HISTOLOGY  . If a

section he made vertically through a tooth all the exposed part or crown is seen to be covered with enamel, which, microscopically, is composed of a number of fine hexagonal prisms arranged at right angles to the surface of the tooth, and formed chiefly of Bone Cement or Alveolar periosteum crustapetrosa or root-membrane From Ambrose Birmingham, in Cunningham's Text-Book of Anatomy . its various parts, and its structure . calcium phosphate with small amounts of calcium carbonate, magnesium phosphate and calcium fluoride, but containing practically no organic matter . The enamel rests on the " dentine," of which hard yet elastic substance by far the greater part of the tooth is composed . It is made of the same salts as the enamel, but contains in addition a good deal of organic matter and forms a structureless mass through which the fine " dentinal tubes " run from the pulp cavity to the periphery . Surrounded by the dentine is the " pulp cavity," which is filled by the tooth pulp, a highly vascular and nervous mass of branched connective tissue cells, which, in a young tooth, has a layer of epithelial cells, the " odontoblasts, " lying close against the wall of the cavity and forming new dentine . Slender processes (" Tomes's fibrils ") project from these cells into the dentinal tubes, and are probably sensory . A nerve and artery enter the apex of the root of the tooth, but it is not understood how the nerve ends . Surrounding the dentine where it is not covered by enamel is the " cement " or " crusta petrosa," a thin layer of bone which is only separated from the bony socket by the alveolar periosteum .