The teeth, differing in part by their texture, from the other bones, have also a peculiar mode of nutrition which we shall now examine. But as the knowledge of this supposes that of the general structure of the teeth, it is proper to explain here that structure, referring their description to the examination of the bones of the face. _Organization of the Teeth._ The teeth are formed by two substances, one external, of a peculiar nature, called enamel, the other internal, which is the common base of it, and the texture of which is the same as that of the other bones. They have besides a cavity which contains a spongy substance, as yet but little known. _Hard Portion of the Teeth._ The enamel of the tooth is only seen around the crown; some anatomists have thought that it extended a little upon the root, an opinion founded no doubt upon the extreme whiteness that the root often has in detached teeth, and which makes it impossible to distinguish the line of demarcation. But a very simple experiment proves this demarcation; it consists in macerating the tooth in diluted nitric acid. The acid immediately attacks both the root and the crown which it softens; but the first becomes yellow like almost all animal substances exposed to its action, whilst the other preserves its colour, and even becomes whiter. This experiment also proves that their respective natures are essentially different. The enamel, thick on the top of the crown, grows thinner towards the root, an arrangement required by its use, which is to defend the tooth, to support principally the efforts of mastication, which are made especially upon the top of the crown. This substance hard, compact, particularly when it has remained a long time in the air, acted upon with difficulty by the file, is composed of very close fibres, the direction of which cannot be traced. The medullary oil does not appear to penetrate it; it does not burn, but breaks by the action of fire, and is thus separated from the other substance, which, exposed to heat, at first becomes black, then burns like the other bones and gives out the same odour.