1212 



THE ORGANS OF DIGESTION 



proper dental substance, called dentine or ivory, sheathed upon the exposed crown by a layer 

 called the enamel, while the dentine of the root is enclosed in a distinct tissue, the cementum or 

 crusta petrosa. Both enamel and cementum are thinnest at the neck and thickest upon their 

 distal portions. 



The enamel (substantia adamantind) (Figs. 924, 925, and 928) is the hardest and most com- 

 pact part of a 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 a congeries of minute hex- 

 agonal rods, columns, or prisms, known as enamel fibres or prisms (prismata odamantina) 

 (Fig. 928). In general they lie parallel with one another, resting by one extremity upon the 

 dentine, which presents a number of minute depressions for their reception, and forming the 

 free surface of the crown by the other extremity. There are additional shorter (supplemental) 

 prisms filling in the spaces between the long diverging prisms. There are occasional collections 

 of prisms which run diagonally. The prisms are directed vertically on the summit of the crown, 

 horizontally at the sides; they are about the ?? Vo f an mcn m diameter, and pursue a more or 

 less wavy course. By reflected light radial striations are visible, best marked near the dentine. 

 These are Schreger's lines, and are due to the fact that the prisms take an undulatory course 







FIG. 925. Longitudinal ground sectipn through the apex of a canine tooth from a three-and-a-half-year-old 

 boy. Ine entrance of the dental canaliculi between the enamel prisms and the course taken by the latter are 

 shown. X 135. (Szymonowicz.) 



and those of two layers may have opposite directions. Another series of lines, having a brown 

 appearance from pigmentation, and denominated the parallel striae or brown striae of Retzius, 

 or the colored lines, are seen on a section of the enamel. These lines are concentric, and cross 

 the enamel rods. They are caused by the mode of enamel deposition. Inasmuch as the enamel 

 columns, when near the dentine, cross each other and only become parallel farther away, a 

 series of radial markings, light and dark alternately, is obtained (Fig. 924). The enamel prisms 

 are themselves calcified and are fixed to each other by a very small amount of cement substance 

 Numerous minute interstices intervene between the enamel fibres near their dentinal surface 

 It is noted that some of the dentinal canals at the crown penetrate a certain distance' between 

 the rods of the enamel; this is considered pathological by some (Fig. 925). No nutritive canals 

 exist in the enamel, except the very few dentinal canals which at the crown penetrate a short 

 distance, and these are found only in a small area. 



Chemical Composition. According to Bibra, enamel consists of 96.5 per cent, of earthy 

 matter and 3.5 per cent, of animal matter. The earthy matter consists of the phosphate and 



