VII.

SHEDDING OF DECIDUOUS TEETH.

BY HENRY S. CHASE, M.D., D.D.S., ST. LOUIS.

THE physiological process by which the roots of the milkteeth are removed, so that their crowns may be easily displaced, thereby giving room for the teeth of replacement, is, to my mind, very simple and beautiful, although so seemingly obscure to those who have heretofore written on the subject.

It has been said that the roots are removed by absorption. This is a term that is often applied to very different physiological motions. Pressure of the advancing tooth of replacement on the roots of the temporary teeth has been said to cause their absorption. This may be so in some cases, but facts bear me out in the enunciation of the law which I stated several years ago; viz., "Absorption of the roots of the deciduous teeth takes place in the ratio of the advancement of the teeth of replacement, independently of their topographical relations."

Deciduous teeth lose their roots even though the teeth which replace them pierce the jaw at an abnormal distance. It cannot be said truthfully, that the deciduous teeth lose their roots at a certain age, independently of the development of the tooth of replacement. When the latter are never developed and do not appear, it is the rule that the temporary teeth are retained without losing their roots, and often for forty years or more.

A child at five and a half years ceased growing. No further development of mind or body took place, and at the age of

twenty-four years the temporary teeth were all retained, and no teeth of replacement had appeared.

But what is this process of absorption? The word has no histological meaning when applied to the movement which it is intended to explain. The change which causes the loss of the roots of temporary teeth is one of differentiation. It is a retrograde physiological process, the reverse of that which caused its development and growth. It is a process of disintegration, caused, not by chemical transformations, but a vital change of form in its microscopical character.

What initiates this change and carries it on? It is the tooth itself. Or it is that Presiding Power which called the tooth into existence. No foreign body has any control over it; no "carneous body," "gum," "blood-vessels," or "absorbents," take away the roots of these teeth, particle by particle. But when the mysterious Presiding Power calls upon the teeth to remove their crowns from the body, in obedience to that mandate commences the beautiful motions which I will now endeavor to describe.

Near the bottom of the alveolus the pericementum becomes thickened by a multiplication of its connective tissue-cells, and a conversion of the cementum-cells into connective tissuecells. A liquefaction of the lime and other carthy salts of the cementum-cells takes place before this differentiation; they are passed by osmotic action from cell to cell until they are carried into the capillaries, and from thence through the general circulation to the kidneys, and cast out in the urine. And this is of necessity true in regard to the differentiation of all the hard dental tissues; for we know it would be impossible for bone-cells, dentine-tubes, and enamel-cells to change their shape when encased in an unyielding armor of phosphate of lime.

Preceding the differentiation, there is always liquefaction of lime-salts. Cell after cell of cementum is changed into connective tissue-cells until the dentine-tubes are reached. Then the

cells of which these are composed are similarly metamorphosed. And so, too, with the enamel-cells of which its prisms are made up.

The differentiation of enamel does not take place to any great extent, because there is no physiological necessity for it, as the end to be obtained is merely a removal of the roots, which would ensure the shedding of the crown. Still the energy of the process is sometimes so great that its motions are carried beyond necessary bounds, and we find portions of the enamel removed high up on the crown.

While these active processes are at work on the exterior of the tooth, the dental pulp and its vessels within the crown and roots are not inactive. The pulp continually grows larger by the addition of cells to itself, differentiated from the dentinal tissue around it, until at last nothing is left of the crown of the tooth but a thin shell composed of enamel tissue only.

The horns of the pulp, which reach up into the cusps, enlarge by the metamorphosis as soon as the body of the pulp, and it would seem in some cases much more rapidly; as may be seen in some specimens of prematurely extracted crowns. The dental pulp itself is converted into a mass of simple cells, excepting its artery and vein, which are the last to be metamorphosed, for they are needed as carriers of food and effete materials, until towards the last of the process.

What becomes of this mass of highly active cells, into which the whole tooth has been changed? Those nearest the walls of the alveolus undergo still another change: processes shoot out from their sides, which thus connect cell with cell, and they become bone-corpuscles. These appropriate the lime-salts which they require from the blood, and thus perfect themselves as a part of the true maxillary and alveolar tissue.

And these gradual motions continue until the whole space lately occupied with cells is found to be bone. The completion of this process is not attained until after the expulsion of the

crown; for we find, on lifting the crown from its bed, that at its base is a mass of red cells, rising above the margin of the gum and forming a slight tumor, showing the hollowed crown to have been filled with it. A microscopical examination of the crown shows portions of it, which had been in contact with this mound of cells, to have the same histological character as the latter, a border decalcified and differentiated. Sometimes the interior metamorphosis of the crown goes on more rapidly than that which is taking place in the roots, and then the latter are retained in the jaw sometimes after the crown has fallen.

Suppose a temporary tooth is decayed and the pulp is dead? Then of course no vital changes take place in the crown, and the roots must do all the work, to liberate themselves and their heads What if the roots are themselves dead? Then of course no vital action can take place in them, or have any effect on them; and the only way they are removed is by chemical disintegration, or by displacement bodily by advancing tissues beneath and around them.

I have spoken of this process of shedding, as though no tooth of replacement was advancing or contiguous to its temporary predecessor. This was merely to simplify the process, and it occasionally actually happens. In the case of temporary molars, the following bicuspids are immediately beneath them, and often in close proximity to their roots. Then, as the dental tissues do not need an extended differentiation, those that are not required for a more or less permanent organization are immediately liquified, and pass into the circulation for removal.

I cannot for a moment entertain the opinion held by some, that the broken-down dental tissues are appropriated by the teeth of replacement to build up their own organizations. I believe that these are effete materials, and that freshly organized matter is necessary for the perfect nutrition of the permanent teeth.