STRUCTURE OP THE TEETH. 



877 



section of dentine often displays a series of lines the incremental lines of Salter 

 which are parallel with the laminae above mentioned. These lines are caused by 

 two facts : (1) The imperfect calcification of the dentinal laminae immediately 

 adjacent to the line ; (2) The drying process, which reveals these defects in the 

 calcification. These lines are wide or narrow according to the number of laminae 

 involved, and along their course, in consequence of the imperfection in the calci- 

 fying process, little irregular cavities are left, which are the interglobular spaces 

 already referred to. They have received their name from the fact that they are 

 surrounded by minute nodules or globules of dentine. 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 tubules. 



Chemical Composition. According to Berzelius and Bibra, dentine consists of 

 28 parts of animal and 72 of earthy matter. The animal matter is resolvable by 

 boiling into gelatin. The earthy matter consists of phosphate and carbonate with 

 calcium, with a trace of fluoride of calcium, phosphate of magnesia, and other 

 salts. 



The enamel is the hardest and most compact 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 

 hexagonal rods, columns, or prisms. They lie parallel with one another, resting 

 by one extremity upon the dentine, which presents a number of minute depres- 

 sions for their reception, and forming the free surface of the crown by the other 

 extremity. These fibres are directed vertically on the summit of the crown, 



FIG. 477. Enamel prisms (350 diameters). A. Fragments and single fibres of the enamel isolated by the action 

 of hydrochloric acid. B. Surface of a small fragment of enamel, showing the hexagonal ends of the fibres. 



horizontally at the sides ; they are about the -5-^-$ of an inch in diameter, and 

 pursue a more or less wavy course. Each enamel rod is crossed by a series of 

 dark transverse lines, which mark the mode of the formation of the rods (Fig. 

 477). Another series of lines, having a brown appearance, and denominated the 

 parallel 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 den- 

 tine, cross each other and only become parallel farther away, a series of radial 

 markings, light and dark alternately, is obtained (Fig. 475). 



Numerous minute interstices intervene between the enamel-fibres near their 

 dentinal surface. It is noted in rare cases that the dentinal fibres penetrate 

 a certain distance between the rods of the enamel. No nutritive canals exist 

 in the enamel. 



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 



