7 2 



HISTOLOGY. 



As they increase in length the enamel layer broadens. Their course does 

 not remain straight. A vertical median section of the enamel shows in its 

 middle part (Fig. 81, c) alternating layers of prisms in cross and longitu- 

 dinal section. At the borders of these layers the prisms are in transition 

 from one layer to the other. At either end the prisms are said to be per- 

 pendicular to the enamel surfaces, but in the midst of their course they 

 bend laterally in opposite directions. Thus they reflect light in such a way 

 as to form alternating light and dark bands (Schreger's lines) which cross 

 the enamel, and are related to the layers of prisms as shown on the right 

 of Fig. 81, c. The lines are seen in reflected light. Contour lines (lines 

 of Retzius) cross the prisms obliquely. They are due to pauses in the 

 enamel formation, and in poorly developed teeth especially they are planes 

 along which the enamel may most readily be fractured. Since they often 





FIG. 



Enamel prisms, 



isolated. 



80. FROM A CHILD 

 AT BIRTH. 



FIG. 8 1. a, Cross section and enamel prisms (after Stohr); b, 

 cross sections of enamel prisms (after Smreker) ; c, Middle 

 part of the enamel from a ground longitudinal section of a 

 canine tooth (after Kolliker). On the right, seen in re- 

 flected light, it shows the light and dark lines of Schreger. 



appear brown in sections they have been ascribed to pigment, but it is said 

 that they are air spaces in the cement. They tend to be parallel with the 

 outer surface of the enamel, on which, however, they terminate between 

 the little encircling ridges which may be seen with a hand lens. A few 

 contour lines but no ridges are shown in Fig. 74. 



In cross section enamel prisms are shown in Fig. 8r. They are from 

 3 to 6 fi in diameter, sometimes five or six sided, but often are concave 

 on one surface and convex on the other, being grooved by the pressure of 

 adjoining prisms. They are said to increase in diameter from the inner 

 toward the outer enamel surface. Nodular enlargements have been 

 described, and transverse bands appear* in isolated prisms treated with 

 dilute acid. 



After birth the tooth pushes out through the tissue of the jaw in 

 which it is embedded, so that its crown becomes exposed. In this process 



