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The teeth

The teeth are hard conical structures, whose roots are embedded in the alveoli of the jaws. The portion of the tooth surrounded by the gums is called the neck (collum dentis), while the portion projecting into the oral cavity is designated as the crown (corona dentis).
The three chief constituents of a tooth are the enamel (substantia adarnantina), the dentine (sub- stantia eburnea), and the cement (substantia ossea). The enamel is found only in the crown, while the cement is present chiefly about the root, although it forms a very thin layer about the neck of the tooth, where the enamel and the dentine become thinner. The enamel has a white glistening surface with a bluish or yellowish shimmer, while the root of the tooth is slightly yellowish and dull.
In the crown of each tooth there may be recognized a masticatory surface directed toward its fellow in the opposite jaw, a labial or buccal surface directed toward the lip or the cheek respectively, a lingual surface in relation with the tongue, and two contact surfaces in apposition with the neighboring teeth.
The root of the tooth is single or multiple and is generally conically shaped. At its apex is a foramen, the external orifice of the canal of the root, which extends throughout the length of the root, and in the region of the neck gradually dilates to a large cavity situated within the crown, the tooth cavity, also known as the pulp-cavity because it is filled by a soft non-calcified tissue, the dental pulp. The shape of the pulp-cavity is in general a reproduction of that of the entire tooth, but it not infrequently presents fine irregular ramifications.
The entire set of teeth, thirty-two in number in the adult, is known as the denture, and is arranged in an upper and a lower row, the superior and the inferior dental arch. The upper row is fixed in the alveoli of the superior maxilla and the lower row in those of the mandible, the form of articulation being that known as gomphosis. The bone and the tooth are separated by a thin layer of tissue common to both structures, the alveolar periosteum, which in the vicinity of the neck of the tooth is also designated the circular ligament. The teeth of the two rows resemble each other in shape and size, although the similarity is not absolute, and the number of teeth in each row is the same, namely, sixteen.
The teeth of each jaw are divided into four different groups according to their shape, the incisors, the canines, the premolars, and the molars, and each of the four varieties possesses such well-defined characteristics that transitional types do not occur, although differences are observed between individual teeth of the same group, particularly between those of the upper and lower jaw.
In each jaw there are four incisors, two canines, four premolars and six molars, the arrangement of the individual groups being the same in both jaws. The incisors are placed most anteriorly, the two central ones being in contact in the median line; then come on either side a canine, two premolars, and three molars, these last being situated most posteriorly.
The incisor teeth have flat, chisel-shaped crowns, convex on the labial surface and concave on the lingual surface, and thicker but narrower at the base and broader and thinner at their free margins. Upon the labial surface are three longitudinal ridges which are not always distinctly marked, and upon the cutting-edge of a recently erupted tooth these ridges terminate as small projections which rapidly disappear as a result of attrition. The inner corners of the cutting-edge are usually sharper than the outer ones, these being generally rounded off.
The crowns of the incisors lie in the frontal plane and present an inner and an outer surface of contact. The roots are rounded, of average length, and usually exactly straight; those of the lateral incisors are somewhat shorter and slightly flattened.
The upper incisors are always larger than the lower, and the upper central incisor is always larger than the lateral, but in the lower jaw this relationship is reversed. The relative size of the incisor teeth is subject to marked individual variations.
The canine teeth are situated between the incisors and the premolars and are of an elongated conical form. Their large, thick, irregularly conical crowns are approximately in the frontal plane, so that they present a labial and a lingual surface, and an inner and an outer surface of contact. The labial surface is markedly convex, and the lingual is characterized by a slight elevation. The roots are very long and conical, although they are distinctly flattened, particularly in the lower jaw. In consequence of their long roots the canines are the longest teeth of the entire dentition, and, moreover, their crowns are higher than those of the other teeth. The cusp of the crown is blunt and not exactly in the middle of the tooth, but some-what nearer its inner side.
The premolars are characterized by bicuspid crowns which are flattened from before backward, and consequently present an anterior and a posterior contact surface, a convex lingual surface, and a larger convex buccal surface. The cusps or tubercles are separated by an almost sagittal groove in such a manner that the lingual cusp is smaller than the buccal one; indeed, the lingual cusp of the lower first premolar is usually poorly developed, but that of the lower second premolar is usually double, so that this tooth, which is usually the largest of the premolars, is frequently tricuspid.
The roots of the lower premolars are always single, of medium length, and distinctly flattened.
Those of the upper premolars vary considerably; that of the first is usually double or at least bifid, while that of the second is generally markedly flattened or furrowed and usually possesses a double root canal.
All the molars possess a number of cusps and roots, and their crowns are low and characterized by their large circumference. The number of the roots and the position of the cusps are different in the upper and lower jaw, the upper molars being usually somewhat smaller than the lower and having three roots, while the lower ones have but two. The first molar in each jaw has the largest and highest crown, while the third has the smallest and the lowest, and, consequently, as a rule; the first lower molar is the largest of the group.
