The hard part of the tooth consists of enamel, dentine, and cement. The Enamel, containing very little organic matter (only about 2 or 3 per cent.), is for the most part passive, and presents great mechanical resistance to destructive processes. In its original formation it is a superadded structure, derived from the epithelium of the surface. The Dentine, consisting of tubules with a hard matrix, possesses a much larger proportion of organic matter (28 per cent.), and is much more directly involved in morbid processes. The Cement, which covers those parts of the tooth which are devoid of enamel, consists of true bone, and is liable to changes of a similar nature to those of bone.

Turning to the soft parts, the Pulp fills the cavity of the tooth, which cavity has in general the shape of the tooth. The pulp is a highly vascular tissue, and richly supplied with nerves whose filaments pass in part into the dentinal tubules. The pulp is so sensitive that in popular language it is called " the nerve." In the original formation of the tooth the dentine is produced by the structure which is afterwards represented by the pulp, and through life the latter retains to a considerable extent its formative power.

The tooth is fitted accurately into the alveolus by means of the cement on the one hand and the Dental periosteum or periodontal membrane on the other. This membrane, lying between the cement and the bone of the jaw, forms a kind of double periosteum, and is, from its position, very liable to the action of mechanical forces, being placed between two rigid structures, one of which (the tooth) is peculiarly exposed to mechanical interference. It is continuous at the apex of the root with the pulp, and at the neck with the mucous membrane of the gums on the one hand and the periosteum of the jaw on the other.