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TEETH [dens, a tooth; quasi edens, from edo, to eat); small bones fixed in the alveoli of the upper and under jaw. In early infancy, nature designs us for the softest aliment, so that the gums alone are then sufficient for the purpose of manducation; but, as we advance in life, and require a different food, she provides u$ with teeth. These are the hardest an^ whitest of our bones, and, at full maturi* ty, we usually find thirtytwo in botk jaws, viz. sixteen above, and as many below. Their number varies, indeed, i" different subjects ; but it is seldom seen tq exceed thirtytwo, and it will very rarelf be found to be less than twentyeight anove me, gums, ana its lang, or root, which is fixed into the socket. The boundary between these two, close to the edge of the gum, where there is usually a small circular depression, is called the neck of the tooth. Every tooth is composed of its cortex, or enamel, and its internal bony substances. The enamel, or, as it is sometimes called, the vitreous part of the tooth, is a very hard and compact substance, of a white color, and peculiar to the teeth. It is found only upon the body of the tooth, covering the outside of the bony or internal substance. When broken, it appears fibrous or striated, and all the stricB are directed from the circumference to the centre of the tooth. The bony part of a tooth resembles other bones in its structure, but is much harder than the most compact part of bones in general. It composes the inner part of the body, and the whole of the root of the tooth. Each tooth has an inner cavity, supplied with bloodvessels and nerves, wThich pass through the small hole in the root. In old people this hole sometimes closes, and the tooth becomes then insensible. The teeth are invested with periosteum from their fangs to a little beyond their bony sockets, where it is attached to the gums. This membrane seems to be common to the tooth which it encloses, and to the sockets which it lines. The three classes into which the teeth are commonly divided, are incisors, canine, and molars, or grinders. The incisors are the four teeth in the fore part of each jaw; they derive their name from their use in dividing and cutting the food in the manner of a wedge, and have each of them two surfaces, which meet in a sharp edge. The canine or cuspidati (eyeteeth) are the longest of all the teeth, deriving their name from their resemblance to a dog's tusk. There is one of these teeth on each side of the incisors, so that there are two in each jaw. Mr. Hunter remarks, that we may trace in them a similarity in shape, situation and use, from the most imperfect carnivorous animal which we believe to be the human speciesto the lion, which is the most perfectly carnivorous. The molars, or grinders, of which there are ten in each jaw, are so called, because, from their size and figure, they are calculated for grinding the food. The canine and incisors have only one fang; bat the three last grinders in the under jaw have constantly two fangs, and the same teeth in near tneir Dase. xne grinders HKewise differ from each other in appearance. The last grinder is shorter and smaller than the rest, and from its coming through the gums later than the rest, and sometimes not appearing till late in life, is called ivisdomtooth* The variation in the number of teeth usually depends on these wisdomteeth. The danger to which children are exposed during the time of dentition, arises from the pressure of the teeth in the gum, so as to irritate it, and excite pain and inflammation. The effect of this irritation is, that the gum wastes, and becomes gradually thinner at this part, till, at length, the tooth protrudes. In such cases, therefore, we may, with great propriety, assist nature by cutting the gum. These teeth are twenty in number, and are called temporary or milk teeth, because they are all shed between the age of seven and four teen, and are supplied by others of a firmer texture, with large fangs, which remain till they become affected by disease, or fall out in old age, and are therefore called the permanent, or adult teeth. Besides these twenty teeth, which succeed the temporary ones, there are twelve others to be added to make up the number thirtytwo. These twelve are three grinders on each side in both jaws; and in order to make room for this addition, we find the jaws grow as the teeth grow, so that they appear as completely filled with twenty teeth, as they are afterwards with thirtytwo. Hence, in children, the face is flatter and rounder than in adults. The denies sapientia, or wisdomteeth, do not pass through the gum till between the age of twenty and thirty. They have, in some instances, been cut at the age of forty, fifty, sixty, and even eighty years; and sometimes do not appear at all. Sometimes, likewise, a third set of teeth appears, about the age of sixty or seventy. The teeth are subject to a variety of accidents. Sometimes the gums become so affected as to occasion them to fall out; and the teeth themselves are frequently rendered carious by causes which have not hitherto been satisfactorily explained. The disease usually begins on that side of the tooth which is not exposed to pressure, and gradually advances till an opening is made into the cavity: as soon as the cavity is exposed, the tooth becomes liable to considerable pain, from the air coming into