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cut between the seventeenth and twenty-fifth year, but are often delayed until a very much later period, and occasionally never appear.
The set of teeth which, as indicated above, begin to appear in the infant about the sixth month, are known as the deciduous teeth (O.T. temporary, or milk teeth); whilst those which succeed them and form the adult equipment are the permanent teeth.
The deciduous teeth are twenty in number, and are named as follows in each jaw, beginning at the median plane :-dentes incisivi, or incisor teeth, central and lateral: dens caninus, or canine tooth; dentes molares, or molar teeth, first and second; or more briefly, two incisors, one canine, two molars. This is conveniently expressed by the "dental formula" for the deciduous teeth in man, which shows the number of each class of teeth above and below onD one side of the mouth, viz. :
i., c. 1, m. 2 = 20.
The permanent teeth, thirty-two in number, are named dentes incisivi, or incisor teeth, central and lateral; dentes canini, or canine teeth; dentes præmolares, dentis premolar (O.T. bicuspid) teeth; dentes molares, molar teeth; and are arranged as follows in each jaw, beginning at the median plane-central incisor, lateral incisor, canine, 1st premolar, 2nd premolar. 1st molar, 2nd molar, and 3rd molar or dens serotinus (O.T. wisdom tooth). The dental formula for the permanent set in man is thus:
i., c. 1, pm. 2, m. =32.
General Form and Structure.-A tooth consists (Fig. 880) of (1) the corona dentis or crown, the portion projecting above the gum. It varies in shape in the different teeth, and in all, except the incisors and canines, bears on its masticating surface a number of tubercles, the tubercula coronæ (O.T. cusps), varying in number from two to five in the different teeth; (2) the collum dentis or neck, the faintly constricted part which is surrounded collar-wise by the gum, and connects the crown with (3) the radix dentis or root, the portion of the tooth embedded in the alveolus of the jaw. In the majority of teeth. namely, in all except the molars, the root, as a rule, is single, or nearly so, consists of a long, tapering, conical, or flattened piece, perfectly adapted to the alveolus in which it lies. In the molar teeth (and in some of the others occasionally) the root is divided into two or three tapering or flattened roots or fangs. At the apex of each root there can be made out, even with the naked eye, a minute opening, the foramen apicis, through which the vessels and nerves enter the tooth.
Alveolar periosteum or root-membrane
FIG. 880.-VERTICAL SECTION OF CANINE TOOTH, to illustrate its various parts, and its structure.
When a section of a tooth is made (Fig. 880), it will be seen that the interior of the body is occupied by a cavity of some size, called the cavum dentis or tooth cavity, which is filled in the natural state by the soft and sensitive tissue known as the pulpa dentis or tooth pulp. The tooth cavity gradually narrows, and is prolonged into each root of the tooth as a slender tapering passage, the canalis radicis (root-canal), which opens at the apical foramen already referred to. Through these root-canals, which also contain some pulp, the vessels and uerves, which enter at the apex, pass to the interior of the tooth.
Short diverticula of the pulp cavity are prolonged into the bases of the tubercles
in the molar and premolar teeth, and in the incisors also there are similar slight prolongations of the cavity towards the angles of the crown.
The roots of the teeth are embedded in the alveoli or sockets of the jaws, to which they are accurately adapted, and firmly united (Fig. 880) by a highly etvascular layer of connective tissue-the periosteum alveolare. This is attached to the wall of the alveolus on the one hand and to the root of the tooth on the other, whilst above it is continuous with the connective tissues of the gum.
So accurately are the root and the alveolus adapted to each other over their whole extent, and so firmly does the periosteum bind them together, that, under normal conditions, the tooth is quite firmly fixed in the bone, and no movement of the root within the alveolus can take place; the vessels and nerves entering at the apex are thus secured against pressure or strain.
