. Comparative anatomy. Anatomy, Comparative. TEETH 197 results in an increase in the number of enamel prisms. The twisting and curvature of the prisms in the developed tooth is a consequence of the torsion of the ameloblast layer during active secretion. While the enamel grows by addition from the outside, the dentine increases in thickness from within. Consequently as the tooth is formed the amelo- blast and odontoblast layers are pushed farther and farther apart. Dur- ing the secretion of the dentine, protoplasmic strands from the odonto- blasts are retained within the dentine to form the de
. Comparative anatomy. Anatomy, Comparative. TEETH 197 results in an increase in the number of enamel prisms. The twisting and curvature of the prisms in the developed tooth is a consequence of the torsion of the ameloblast layer during active secretion. While the enamel grows by addition from the outside, the dentine increases in thickness from within. Consequently as the tooth is formed the amelo- blast and odontoblast layers are pushed farther and farther apart. Dur- ing the secretion of the dentine, protoplasmic strands from the odonto- blasts are retained within the dentine to form the dental canaliculi. The odontoblast cells persist throughout life, and by their continued secretion may in old age entirely obliterate the pulp cavity of the tooth. blood vessel in pulp dentine enamel blood vessel in mesenchyme. odontoblast' dentinal Time's ameloblast outer epithelium layer of enamel organ Fig. 150.—Projection drawing of small segment of developing incisor from 130 mm. pig embryo to show formation of enamel and dentine. X3S0. (From Patten's "Em- bryology of the ;) The crown of the tooth is the first to develop, and for a while the tooth resembles a silver-plated thimble, the thin enamel coating cor- responding to the silver plate covering, the dentine to the bare metal. As the tooth grows, it increases in length as well as in thickness, adding first a neck and later a root. The opening into the inner pulp cavity becomes more and more restricted as the root elongates until finally only a minute foramen remains to admit blood-vessels and nerves. The nerves grow into the pulp and acquire free terminations among the odontoblast cells. The cement layer is the last to be added. Cement is secreted by bone-cells which penetrate the connective tissue sac enclosing the tooth. Membrane bone is formed around the root of the teeth to form the alveoli of the jaw-bone and to hold the teeth firmly in place. The mechanics of the eruption of teeth is a problem which
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