A concise illustrated overview over the structure of teeth compiled by Healing Teeth Naturally
All true teeth have the same general structure and consist of three layers. In mammals an outer layer of enamel, a white hard surface, in fact the hardest tissue in the body, covers part or all of the crown of the tooth.
Enamel owes its hardness to the fact that 96% of it consists of minerals, mostly hydroxyapatite which is composed of calcium and phosphate. The remaining 4% consist of water and organic material.
The middle yellowish layer of the tooth is composed of dentine, which is less hard than enamel and similar in composition to bone. Dentin is less mineralized than enamel and consists only 45% percent of hydroxylapatite, the remainder being organic material and water.
The dentine forms the main bulk, or core, of each tooth and extends almost the entire length of the tooth, being covered by enamel on the crown portion and by cementum on the roots. Together with the enamel, the dentine serves to protect the innermost portion of the tooth, a living tooth tissue called the pulp, where tiny blood vessels, connective tissue and a nerve reside.
Dentine is nourished by the pulp. The pulp or root canal is long and narrow with an enlargement, called the pulp chamber, in the coronal end. The pulp canal extends almost the whole length of the tooth and communicates with the body's general nutritional and nervous systems through the apical foramina (holes) at the end of the roots.
Below the gumline extends the root of the tooth, which is covered at least partially by cementum. The latter is similar in structure to bone but is less hard than dentine. Cementum affords a thin covering to the root and serves as a medium for attachment of the fibres that hold the tooth to the surrounding tissue (periodontal membrane). Gum is attached to the adjacent alveolar bone and to the cementum of each tooth by fibre bundles.
The anatomy of the root canal system is often very complex and elaborate, with numerous accessory or lateral canals and pores in addition to the main root canal. Comparable to the root system of a plant, tiny filaments may branch off the main root canal system of a tooth.
While these accessory canals are usually found at the lower third of the root or where the roots of multi-rooted teeth connect, they can also be found in other locations along the length of a main root canal and open into the periodontal membrane.
Partially excerpted from "The structure of teeth", copyright © 1994-2000 Encyclopædia Britannica, Inc.
Also refer to these (eye-opening) high-magnification images of enamel and dentine.