When, however, the alveolar periosteum is inflamed it becomes swollen and exquisitely sensitive; the tooth, as a result of the swelling, is pushed partly out of its socket, its crown projects above those of its neighbours, and strikes against the opposing tooth when the mouth is closed, giving rise to much pain and discomfort.
The neck, although the term is useful, can scarcely be recognised as a distinct constriction in the permanent teeth; it corresponds to the line along which the gum and alveolar periosteum meet, or along which the gum is united to the tooth; but, as already pointed out, the gum does not stop at the neck, but forms a free fold which surrounds the base of the crown collar-wise for a short distance. The outline of the margin of the gum opposite the labial and lingual surfaces of the crown is usually concave, but opposite the contact surfaces of the tooth it is convex, and reaches much nearer to the edge of the crown than on the other surfaces.
In the incisors and canines the tooth cavity, which is about to the diameter of the tooth, passes very gradually into the root-canal (Fig. 880), so that it is difficult to say where one ends and the other begins. The reverse is the case in the molars, whilst the premolars are somewhat variable in this respect.
Tartar is a hard calcareous deposit from the saliva (salivary calculus), often found on the teeth near their necks. It is composed of lime salts, and its deposit is largely determined by the presence of organisms (leptothrix, etc.) in the mouth.
The permanent teeth (Figs. 881 and 882) are thirty-two in number, sixteen above and sixteen below, or eight in each half of both jaws; and, although they can be grouped under four heads-incisors, canines, premolars, and molars-the individual teeth differ so much in their characters that each tooth requires a separate description.
Descriptive Terms.-Before describing the permanent teeth, it is requisite that certain terms which are employed to denote the surfaces of the teeth should be defined. This is a matter of some importance, seeing that the terms medial and lateral, anterior and posterior, cannot, owing to the curvature of the dental arches, be properly applied to all the teeth in the same sense. The terms given below have been adopted seeing that they are free from the danger of misconception.
The part of a tooth which comes in contact with the teeth of the opposite jaw is known as the facies masticatoria (grinding or masticating surface) (Fig. 883). The surface in contact with or looking towards its predecessor in the row is known as the facies medialis in incisors and canines, facies anterior in premolars and molars; the opposite surface, namely, that which looks towards its successor in the row, is known as the facies lateralis in incisors and canines, facies posterior in molars and premolars. The surface which looks towards the tongue is the facies lingualis (lingual surface), and that looking in the opposite direction, i.e. towards the lips and cheek, the facies labialis (labial surface). The portion of a tooth which touches its neighbour in the same row is known as the facies contacta (contact surface).
Dentes Incisivi (Figs. 881 and 882).-The incisor teeth, four in number in each jaw, are used specially for cutting the food, hence their name. of each is chisel-shaped, and presents a labial surface which is convex in all directions, a concave lingual surface, and a chisel-like edge, which, when first cut, is surmounted by three small tubercles separated by two grooves. These tubercles, however, are soon worn down, and the edge becomes straight or nearly 80. Owing to the fact that the upper incisors overlap those in the mandible, the
cutting edge is worn away, or becomes bevelled, on the lingual aspect in the former, but on the labial aspect or summit in the latter. The upper, and par ticularly the central upper incisors, are of large size, and slope somewhat forward whilst the lower incisors, all of nearly equal size, are much smaller-being
FIG. 881. THE PERMANENT TEETH OF THE RIGHT SIDE, LABIAL ASPECT. The upper row shows the upper teeth, the lower row the lower teeth. The wide vertical "labial ridge" is distinct on the upper canine and premolar teeth.
