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Tooth

one of the hard bodies in the mouth, attached to the skeleton, but not forming part of it, and developed from the dermis or true skin.

True teeth consist of one, two, or more tissues, differing in their chemical composition and in their microscopical appearances. Dentine, which forms the body of the tooth, and "cement," which forms its outer crust, are always present, the third tissue, the "enamel," when present, being situated between the dentine and cement. The dentine, which is course, nearly at right angles to the outer surface. When a part of the primitive vascular pulp from which the dentine is developed remains permanently uncalcified, red blood is carried by "vascular canals" into the substance of the tissue. Such dentine is called vaso-dentine, and is often combined with true dentine in the same tooth, as, for example, in the large incisors of certain rodents, the tusks of the elephant, and the molars of the extinct megatherium. When the cellular basis is arranged in concentric layers around the vascular canals and contains "radiated cells," like those of bone, this is termed osteo-dentine: resembles true bone very closely. The cement always corresponds in texture with the osseous tissue of the same animal, and wherever it occurs in sufficient thickness, as on the teeth of the horse or ox, it is traversed like bone by vascular canals. The enamel is the hardest of all the animal tissues, and contains no less than 96.4 per cent. of earthy matter (mainly phosphate of lime), while dentine contains only 72 per cent. and cement and ordinary bone only 69 per cent. of earthy matter.

In a few fishes the teeth consist of a single tissue a very hard kind of nonvascular dentine. Teeth consisting of dentine and vaso-dentine are very com- TOOTH mon in fishes. In all fishes the teeth are shed and renewed, not once only, as in mammals, but frequently during the whole course of their lives. Tortoises and turtles, toads, and certain extinct saurians are toothless. Frogs have teeth in the upper, but not in the lower jaw.

Newts and salamanders have teeth in both jaws and upon the palate; and teeth are found on the palate as well as on the jaws of most serpents. In reptiles, as a general rule, the base of the tooth is anchylosed to the bone which supports it. The completion of a tooth is soon followed by preparation for its removal and succession, the faculty of developing new tooth germs being apparently unlimited in this class. The extinct Odontornithes are the only birds with teeth. Of mammals there are a few genera and species devoid of teeth. The 8 9 SECOND TEETH 11-12 16 -12-15 12-16 20-25

The figures refer to years after birth true ant-eaters, the pangolins, and the echidna are strictly toothless. It is only in the mammals that we have a wellmarked division of the teeth into the four kinds of incisors, canines, premolars, and molars. The teeth are so admirably adapted for the special purposes which they are called on to fulfill that it is generally easy, from a careful examination of them, to say to what class of animals they belong, and to draw various conclusions regarding the habits and structure of the class generally. Thus, in carnivorous animals the molars are not grinding teeth, but present sharp cutting edges, and those of the upper and lower jaw overlap each other, resembling a pair of scissors in their action. In insectivorous animals the molars have a tuberculater surface, with conical points and depressions, so arranged as to lock into each other. In frugivorous animals, living on soft fruits, these teeth are provided with rounded TOOTH tubercles, while in herbivorous animals they have a broad, rough surface, resembling a millstone.

Diseases of the Teeth. - Decay (caries) is by far the most common of the diseases which affect the teeth, and consists in a gradual and progressive disintegration of the tooth substance. The exciting cause of caries has been proved to be due to the action of micro-organisms producing lactic acid.

Decay is rarely met with on smooth surfaces exposed to the friction of food and the direct washings of the saliva.

It usually begins in some pit or groove in the enamel or between the teeth, such points forming a lodgment for the development of the organisms. Once the enamel has been penetrated the decay proceeds more rapidly, spreading laterally beneath the as yet healthy enamel and toward the pulp. Caries is most common in early life, by far the greater number of cavities making their appearance between the ages of 6 and 18. Pain may be felt soon after the enamel has been penetrated, or may be delayed until the nerve (pulp) has become almost or quite exposed. Ultimately the pulp becomes exposed when the pain increases and may become very violent, especially if the nerve be pressed on by food forced into the cavity by mastication. Should this take place the pulp becomes acutely inflamed and soon dies, when the pain may either cease or go on till an alveolar abscess is formed.

Periostitis and Alveolar Abscess. -Periostitis is an inflammation of the membrane (periosteum) which covers the roots of the teeth and lines their sockets.

