OPERATIVE DENTISTRY.

THE ALVEOLAR PROCESSES AND ARTICULATION OF THE

TEETH.

For a description of the bones of the face the student is referred to works on anatomy.

By the form of articulation known as gomphosis, the teeth are implanted in the alveolar processes, portions of the maxillary bones, which in regard to their development, form and duration are entirely subservient to the teeth. These portions of the maxillary bones, with which the teeth have so intimate a connection, and which have so many important relations to operative dentistry, should receive special attention. These are best described as consisting of two plates of bone, an outer and an inner plate, the alveoli being formed by septa passing across between the plates.

The outer plate, continuous with the facial surface of the maxillary bone in the upper jaw, is very thin, and marked by eminences corresponding to the alveoli which contain the roots of the teeth, that over the cuspid being specially prominent, and known as the canine eminence, behind which is the canine fossa, and in front the incisive or myrtiform fossa.

The inner plate is much thicker and stronger and merges into the palate processes. After extraction of the teeth of the upper jaw, the thin external alveolar plate is absorbed more quickly and to a greater extent than the inner.

In the lower jaw the external plate is thick and strong, except in front, and after extraction of the teeth the absorption of the two plates is nearly uniform.

The socket for each tooth conforms exactly to the root or

roots which it contains, and consists of a thin shell of dense bone, which is surrounded by spongy tissue, and is continuous at the free margin with the dense cortical bone of the jaw. The walls are perforated by numerous foramina, and at the bottom of each alveolus or each division is a larger foramen for the transmission of vessels and nerves to the dental pulp and pericementum. The bony septa between the alveoli rise to a higher level on the teeth in both jaws than the outer and inner walls. This should not be forgotten in the fitting of bands and crowns to the roots of teeth.

OCCLUSION OF THE TEETH.

In the normal human denture the teeth are arranged in a parabolic curve; every tooth touches those at each side, and the cusps and cutting edges of all are on nearly the same level. The plane of the grinding surfaces of the molars rises a little, however, from the first to the third, so that, viewed from the side, a gentle curve is seen, with the concavity directed upward. The arch of the upper denture is a somewhat larger curve than that of the lower, and in normal occlusion the upper front teeth shut over and in front of those of the lower jaw. The bicuspids and molars of the upper jaw also project beyond the corresponding lower teeth, so that the buccal cusps of the latter, in the closure of the mouth, come between the buccal and lingual cusps of those above.

It will be seen, also, that no tooth in either denture is exactly opposed to any one in the other, but that in normal occlusion each tooth comes in contact with two in the opposite jaw, except the upper third molar, which, being smaller than the lower third molar, occludes with the posterior two-thirds of this tooth, and the inferior central incisor, which is opposed to the superior central incisor only.

By the arrangement thus briefly described the cusps of the bicuspids and molars of either jaw shut into the depressions between the cusps of the teeth in the opposite jaw, thus securing a firmer occlusion and greater efficiency in mastica

tion than could otherwise be obtained. Moreover, if any one tooth be lost, those in the opposite jaw will still retain a partial occlusion with the contiguous teeth.

THE DENTAL TISSUES.

Three kinds of calcified tissue-dentine, enamel and cementum-enter into the structure of a human tooth, and these, though not absolutely peculiar to the teeth, are appropriately called dental tissues.

Dentine incloses the pulp chamber and makes up the greater part of the tooth. That portion above the neck is invested with a layer of enamel of varying thickness, and is described as the crown of the tooth, while a layer of cementum covers the root and usually overlaps the enamel to a slight extent. If the two last-named tissues be entirely removed, the dentine will still show the general form of the tooth, reduced in size and with the extremities of the roots and cusps quite thin and pointed.

As to its physical properties, dentine is hard, dense and highly elastic, yellowish-white in color and somewhat translucent, fractured surfaces showing a silky lustre.

Chemical analysis of perfectly dry dentine shows it to consist of about 28 per cent. animal and 72 per cent. mineral matter, the latter being mostly phosphate of lime. About 10 per cent. of the fresh tooth is water. The constituents vary, not only in different individuals, but probably in the same individual at different ages and under different conditions.

The structural elements of dentine are the matrix, richly impregnated with calcareous salts, the dentinal tubes, and the soft fibrils. Each tube passes, with some variation in direction and curvature, from its opening on the wall of the pulp cavity toward the surface of the dentine. These tubes contain the soft fibrils, which are prolongations of the cells at or near the surface of the pulp.

Enamel.-Well-formed enamel is by far the hardest of all animal tissues, containing also the smallest proportion of or

ganic constituents-from 3 to 5 per cent. Nearly 90 per cent. is phosphate of lime.

Human enamel is translucent, pearly-white in color, with often a tinge of yellow.

In structure it consists of enamel fibers or prisms, hexagonal in shape and in very close apposition, if not in actual contact. The course of these is, in a general way, from the dentine outward, at right angles with its surface. Enamel has a definite cleavage, the line of fracture passing not between the prisms, but through their centres.

Cementum. This tissue is in all respects very much like true bone. It differs from bone in having lacunæ more variable in size and form, and canaliculi in greater number and of greater length. When the cementum is very thin, as at the neck of a tooth, it is apparently structureless.

Pericementum.-This membrane covers the root of the tooth, lying between it and the bony socket. It serves as a means of attachment and a medium of nutrition to the cementum, and also acts as a cushion to lessen the shock of occlusion. It consists of fibrous connective tissue, and is richly supplied with vessels and nerves. It is thicker near the neck of the tooth, where it is continuous with the gum and periosteum of the alveolar process. It is also thicker nearer the apex of the root. The direction of the fibers is obliquely across from the alveolar wall to the cementum, and, although the membrane varies histologically in different parts of its thickness, it is probably but a single membrane.

THE DECIDUOUS AND PERMANENT DENTITIONS.

Man, in common with most mammals, is provided with two sets of teeth, known as the deciduous and the permanent set. The deciduous teeth, which are adapted to the requirements of childhood, are fewer in number and, as regards teeth of the same class, smaller than their successors. They constitute the dental apparatus of the child from the time of their eruption to the age of six years, when the first permanent molars ap

pear. The teeth last named are thus accessory, as regards function, to the deciduous dentition, though anatomically they are quite distinct, and have relations entirely with the permanent set.

Formula of the deciduous dentition

The ten anterior permanent teeth in

each jaw replace the deciduous teeth, while the twelve molars appear posterior to

these and have no predecessors.

Formula of the permanent dentition—

i, c, pm, m = 1; = 32.

In both deciduous and permanent dentitions the teeth are symmetrical on the two sides of each jaw, and equal in number though not symmetrical in the two jaws.

Eruption of the Teeth.-The time of eruption of the deciduous and permanent teeth is subject to great variation, hence tables prepared by different observers do not agree. It is believed that the following tables are correct for the usual or average age at which the several classes of teeth appear:—

The lower teeth in each dentition appear a little earlier than

the upper.

Periods of Dentition.-A comparison of the above tables will show that the first dentition is completed at the age of two and a half years. These twenty teeth exercise their functions during the three or four years that follow; at six years of age the first permanent molars appear, and the child then has twenty-four teeth; during the six years between this period and the age of twelve the deciduous teeth are all replaced by their permanent successors, and the second permanent molars