the smallest of all the teeth-and are placed vertically. The roots of the incisors are single, though a groove is occasionally seen on each side, suggesting a division The central upper incisors are very much larger than the lateral upper incisors (Fig. 881), but in the mandible the opposite is the case, the lateral incisors being slightly the larger. In all incisors
FIG. 882.-THE PERMANENT TEETH OF THE RIGHT SIDE, LINGUAL ASPECT. The upper row shows the upper teeth, the lower row the lower teeth. The cingulum is distinct on the upper incisors and both canines, the lingual tubercle on the upper lateral incisor and the upper canine,
the lateral angle of the crown is more rounded than the medial. The concave lingual surface of the crown in the upper incisors is usually limited towards the gum by a A-shaped ridge (Fig. 882), known as the cingulum. The two limbs of the A are continued up along the sides of the lingual surface, whilst the apex is turned towards the gum; and here, particularly in the lateral incisor, there is often developed a small lingual tubercle (Fig. 882). The cingulum.is rarely found on the lower incisors.
The roots of the upper incisors and canines are conical and rounded (the lateral incisors and canines not so distinctly as the central incisors) (Fig. 881), whilst those of the mandible are flattened from side to side (medio-laterally).
Dentes Canini.-In the four canine teeth, which succeed the incisors in each row (Figs. 881 and 886), the crown is large and conical, corresponding closely in general form to a very large central incisor with its angles cut away, so that the crown assumes a pointed or conical shape. The labial surface is convex, the lingual usually somewhat concave. The root is single and long, particularly in the upper canine, the root of which is longer than that of any other tooth, and produces the canine eminence on the anterior surface of the maxilla. The upper canines are larger than the corresponding lower teeth, behind which they bite; and they are sometimes known as the "eye teeth."
The upper canine presents on its lingual surface a well-marked cingulum, and often a distinct lingual tubercle; in addition, there is usually a median ridge running from the point of the crown to the apex of the cingulum, which is separated from the lateral part of the cingulum on each side by a slight depression. These points are neither so well marked, nor so constant, in the lower as in the upper canine.
Of the two margins sloping away from the apex of the crown, the lateral is the longer in both teeth. After it has been a little worn the lower canine is less distinctly pointed than the upper; its root is also more flattened. On the labial surface of the crown, of both canines and premolars, a wide low vertical ridge (labial ridge) can generally be made out (Fig. 882); it is most distinct on the canine and first upper premolar.
Sutura palatina transversa
Dentes Præmolares (Figs. 881 and 882). premolar teeth, teeth, eight in number, two in each jaw above and below, are placed posterior to the canines, and anterior to the molars, as the name indicates. The crown, which, unlike that of the incisors and canines, is flattened anteroposteriorly, is characterised by the presence of two tubercles (O.T. cusps) (Fig. 883). One of the tubercles, the larger, is placed on the labial, the other on the lingual side. The labial and lingual surfaces are both convex. The root is single, but it is, as a rule, flattened antero-posteriorly and grooved, showing in this a tendency to division, which often actually takes place in the first upper premolar. The upper premolars are easily distinguished by the fact that their two tubercles are large and are separated from one another by a distinct antero-posterior fissure (Fig. 883); whilst in the lower premolars, on the other hand, the separation between the two tubercles is not effected by a continuous fissure as in the upper teeth, but by two dimple-like depressions separated by a ridge which joins the two tubercles (Fig. 884). In the upper premolars, therefore, the two tubercles are separated by a fissure, in the lower they are united by a ridge.
FIG. 883. THE HARD PALATE AND UPPER PERMANENT TEETH,
VIEWED FROM BELOW.
The first upper premolar is often slightly larger than the second; the reverse is the case in the mandible. The labial surface of the crown is usually somewhat larger than the lingual surface in all premolars. As a general rule in the lower premolars the labial surface of the crown is sloped medially near the masticating surface. The first can usually be distinguished from the second by the fact that, while the lingual tubercle and surface are smaller than the labial in the first premolar, they are nearly of the same size in the second. In addition, the root of the first upper premolar is bifid or nearly so, and its labial ridge is fairly distinct, but is indistinct in the second. In the first lower premolar the lingual tubercle and surface are very small, in fact the tubercle is quite rudimentary. It should, however, be added that it is often extremely difficult to identify the various premolars.
