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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 on one side of the mouth, viz. :
i., c. 1, m. = 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. 1, 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, and 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 nerves, 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 vascular 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 a 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. The crown 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
III. Accessory Digestive Glands. The largest of these is the liver (hepar) which occupies the upper and right portion of the abdominal cavity, immediately below the diaphragm, and its secretion-the bile-is conveyed into the duodenum by the bile duct (ductus choledochus). The pancreas, next in size, lies across the front of the vertebral column, with its right end or head resting in the concavity of the duodenum, into which its secretion flows through the pancreatic duct. The salivary glands consist mainly of three large paired glands, parotid, submaxillary, and sublingual (glandula parotis, submaxillaris, and sublingualis), and their ducts, which convey the saliva, open into the mouth. The saliva is a mechanical lubricant, which facilitates swallowing and the movements of the tongue in speaking and masticating, and also plays an important part in the chemical processes of digestion.
1. Parts.-Rima oris, vestibulum oris, cavum oris proprium.
2. Boundaries.-1. Labia oris-their structure.
2. Bucca-their structure.
3. Palatum (palatum durum, palatum molle)-arrangement and structure. 4. Isthmus faucium.
3. Structures found in the Mouth.-Gingivæ, gums.
4. Glandulæ oris.-Buccal and salivary glands.
The mouth is the upper expanded portion which forms the first division of the alimentary canal. It lies between the maxillæ and mandible, bounded externally by the lips and the cheeks, and roofed in by the palate. It contains the teeth and greater part of the tongue; and the ducts of the salivary glands open
into it. The cavity is divisible into two portions, the vestibule and the cavity proper of the mouth. These are separated from one another by the alveolar ridges, gums, and teeth of the maxillæ and mandible. The cavity of the mouth narrows at the back to a slight constriction, marked by a vertical fold on each side, called the arcus glosso-palatinus (O.T. anterior pillar of the fauces), and between them the cavity of the mouth is continuous with that of the pharynx.
Rima Oris.-The aperture of the mouth is bounded above and below by the corresponding lips, which, by their junction at the sides, form the labial commissures. In a state of rest, with the lips in apposition, the rima appears as a slightly curved line, corresponding in length to the interval between the first premolar teeth, and in level to a line drawn across just below the It also shows the two palatine arches, and the pharyngo- middle of the upper incisor crowns nasal isthmus, through which the naso-pharynx, above, The shape of the rima varies with communicates with the oral portion of the pharynx, every movement of the lips, from the resting linear form, curved like the conventional bow, to a circular or oval shape when the mouth is widely open. or the "pursed-up" condition produced by the contraction of the orbicularis oris muscle.
Vestibulum Oris.-The vestibule of the mouth lies immediately internal to the
aperture of the mouth. It is that portion of the cavity which occupies the interval between the lips and cheeks externally, and the teeth and gums internally.
In the normal resting condition, when the mouth is closed and the lips and the teeth are in contact, its cavity is practically obliterated by the meeting of its walls, and it becomes merely a slit-like interval, with a narrow roof and floor formed by the reflection of the mucous membrane from the deep surface of the lips and cheeks to the corresponding gum. This reflection is interrupted in the median plane by a small but prominent fold of the mucous membrane, the frenulum, which connects the back of each lip to the front of the gum. The upper frenulum is the better developed, and is readily brought into view by everting the lip. The frenulum of the lower lip is not always present.
On the outer wall of the vestibule, opposite the crown of the second upper molar, upon a variably developed eminence, is placed the small opening of the duct
of the parotid gland, which conveys the saliva from the parotid gland to the mouth.
When the teeth are in contact the vestibule communicates with the cavity of the mouth only through the small and irregular spaces left between the opposing teeth, and posteriorly, on either side, by a wider but variable aperture between the last molars and the ramus of the mandible.
Advantage is some
times taken of the pres
ence of this aperture for the introduction into the cavity of the mouth of liquid food in certain cases-trismus, anchylosis, etc.-in which the jaws are rigidly closed.
On the outer wall of the vestibule, the anterior border of the masseter can be distinctly felt with the finger, when the muscle is thrown into a state of contraction. Still further back, the front of the coronoid process, bearing the lower part of the insertion of the temporal muscle, can also be made out. The spheno-mandibular ligament, which corresponds to, and is felt along with, the anterior border of the internal pterygoid muscle, is distinguishable as a pliant ridge when the finger is carried from the front of the coronoid process behind the last molar tooth into the cavity of the mouth.
