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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
  The matrix (intertubular dentin) is translucent, and contains the chief part of the earthy matter of the dentin. In it are a number of fine fibrils, which are continuous with the fibrils of the dental pulp. After the earthy matter has been removed by steeping a tooth in weak acid, the animal basis remaining may be torn into laminæ which run parallel with the pulp cavity, across the direction of the tubes. A section of dry dentin often displays a series of somewhat parallel lines—the incremental lines of Salter. These lines are composed of imperfectly calcified dentin arranged in layers. In consequence of the imperfection in the calcifying process, little irregular cavities are left, termed interglobular spaces (Fig. 1008). Normally a series of these spaces is found toward the outer surface of the dentin, where they form a layer which is sometimes known as the granular layer. They have received their name from the fact that they are surrounded by minute nodules or globules of dentin. Other curved lines may be seen parallel to the surface. These are the lines of Schreger, and are due to the optical effect of simultaneous curvature of the dentinal fibers.
  Chemical Composition.—According to Berzelius and von Bibra, dentin consists of 28 parts of animal and 72 parts of earthy matter. The animal matter is converted by boiling into gelatin. The earthy matter consists of phosphate of lime, carbonate of lime, a trace of fluoride of calcium, phosphate of magnesium, and other salts.


FIG. 1008– Transverse section of a portion of the root of a canine tooth. X 300. (See enlarged image)

  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.
  The crusta petrosa or cement (substantia ossea) is disposed as a thin layer on the roots of the teeth, from the termination of the enamel to the apex of each root, where it is usually very thick. In structure and chemical composition it resembles bone. It contains, sparingly, the lacunæ and canaliculi which characterize true bone; the lacunæ placed near the surface receive the canaliculi radiating from the side of the lacunæ toward the periodontal membrane; and those more deeply placed join with the adjacent dental canaliculi. In the thicker portions of the crusta petrosa, the lamellæ and Haversian canals peculiar to bone are also found.
  As age advances, the cement increases in thickness, and gives rise to those bony growths or exostoses so common in the teeth of the aged; the pulp cavity also becomes partially filled up by a hard substance, intermediate in structure between dentin and bone (osteodentin, Owen; secondary dentin, Tomes). It appears to be formed by a slow conversion of the dental pulp, which shrinks, or even disappears.

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