Enamel

The enamel (substantia adamantina) is the hardest and most compact part of the tooth, and forms a thin crust over the exposed part of the crown, as far as the commencement of the root. It is thickest on the grinding surface of the crown, until worn away by attrition, and becomes thinner toward the neck. It consists of minute hexagonal rods or columns termed enamel fibers or enamel prisms (prismata adamantina). They lie parallel with one another, resting by one extremity upon the dentin, which presents a number of minute depressions for their reception; and forming the free surface of the crown by the other extremity. The columns are directed vertically on the summit of the crown, horizontally at the sides; they are about 4µ in diameter, and pursue a more or less wavy course. Each column is a six-sided prism and presents numerous dark transverse shadings; these shadings are probably due to the manner in which the columns are developed in successive stages, producing shallow constrictions, as will be subsequently explained. Another series of lines, having a brown appearance, the parallel striæ or colored lines of Retzius, is seen on section. According to Ebner, they are produced by air in the interprismatic spaces; others believe that they are the result of true pigmentation.

Numerous minute interstices intervene between the enamel fibers near their dentinal ends, a provision calculated to allow of the permeation of fluids from the dental canaliculi into the substance of the enamel.

Chemical Composition.—According to von Bibra, enamel consists of 96.5 per cent. of earthy matter, and 3.5 per cent. of animal matter. The earthy matter consists of phosphate of lime, with traces of fluoride of calcium, carbonate of lime, phosphate of magnesium, and other salts. According to Tomes, the enamel contains the merest trace of organic matter.