When digestion is impaired, and acidity prevails in the stomach, the mucous secretions in the mouth also become altered in character, and by their incessant contact injure and even destroy the teeth. From this cause we often see the teeth in young people in a state of complete decay. They are in reality the subjects of chemical decomposition, and eaten away by the morbid secretions of the mouth; and hence, in such cases, we generally find the individual complaining of heat and soreness of the tongue, gums, and mouth, and occasionally of the teeth being “set on edge.” Considered as _living_ parts, the teeth require some additional care. In that capacity they are exceedingly apt to suffer from sudden changes of temperature. Being from their solidity rapid conductors of heat, their internal nerve speedily becomes affected by the alternations of temperature to which they are daily exposed, both in taking food and in the change from a warm to a cold atmosphere. It is a not uncommon practice, for example, to take a glass of cold wine or water immediately after finishing a plateful of very hot soup; and it is quite usual to take tea and coffee and every kind of meat as hot as they can possibly be swallowed--than which practices it would be difficult to imagine any thing more hurtful to the teeth. For the same reason, in going out at night from a warm room to the coil air, it is desirable to protect the teeth from the influence of the sudden change, by breathing through two or three folds of a silk handkerchief, or through a woollen _comforter_. When the teeth and lower part of the face are left exposed in such circumstances, rheumatism and toothach not unfrequently ensue from the direct impression of the cold air upon parts rendered more susceptible by the preceding heat. The great source of injury to the teeth, however, both in childhood and in mature age, is disordered digestion. If the health be good, and the stomach perform its functions with vigour, the teeth will resist much exposure without sustaining injury. But if these conditions fail, they will rarely continue long unscathed. It is almost always from the latter cause that, in infancy, teething so often gives rise to serious constitutional disorder. * * * * *