It may be either general or local. When general the majority of all of the teeth are commonly involved. When the disease is local-confined to one or two teethit may result from a blow or some such injury, or it may proceed from an inflamed pulp; but by far the most common cause is the presence of a dead nerve, the poisonous products of which are liable at any time to cause violent inflammation at the end of the root.

Alveolar abscess may be defined as a suppuration around the root or roots of a tooth. It is of two varieties, acute and chronic. Its causes are those of periostitis, which precedes it, the continuous and throbbing nature of the pain indicating the formation of matter (pus) within the surrounding bone.

Exostosis is characterized by an increase in the thickness of the cementum, the external of the two hard tissues forming the roots of the teeth. Its forms vary from a small nodule or patch to a quantity sufficient to invest the entire TOOTH TOPAZ root or roots of the same or adjoining of an alveolar abscess the tooth will teeth. Exostosis may often cause neuralgic pains about the jaws. Impaction and Difficult Eruption of the Wisdom and other Teeth. It is not uncommon to find certain of the temporary teeth firmly set in the adult jaw, and occupying the place of the permanent ones. In such cases the permanent tooth is usually present in the body of the jaw, but it has been retarded in eruption by being too deeply imbedded in the bone. Impaction may also be due to an abnormal direction of growth.

Such teeth may appear late in life after all the others are lost, and the bone overlying them has been absorbed and so exposed them. When these cases do occur they are responsible for the popular but incorrect idea of a third set of teeth. An impacted tooth seldom gives rise to any trouble, unless it be an upper or lower wisdom, particularly the latter.

The cutting of these teeth is sometimes accompanied by distressing symptoms which may be protracted for months or years, unless they are removed by extraction of the tooth.

Absorption of the Alveoli.- The gradual wasting of the bone which surrounds and supports the roots of the teeth, accompanied by a simultaneous recession of the gums, is one of the changes which mark the approach of old age. This wasting may, however, occur in middle life without any visible cause, though the majority of such cases are due to chronic inflammation of the gums, with or without the deposition of tartar. Heredity, or the use of too hard a tooth brush, may likewise be accountable. The teeth most affected are the front ones; but the reverse of this is not unusual. The gums, especially in front, gradually recede and lay bare the roots; the teeth now become loosened and finally drop out.

Toothache is not so much a disease as a symptom. Its chief causes are mentioned under caries and alveolar abscess. When toothache is due to caries with or without simple exposure of the pulp, the attack is brought on by taking hot or cold, sweet or acid fluids, and is seldom of long duration. To afford relief in such cases as these, gently wash out the cavity with a solution of carbonate of soda; then, drying it carefully with a piece of cotton-wool, take a very small pellet of wool dipped in eucalyptus oil and place it in the bottom of the cavity; over this place a piece of cotton wool large enough to fill the cavity and saturated with the following solution: drachm of mastic in 1½ ounce of eau de cologne. This should be changed daily.

When the pain is caused by the forming be found insensitive to change of temperature but very susceptible to pressure. The patient now becomes feverish, and the pain, which is at first of a dull heavy character, becomes more intense, throbbing, and continuous, till pus has been formed and discharged through the gum. Provided the tooth is likely to prove useful and the patient cannot consult a dentist, the gum should be carefully painted with tincture of iodine, or the old-fashioned plan of placing a roasted fig over the root may be resorted to; at the same time it is well to give an aperient such as epsom salts, followed by a full dose of quinine-6 to 8 grains for an adult. Great relief follows this treatment, which is, of course, only temporary. the diseases of the teeth and gums might Hygienic Care of the Teeth. Many of be prevented or greatly retarded by proper attention to the cleansing of these organs. The implements best fitted for this purpose comprise the quill toothpick, waxed silk thread and brushes, with suitable powders. An excellent tooth powder is composed of precipitated chalk, 2 ounces; light magnesia, 2 ounces; oil of cinnamon, 8 drops; thymol crystals, 4 grains; otto of roses, 10 drops. The teeth should be brushed twice daily, in the morning and in the evening. The brush, used properly, should be pressed against the teeth and the handle rotated so as to make the bristles sweep vertically between and over them; this, coupled with an up-and-down motion, will thoroughly cleanse the interspaces; the inner surfaces of the back teeth are best cleaned in a like manner, while the corresponding parts of the upper and lower incisors are effectually reached by a vertical drawing movement. The brush should be of medium texture, and the bristles of unequal length, and not too closely placed.