contact with the nerve. The enamel of the teeth, as we have al~ ready said, is very hard, but liable to be cracked by the pressure of very hard substances, or by exposure to great heat or cold, and, more peculiarly, by sudden changes from one to the other. The bony substance below, being thus exposed, begins to decay; the nerve and bloodvessels are at length laid bare, and toothache ensues. Rheumatism, gout, and venereal disorders, exert a very prejudicial influence on the teeth. To preserve the teeth, we must guard against too hot or too cold drinks; violent changes of temperature ; biting of very hard substances, as in cracking nuts, also biting off threads, and untying knots with the teeth, as the former injures the enamel, the latter tends to loosen the teeth in their sockets. Acids, of all sorts, particularly the stronger ones, injure the enamel. Therefore, all toothwashes which contain them are eventually prejudicial to the teeth, although the immediate effect is to clean and whiten them. Roughpointed substances also injure the enamel, so that we should avoid the use of metallic toothpicks, and toothpowder made of pumice stone, coral, cream of tartar, &c. People who eat much meat and little bread, or have a bad digestion, or smoke tobacco, find that a deposit of earthy particles collects around the teeth, and forms tartar, particularly about the parts which are least exposed to the action of the foodthe lower and inner parts, near the gums. The gums gradually separate from the teeth ; the consequence is, that these decay, and the breath is rendered offensive. To avoid these effects, the teeth should be daily cleaned with tepid water and a hard brush. A proper powder should also be occasionally applied to them. Where tartar has been formed, it should be removed by the dentist, and its return carefully guarded against. Decay can often be checked by the removal of the parts which have turned black, and filling the cavity with gold, so that the teeth may be preserved for many years or for life. Every one should have his teeth examined at intervals of a few months, to detect incipient decay. ^Artificial teeth are often inserted to remedy, as far as possible, the loss of the natural ones. These were formerly taken from the corpses of healthy men (though this point of healthiness was often far too little attended to): they are now, more generally, prepared from the teeth of the walrus or seacow, from ivory, from porcelain, &c. Artificial teeth are either secured in the stumps of natural ones, by means of a gold or silver support, or, where such stumps do not exist, they are fastened to neighboring teeth by gold or silk thread. The porcelain teeth have an advantage over the other kinds, which lose their color, and acquire a disagreeable smell, in the course of time. Then* hardness may, perhaps, however, make them injurious to the contiguous natural teeth. Besides the accidental means by which the teeth are affected, old age seldom fails to bring with it sure and natural causes for their removal. The alveoli fill up, and the teeth, consequently, fall out. The gums then no longer meet in the fore part of the mouth, the chin projects forwards, and, the face being rendered much shorter, the whole physiognomy appears considerably altered. The great variety in the structure of the human teeth, fits us for a variety of food, and, when compared with the teeth given to other animals, may, in some measure, enable us to explain the nature of the aliment for which man is intended by nature. Thus, in ruminating animals, we find incisors only in the lower jaw, for cutting the grass, and molars' for grinding it; in graminivorous animals, we see molars alone; and in carnivorous animals, canine teeth for catching at their prey, and incisors and molars for cutting and dividing it. But as man is not designed to catch and kill his prey with his teeth, we observe that our canine are shaped differently from the fangs of beasts of prey, in whom we find them either longer than the rest of the teeth, or curved. The incisors, likewise, are sharper in those animals than in man. Nor are the molars in the human subject similar to the molars of carnivorous animals: they are flatter in man than in these animals; and in the latter, we likewise find them sharper at the edges, more calculated to cut and tear the food, and, by their greater strength, capable of breaking the bones of animals. From these circumstances, therefore, we may consider man as partaking of the nature of these different classes ; as approaching more to the carnivorous than to the herbivorous tribe of animals; but, upon the whole, formed for a mixed aliment, and fitted equally to live upon flesh and upon vegetables. Those philosophers, therefore, who would confine a man wholly to vegetable food, do not seem to have studied nature. As the molars are the last teeth that are formed, so they are usually the first that fell out. This would seem to prove that we require the same kind of aliment in old age as in infancy. Besides the use of the teeth in mastication, they likewise serve a secondary purpose, by assisting in the articulation of the voice. Albin, Hunter, Blake, Fox, and many others, have written on the teeth.See, also, A. Serres, Essai sur VAnalomie et la Physiologie des Dents, ou JYouvelk Theorie de la Dentition (Paris, 1817). For zoologists, Cuvier's Des Dents des Mammiferes (Paris, 1825) is of much interest.