The cusps of the molar teeth are four, rarely five in number, two being lingual and two buccal. In the lower molars the four cusps are separated by a tolerably regular cruciform groove, and since the Ungual cusps are higher than the buccal ones, the lower molars look as though they were composed of two fused premolars. The lower first molar usually has five cusps, three buccal and two lingual. In the upper molars the buccal cusps are higher than the lingual and the separating sulci have the form of a slanting H, so that the lingual and buccal cusps hold an oblique relation to each other. Irregularities in the number and arrangement of the cusps are common, particularly in the third molar (wisdom tooth, see below), which may have from three to five cusps. As in the case of the premolars, the frontal surfaces of the crowns of the molars are in relation with each other, so that an anterior and a posterior surface of contact may be recognized. Both the lingual and the buccal surfaces of the molar crowns are convex, and both surfaces of the upper molars (at least of the first) have a longitudinal sulcus, while in the lower molars only the buccal surface presents this marking.
The lower molars have two roots, an anterior and a posterior, which are sometimes grooved.
They are of moderate length, compressed in the frontal plane, and their apices are usually bent backward. The grooves are an indication that each root is formed by the fusion of two halves, and in rare instances more than two roots may consequently be present.
The upper molars have three conical roots the ends of which are also bent backward. Two are buccal and one is lingual (or palatine, i. e. } directed toward the palate, posterior). All three roots are well developed in almost all cases in the first upper molar, while they may be more or less fused in the second. The latter condition is the rule in the third.
The third molars do not make their appearance until from the twentieth to the twenty-fifth year, and have consequently been called the "wisdom teeth" {denies serotini). They are only rudimentary structures in civilized nations, but in ancient skulls and in those of many savage races they are well developed and frequently but slightly smaller than the second molars. The upper wisdom tooth is always much smaller than the lower, and its roots are usually fused together, although the original number is frequently indicated, particularly by the number of the root-canals. There are frequently only three cusps present. The lower wisdom tooth usually has two short roots and the crown seems better developed than that of the upper jaw.
The upper teeth, particularly the front ones, are normally directed slightly outward; those of the lower row slightly inward, so that the somewhat larger superior dental arch overlaps the smaller inferior one throughout its entire circumference. When the dental arches are in apposition (the so-called position of articulation of the teeth) every tooth is opposed to two teeth of the other jaw, except in the cases of the upper third molars, which are in contact only with the lower third molars; this condition is due to the front teeth of the upper row being considerably wider than those of the lower.
In contrast to the thirty-two permanent teeth forming the adult dentition is the deciduous or "milk" dentition of childhood, which contains but twenty deciduous teeth, namely, eight incisors, four canines, and four molars.
The deciduous incisor and canine teeth, although smaller, correspond to those of the permanent denture not only in number but likewise in peculiarities of shape. They are also found in the same situation as the permanent teeth of the same name, while the deciduous molars appear at the site of the subsequent premolars. They resemble the permanent molars in having several roots, and in being provided with several cusps. The second (posterior) milk-molars are always larger than the first. The upper molars, like those of the permanent dentition, have three roots, two buccal and one lingual, but these roots present a marked tendency toward fusion. The lower molars have two roots, and the crowns of both the upper and lower teeth have from four to five irregularly situated cusps.
The teeth of the lower jaw erupt normally before those of the upper.
The lower central incisor makes its appearance usually in the sixth or seventh month and is soon followed by the corresponding tooth in the upper jaw (in the seventh to the eighth month). The lateral incisors usually erupt in the eighth to the twelfth month, and the lower first molars in the twelfth to the sixteenth month, followed several months later by those of the upper jaw. After the first molars come the canines (sixteenth to twentieth month) and finally the second molars (twentieth to thirtieth month).
The milk-teeth are gradually replaced by the permanent dentition, so that during a certain period in childhood teeth of both sets may be seen alongside of each other, and at this stage the jaws consequently contain a large number of teeth in different stages of development. The first permanent tooth to erupt is the lower first molar, which makes its appearance from the fifth to the eighth year and is speedily followed by the corresponding tooth in the upper jaw. The deciduous central incisors are not replaced until the sixth to the ninth year, and the lateral from the seventh to the tenth year. The first premolars erupt from the ninth to the thirteenth year, the permanent canines from the ninth to the fourteenth year, and the second (posterior) premolars from the tenth to the fourteenth year, these last being almost immediately followed by the second molars. The third molars frequently do not make their appearance until quite late (the sixteenth to the fortieth year). The upper premolars usually erupt before the lower, but with this exception the teeth of the lower jaw always appear first.
The crown of the tooth is formed first and the so-called enamel-organ takes part in its formation only, since it alone possesses enamel. The roots are formed by the dental papillae and are not complete when the eruption of the tooth occurs and the cement and the circular ligament are formed by the wall of the dental sac. During the development of the tooth the pulp-cavity and particularly the root-canals are relatively large.
The roots of the deciduous teeth are eventually absorbed by the action of osteoclasts, and their crowns either fall out or are broken off mechanically.
Dental anomalies are not common, although supernumerary teeth may be observed and normal teeth may be wanting. The upper lateral incisors are most frequently absent, in which case the central incisors are correspondingly enlarged, and supernumerary teeth most frequently occur in the incisor set. Anomalies of position are common. Very rarely there is observed the beginning of a third dentition.

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