Dentes Molares.-The molar teeth are twelve in number-three on each side
above and below-and are distinguished as first, second, and third molars. The last in each jaw is also known as the dens serotinus. All the molars are characterised by the large size of the crown and the possession of three or more trihedral tubercles on the masticating surface (Figs. 883 and 884). They are the largest of all the teeth, but they diminish in size, as a rule, from the first to the third In shape the crown is more or less quadrangular, with convex labial and lingual surfaces. The roots are either two or three in number, but frequently in the last molars they are united to a varying degree.
The molars of the maxilla and mandible differ so considerably in their further details that they must be considered separately. They may be most readily distinguished from one another by the fact that normally the upper molars possess three roots (Figs. 881 and 882), whilst the lower molars have two at most. The number of tubercles, though not so reliable a guide as the form of the root, is also generally sufficient to distinguish them. In the upper molars there are either three or four tubercles, whilst in the lower the number is most commonly five (see, however, below).
In the upper molars, the crown, viewed from the masticating surface (Fig. 883) is rhomboidal in shape (i.e. quadrangular with the angles not right angles). The labial and the lingual surfaces are convex. The number of tubercles is either four or three. On the first there are invariably four-two on the labial and two on the lingual side--the anterior lingual of these being connected with the posterior labial by an oblique ridge (Fig. 883), which is also found on the second and third molars when these bear four distinct cusps. The second upper molar has either four or three tubercles in about an equal proportion of European skulls, whilst in the third the number is much more frequently three than four. The roots in the upper molars are three in number (except, occasionally, when the three roots of the third are confluent), two being labial, and the third lingual (Figs. 881, 882, and 885).
In the lower molars, the crown, viewed from above (Fig. 884), is somewhat cubical. The labial and lingual surfaces are convex, as in the upper molars. The first, as a rule, bears five tubercles, two being on the labial side, two on the lingual. and the fifth behind and lateral, that is, between the two posterior tubercles and somewhat to the labial side. The second has usually only four tubercles; a fifth, however, is sometimes present. The third has either four or five, the former number more frequently than the latter.
The roots of the lower molars are two in number, each wide, grooved, and flattened antero-posteriorly. One root is anterior, the other posterior, and both are usually recurved in their lower portions (Fig. 885). As in the corresponding teeth of the maxilla, the roots of the lower last molars are often more or less united into a single mass.
The chief characters of the upper and lower molars may be summarised thus:The first molar is usually larger than the second, and the second than the third. The upper molars are directed downwards and laterally; whilst the lower molars, which the former partly overlap, slope upwards and medially, with the result that the labial tubercles of the lower molars lie in the groove separating the lingual from the labial tubercles of the (Fig. 886, p. 1120). As a result of this overlapping, the labial edge of the crown is sharp and the lingual edge rounded in the upper molars; whilst the lingual edge is sharp and the labial edge rounded in the lower set.
The fissures which separate the cusps on the grinding surfaces of the molar teeth are generally continued as faint grooves on the labial and lingual surfaces.
Upper Molars. The crowns, as already stated, are rhomboidal in shape, and when viewing their masticating surfaces, as in Fig. 883, if the planes of separation between them be prolonged. they would strike the median plane near the posterior part of the hard palate; in other words. their anterior and posterior surfaces are not in transverse but in oblique planes, sloping strongly postero-medially, and converging in that direction. A knowledge of this is useful in determining the side to which an upper molar belongs, as is the fact that the anterior labial roct is broader than the posterior (Fig. 882).
As regards the number of tubercles (Fig. 883):-The first upper molar has four tubercles in practically all skulls (99 per cent); occasionally, indeed, another, but very rudimentary, tubercle is present on the lingual side of the antero-lingual tubercle. The second molar has either three or four in an almost equal proportion of Europeans, but more frequently four, taking the teeth of all nations together. (According to Topinard, four are present in 66 per cent. of all races