striking them with a steel, he was able to extract sparks from them.[361] JEAN DUVERNEY (1648 to 1730), a celebrated French anatomist, wrote a good monograph[362] about the teeth. As different anatomists of the sixteenth century had already done, he examined many fetal jaws in order to study in them the formation of the teeth. In relating his observations, he says that he found in every alveolus a mass of soft viscous matter, having the form of the tooth that is to derive from it, and which may be considered as its nucleus. This nucleus is entirely surrounded by a membrane, which the author likens to that which surrounds the fetus, and to which he gives the name of _choroid membrane_. From the surface of the nucleus a gelatinous juice transpires, which, thickening in layers, forms the enamel and the rest of the tooth. The choroid membrane is abundantly furnished with nerves, and with blood and lymph vessels. Into the interior of the teeth penetrate vascular and nervous branches which serve to maintain its vitality. In fetal jaws one finds, besides the germs of the deciduous teeth, also those of the permanent ones. The “choroid membrane” does not follow the tooth when it issues from the alveolus; it remains instead within the latter, forming the peridental membrane. Duverney says that in old people the root cavity diminishes in so considerable a manner, and the vessels are so compressed that they almost entirely disappear. It is then that a period of decadence begins in the tooth, it is more feebly nourished, wears away more rapidly than hitherto, and becomes shorter. The author also speaks of senile atrophy of the jaws, especially of the alveolar processes. With regard to this, he observes that if in old age the lower jaw advances beyond the upper, this depends wholly on the disappearance of the alveolar border, which projected more in the upper than in the lower one. Duverney admits the existence of direct vascular relation between the gums and the teeth; because in the case of diseases of the gums it is rare not to find the teeth altered as well. From the point of view of the development and nutrition of the teeth, Duverney finds much analogy between the tusks of the elephant, the teeth, properly so called, the feathers of birds, and the hair of mammifera.[363]