586. Each tooth is divided into body, neck, and root (fig. CLVIII. 1, 2, 3). The body is that part of the tooth which is above the gum, the root that part which is below the gum, and the neck that part where the body and the root unite (fig. CLVIII.). The body, the essential part, is the tooth properly so called, the part which performs the whole work for which the instrument is constructed, to the production and support of which all the other parts are subservient. [Illustration: Fig. CLVIII. Views of different kinds of teeth, showing their anatomical division into, 1. The body or crown. 2. The fang or root. 3. The neck.] [Illustration: Fig. CLIX.—Sections of Teeth, exhibiting their Structure. 1. The bony substance. 2. The enamel. 3. The internal cavity. 4. The foramen, or hole at the extremity of the root.] 587. When a vertical section is made in the tooth, it is found to contain a cavity of considerable size (fig. CLIX, 3), termed the dental cavity, which, large in the body of the tooth, gradually diminishes through the whole length of the root (fig. CLIX. 3). The dental cavity is lined throughout with a thin, delicate, and vascular membrane, continued from that which lines the jaw. It contains a pulpy substance. This pulp, highly vascular and exquisitely sensible, is composed almost entirely of blood-vessels and nerves, and is the source whence the bony part of the tooth derives its vitality, sensibility, and nutriment. The blood-vessels and nerves that compose the pulp enter the dental cavity through a minute hole at the extremity of the root (fig. CLIX. 4). The membrane which lines the dental cavity is likewise continued over the external surface of the root, so as to afford it a complete envelope. 588. Provision having been thus made for the organization of the tooth, for the support of its vitality, and for its connexion with the living system, over all that portion of it which is above the gum, and which constitutes the essential part of the instrument, there is poured a dense, hard, inorganic, insensible, all but indestructible substance, termed enamel (fig. CLIX. 2); a substance inorganic, composed of earthy salts, principally phosphate of lime with a slight trace of animal matter: a substance of exceeding density, of a milky-white colour, semi-transparent, and consisting of minute fibrous crystals. The manner in which this inorganic matter is arranged about the body of the tooth is worthy of notice. The crystals are disposed in radii springing from the centre of the tooth (fig. CLX. 3); so that the extremities of the crystals form the external surface of the tooth, while the internal extremities are in contact with the bony substance (fig. CLX. 3). By this arrangement a two-fold advantage is obtained; the enamel is less apt to be worn down by friction, and is less liable to accidental fracture. [Illustration: Fig. CLX.