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178 THIRD PERIOD-MODERN TIMES

Fallopius' reference to the analogy between the development of teeth
and that of feathers was highly important, as a point of departure for
embryological researches which showed clearly the real nature of teeth,
thus destroying the mistaken idea—held by Galen and many other authors
—that these organs were bones.
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 sav, deeper down, more distant from the surface of the gums), to which
is joined a small nerve, a small artery, and a small vein {cm nervulus,
et arteriola, et venida applicaiitur); the other anterior (that is more super-
ficial), 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 develop-
ment of the teeth were carried out with great accuracy, and he is, there-
fore, 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 dcntis or gubernaciiliim 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 nar-
rowest 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 deutis (the way of the tooth) or guhernaciiliun
dentis (helm or guide of the tooth).
I^.'VRTHOLOMHUS 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 (ItaU), and a contemporary of Vesalius, Ingrassia,
Realdo Colombo, and Fallopius; he died in 1574, after having immortal-
ized his name through many anatomical discoveries and writings of the
highest value.
His book on the teeth, Lihellus de detitihus, published at Venice in 1563,