adapted to feed on fruits, grain, or flesh, as they are less pointed than those of the cat, and more pointed than those of the sheep.

217. The teeth are composed principally of two substances -the i'vo-ry and the en-am'el. The internal part of the tooth or the ivory, is harder and more enduring than bone, and forms the body of the tooth. The enamel is remarkable for its hardness, and varies somewhat in color with the age, temperament, habits, and manner of living of different individuals. When any part of the enamel is destroyed, it is never regenerated.

Fig 56. A side view of the body and enamel of a front tooth.

Fig 57. A side view of a molar tooth. 1, The enamel. 2, The body of the tooth. 3. The cavity in the crown of the tooth that contains the pulp. 4, A nerve that spreads in the pulp of the tooth. 5, An artery that ramifies in the pulp of the tooth.

218. Each tooth is divided into two parts, namely, crown and root. The crown is that part which protrudes from the jaw-bone and gum, and is covered by the highly polished enamel. The root, or " fang," is placed in the sockets of the jaw, and consists of bony matter. Through this bony substance several small vessels pass, to aid in the growth and

217. Give the structure of the teeth. What is said of the enamel ? 218. Into how many parts are the teeth divided? Describe the crown. The root. What vessels pass through the bony matter? What is their use?

also in the removal of the tooth. There are, beside these vessels, small white cords passing to each tooth, called nerves. (See fig. 57.) When these nerves are diseased, we have the toothache.

PHYSIOLOGY OF THE TEETH.

219. The use of the teeth is twofold. 1st. By the action of the incisors the food is divided, while the molars grind or break down the more solid portions of it. By these processes, the food is prepared to pass more easily and rapidly into the stomach.

220. In the mastication of food there are two movements of the lower jaw-the action by which the teeth are brought together, and the lateral motion. In the former, the food is cut or divided, the jaws acting like shears. This movement is produced by the action of two large muscles situated on each side of the head and face.

Observation. The muscles attached to the lower jaw are of great strength; by their action alone, some persons are enabled to bite the hardest substances. By putting the fingers upon the side of the head above and in front of the ears, and upon the face above the angle of the jaw, while masticating food, the alternate swelling and relaxation of these muscles will be clearly felt.

221. The lateral, or grinding movement of the teeth, is produced by the action of a strong muscle that is attached to the lower jaw on the inside.

Observation. Those animals that live solely on flesh, have only the cutting, or shear-like movement of the jaws. Those

How pro

219–222. Give the physiology of the teeth. 219. Give one of the functions of the teeth. 220. How many movements of the lower jaw in masticating food? What effect has the first movement upon the food? duced? What is the character of the masticating muscles? 221. How is the grinding motion of the teeth produced? What is said of the movements of the teeth in different animals ?

that use vegetables for food, have the grinding motion; while man has both the cutting and grinding movement.

222. 2d. The teeth aid us in articulating with distinctness certain letters and words. An individual who has lost his front teeth cannot enunciate distinctly certain letters called dental. Again, as the alveolar processes are removed by absorption soon after the removal of the teeth, the lips and cheeks do not retain their former full position, thus marring, in no slight degree, the symmetry of the lower part of the face. Consequently, those simple observances that tend to the preservation of the teeth are of great practical interest to all persons.

HYGIENE OF THE TEETH.

223. To preserve the teeth, they must be kept clean. After eating food, they should be cleansed with a brush and water, or rubbed with a piece of soft flannel, to prevent the tartar collecting, and to remove the pieces of food that may have lodged between them. Toothpicks may be useful in removing any particles inaccessible to the brush. They may be made of bone, ivory, or the common goose-quill. Metallic toothpicks should not be used, as they injure the enamel.

224. The mouth should be cleansed with pure tepid water at night, as well as in the morning; after which the teeth should be brushed upward and downward, both on the posterior and anterior surfaces. It may be beneficial to use refined soap, once or twice every week, to remove any corroding substance that may exist around the teeth; care being taken to thoroughly rinse the mouth after its use.

225. Food or drink should not be taken into the mouth when very hot or very cold. Sudden changes of temperature will crack the enamel, and finally produce decayed teeth.

222. What is another use of the teeth? 223-232. Give the hygiene of the teeth. 223. How can the teeth be preserved? By what means? 224. How often should they be cleansed? 225. What is said of very hot or cold drinks?

Observation. On this account, smoking is pernicious, because the teeth are subjected to an alternate inhalation of both cold and warm air.

226. The temporary teeth should be removed as soon as they become loose. If a permanent tooth makes it appearance before the first is removed, or has become loose, the milk tooth, although not loose, should be removed without delay. This is necessary that the second set of teeth may present a regular and beautiful appearance.

227. In general, when the permanent teeth are irregular, one or more should be removed. If the teeth are crowded and irregular, in consequence of the jaw being narrow and short, or when they press so hard upon each other as to injure the enamel, remove one or more to prevent their looking unsightly, and in a few months the remaining teeth, with a little care, will fill the spaces.

Observation. When it is necessary to remove a tooth, apply to some skilful operator. It requires as much skill and knowledge to extract teeth well, as it does to amputate a limb; yet some persons, who possess strong arms, will obtain a pair of forceps, or a tooth-key, and hang out the sign of "surgeondentist," although ignorant of the principles that should guide them.

228. It is not always necessary to have teeth extracted when they ache. The nerve, or the investing membrane of the root, may be diseased, and the tooth still be sound. In such instances, the tooth should not be extracted, but the diseased condition may be remedied by proper medication. There are many sound teeth, that become painful, as already mentioned, which are unnecessarily removed.

Why is smoking injurious to the teeth? 226. What remark respecting the temporary teeth? 227. What remarks respecting the permanent teeth? Do those persons that extract teeth require skill as well as knowledge? 228. Why should not teeth be extracted at all times when they are painful?

Illustration. Dr. H. M., of Belfast, Me., related to me that an individual in that vicinity had his teeth, (all of them sound,) on one side of the lower jaw, extracted by an ignoramus of a "tooth-puller," and this without any relief from pain. The disease was tic douloureux, which was relieved by Dr. M.

229. The preservation of the teeth requires that they be frequently examined. When a part of the enamel is removed, and a small portion of the body of the tooth has become carious, in many instances such teeth may be preserved from further decay by having them filled or "plugged " with gold foil. All amalgams, pastes, and cheap patent articles for filling, should be avoided, if you would preserve both the teeth and the general health.

230. The practice of cracking nuts with the teeth, or of lifting heavy bodies, and the constant habit of biting thread, should be avoided, as they finally destroy the enamel.

231. All acidulated drinks and mineral waters, that "set the teeth on edge," are injurious. All tooth-powders and washes that contain any article that is acid, corrosive, or grinding, should be banished from the toilet. Tobacco is not a preservative of the teeth. It contains "grit," which wears away the enamel ; beside, when chewed, it debilitates the vessels of the gums, turns the teeth yellow, and renders the breath and the appearance of the mouth disagreeable.

232. Healthy persons have generally sound teeth, while feeble persons have decayed teeth. For this reason, we should try to learn and practise the few simple rules that promote health.

Give an illustration of the removal of sound teeth. 229. How may decaying teeth be preserved? What should be avoided in the filling of teeth? 230. What practices should also be avoided? 231. What is said of acidulated drinks? What effect has the chewing of tobacco upon the teeth? 232. What is one reason for preserving health?