Teeth, hard, calcified structures, attached to the upper and lower jaws of vertebrates and of a few lower animals, and used primarily for mastication. In some animals teeth serve other functions, such as gnawing, digging, and fighting. In the course of evolution various forms of teeth have developed, from the simple, scalelike rows of cone-shaped teeth found in sharks to the more complex structures common in mammals.
Human teeth also serve major functions other than chewing. The teeth are directly involved in the process of speech, acting as a brace against which the tongue presses in the formation of certain sounds. The teeth also affect the dimensions and the expression of the face, the appearance of which may be unpleasantly modified by the loss of neighbouring teeth or by any irregularity in tooth growth or colouring
Structure of a Tooth
The sensitive nerves and blood vessels at the centre of each tooth are protected by several layers of tissue, the outermost (the enamel) being the hardest substance in the body. Under the enamel, surrounding the pulp from crown to root, lies a layer of bonelike dentin. A hard tissue called cementum separates the root from the periodontal ligament, which in turn holds the root in place and cushions the tooth against the gum and jaw during the grinding, jarring activity of chewing.
Human teeth consist of an external portion, called the crown, and a root that is embedded within the jaw. The outer layer of the crown is composed of calcified tissue known as enamel—the hardest substance in the body. Inside the enamel is the dentine, a bonelike substance extending from the inner surface of the enamel into the jaw to form the root. Covering the dentine of the root is a thin layer of a hard tissue called cementum. The roots are held in place by elastic fibres that constitute the periodontal membrane, which extends from the cementum to a thickened layer of bone, known as the lamina dura, within the jaw.
The dentine of the crown encloses the pulpal chamber, which penetrates into the root as the root canal. Passing through the root-canal opening at the end of the root are blood vessels, nerves, and connective tissue, which fill the root canal and pulpal chamber.
The development of the tooth bud in the human embryo begins in the second month after conception. The tooth bud is composed of outer, or ectodermal, and inner, or mesodermal, tissues. The ectoderm calcifies into enamel rods that develop to cover the crown. Following the depositing of enamel, the mesoderm differentiates into the dentinal portion of the crown and the pulpal chamber. As the embryo develops, continuing calcification results in the formation of the root and a wide root canal, through which blood vessels, nerves, and connective-tissue elements extend into the pulpal chamber. As the tooth crown erupts and the root lengthens, the pupal chamber and canal become more constricted because of the continuous production of dentine by special cells within the pulp. The crown is pushed through the gum by an eruptive force
Deciduous and Permanent Teeth
In humans, one set of 20 teeth is produced for use during early jaw development; these are called the deciduous teeth or, more popularly, milk or baby teeth. A second set of 32 larger permanent teeth replaces the deciduous teeth as the jaw matures. As a result of the growth and enlargement of the jaw, the roots of the deciduous teeth separate, allowing space for the larger permanent teeth to develop between them. The pressure of developing permanent teeth causes the tissues of the jaw to resorb, or suck up, the roots of the corresponding deciduous teeth, leaving only the crowns. As each permanent tooth erupts, it dislodges the crown of the deciduous tooth.
The 20 deciduous, or baby, teeth in a child's mouth are gradually replaced by the permanent array of 32 teeth illustrated here. The 8 incisors (4 on the upper jaw and 4 on the lower) have a straight, sharp edge for cutting and biting. The 4 pointed canines are specialized for tearing. The 8 bicuspids, or premolars, missing among the deciduous teeth, have grinding surfaces, as do the 12 more massive molars (first, second, and third molars). The third molars, absent in some people, are called wisdom teeth.
Types of Teet
The crowns of the permanent teeth are of three general types: the incisors, the cuspids, and the molars. The front teeth, or incisors, are spade shaped to facilitate the cutting of food. In each quarter of the mouth is a central and lateral incisor. Behind the incisors are the three cuspid, or ripping, teeth. The first cuspid, or “canine”, situated directly at the back of the lateral incisor, has a single, pointed cusp. The two teeth at the back of the cuspid are called the bicuspids, as each has two cusps. Behind the biscuspids are the first, second, and third molars, which have a relatively flat chewing surface, permitting the grinding or milling of the food. In general, food is cut with the front incisal teeth, reduced in size by the cuspids and bicuspids, and ground to a digestible size by the molars. Human teeth are still evolving; dental scientists expect that third molars, or wisdom teeth, will eventually disappear as the human jawbone continues to shrink and as refined foods eliminate the need for extra molars.
Teeth are anchored in the jaw by their roots, which fit into sockets of the spongy bone. In a growing child, the roots of the deciduous teeth are gradually absorbed into the bone. When a baby tooth “falls out”, it is really just the loose crown, dislodged by an incoming permanent tooth, that emerges from the gum line. Left to right are incisor, canine, premolar, and molar teeth.
The sequence of tooth eruption in the upper and lower jaws occurs in an ordered pattern. Occasional irregularities in the sequence of eruption may cause faulty alignment of the teeth. In some instances, the deciduous tooth is not shed, or the succeeding permanent tooth may be lacking. In other cases the permanent tooth may be impacted in the bone and incapable of eruption; supernumerary, or extra, teeth may also be present. Misalignment, or malocclusion, of the teeth also may occur after eruption. Because the position of a tooth in the jaw is not static, the loss of a tooth may permit the adjoining teeth to tilt towards the empty space, and the corresponding tooth in the opposite jaw may continue to erupt into the space.
Such a shift in position is possible because the tooth is attached to the jaw by the short elastic fibres of the periodontal membrane. The teeth are subjected to a wide range of jaw movements, which are permitted by the ball-and-hinge joints in the human jaw. Normally, each tooth is afforded protection by the adjoining and opposite teeth, which serve to equalize the forces of jaw motion and prevent shifts in position. When severe malocclusion occurs, the teeth should be brought into position by a dental specialist known as an orthodontist.
Teeth are extremely susceptible to a process of decay (dental caries). The ever-present oral acidogenic bacteria react with the carbohydrates in the mouth to form acids capable of dissolving the enamel. The breakdown of the enamel permits other bacteria to penetrate the dentine. Decay in a tooth eventually produces a cavity, or hole, in the structure. Extension of the decay causes infection of the tissue within the pulpal chamber, resulting eventually in necrosis or abscess formation, which, if unabated, may affect the jawbone. The decay process is accompanied by the formation of putrefactive gases. If the opening into the pulp is obstructed, severe pain ensues as the pressure of the gases increases. In many instances, the tooth can be treated by root-canal therapy, which removes the infected material from the root canal. In severe cases, the tooth is extracted.
Because teeth, unlike most other organs, are not self-reparative, early dental treatment is required to prevent serious complications. The removal of decayed material in the tooth and its replacement with an inert filling material restores the tooth. Fillings may be made of gold, silver amalgam, porcelain, synthetic cement, or plastic. Sometimes damaged or diseased teeth are capped, that is, given a new crown or a cover of appropriate material.
Proper oral hygiene and periodic dental examination help to prevent dental disease. A well-balanced diet with a minimum of carbohydrates may minimize dental infections. Brushing the teeth after meals to remove food residues helps to reduce decay. Teeth should be brushed in the direction of their growth to avoid gum irritation.
Teeth with Dental Caries
This view of the inside of the mouth shows dental caries, or cavities, appearing as dark areas on the front teeth. The term dental caries refers to the destruction, or necrosis, of teeth usually by bacterial action resulting in a condition commonly known as tooth decay. Cavities have been filled on the rear teeth to prevent further tooth damage.
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