In addition to the duct of the parotid, the ducts of numerous small glands which are embedded in the lips and cheeks open into the vestibule.
Under normal conditions, as pointed out above, the lips and cheeks lie against the teeth and gums, obliterating the cavity of the vestibule, and helping, with the aid of the tongue, to keep the food between the grinding surfaces of the molar teeth during mastication. In facial palsy, however, owing to the paralysis of their muscles, and particularly of the buccinator muscle, the lips and cheeks fall away from the dental arches, and allow the food to pass out from between the teeth and to accumulate in the vestibule.
Cavum Oris Proprium.-The cavity proper of the mouth is the space situated within the dental arches, extending backwards to the glosso-palatine arches (O.T. anterior pillars of the fauces). Its boundaries consist of a roof, a floor, and a margin, formed by the teeth and gums. The roof is formed by the hard palate and the anterior portion of the soft palate, while the floor is formed by the anterior part of the tongue in the middle, and on each side by the reflection of the mucous membrane from the side of the tongue to the mandible.
On each side of the tongue, and in front of it, when it is at rest, there is only a slit or sulcus between the tongue and the gums, into which the ducts of the submaxillary and sublingual glands open.
If, however, the tongue is raised, there is exposed a limited space to which the term sublingual space is more usually applied (Fig. 876).
The term "floor of the mouth," or sublingual region, is frequently applied to the muscular and other structures, especially the mylo-hyoid muscles, which fill in the interval between the two halves of the body of the mandible. These structures, with the hyoid bone, form the basis upon which the tongue and the mucous membrane of the sublingual space are supported, and they extend from the symphysis menti, in front, to the body of the hyoid bone, behind.
The sublingual region (Fig. 876) is covered by the mucous membrane between the deep surface of the gum and the inferior aspect of the tongue. When the tip of the tongue is raised the membrane forms in the median plane a prominent fold, the frenulum linguæ, stretching from the floor of the mouth to the inferior surface of the tongue. On each side of the frenulum, near its junction with the floor, there can be readily made out a prominent soft papilla, the caruncula sublingualis, on which the opening of the duct of the submaxillary gland (O.T. Wharton's duct) may be seen (Fig. 876). Running laterally and posteriorly, on each side, from this, Layer of muscle cut and occupying the greater part of the floor of the mouth, there is a well-marked ridge, plica sublingualis, due to the projection of the underlying sublingual gland. Most of the ducts of this gland open near the crest of the ridge on each side. There is also another fold, called the plica fimbriata, medial to each of the others, on the inferior surface of the tongue.
Duet of the submax
Openings of ducts of
FIG. 876.-OPEN MOUTH WITH TONGUE RAISED, AND THE
SUBLINGUAL AND ANTERIOR GLANDS EXPOSED.
When the mouth is closed, and respiration is carried on through the nose, the cavum oris is reduced to a slit-like space, and practically obliterated by the tongue coming in contact with the palate above, and with the gums and teeth laterally and in front. When the mouth is slightly open and the teeth nearly in contact, the tongue becomes somewhat concave or grooved along the median plane, and leaves a channel-like space between it and the palate, while it remains in contact with the roof and gums laterally. By depressing the hyoid bone together with the root of the tongue, the cavum oris can be increased to a considerable size even when the teeth are in contact. Finally, by the simultaneous descent of the mandible and hyoid bone with the tongue, and the ascent of the soft palate, the cavity is increased to its greatest dimensions (Fig. 874).
The sublingual gland of the left side has been laid bare by the removal of the mucous membrane; to expose the anterior lingual gland of the right side a thin layer of muscle, in addition to the mucous membrane, has been removed. A branch of the lingual nerve is seen running on the medial aspect of the gland. The profunda vein also is faintly
indicated on this side.
Labia Oris. The lips are the two movable folds, covered superficially by skin, and on their deep surface by mucous membrane, which surround the rima oris. The meeting of the lips at each side constitutes the commissure, and bounds the angle of the mouth (angulus oris). Laterally, they are prolonged into the cheeks, with which they are continuous. The junction of the lips and cheek is marked on the surface by the sulcus naso-labialis, which passes downwards and laterally from the margin of the nose towards the angle of the mouth, while the sulcus mento-labialis separates the lower lip from the chin. The upper lip presents on its superficial surface a well-marked vertical groove, the philtrum, bounded by two distinct ridges descending from the columella nasi (Fig. 876); inferiorly the groove