DIGESTIVE SYSTEM. 149 



They are so arranged that the expansion of adjacent 

 rods lie opposite each other; that is, the expansions 

 do not interlock with the constrictions. The cement 

 substance, therefore, has a reciprocal arrangement. 

 Sections ground parallel to the rods show, therefore, 

 dark and light lines, described as the striations of the 

 enamel, which are caused by the difference in the 

 refracting power of the prismatic and interprismatic 

 substances. In a ground section across the rods, the 

 tissue presents a mosaic pattern, which becomes 

 more distinct if treated with acid. 



Fig. 106. Transverse section of enamel (Noyes). 



Lines of Retzius. These lines or stratification 

 bands begin at the tip of the dentin cusps and sweep 

 around in larger and larger zones. They thus pass 

 obliquely through the enamel and record the growth 

 of the crown, as each line was at one time the surface 

 of the enamel. They are to be regarded as traces 

 of the strata caused by the periodic deposition of 

 lime salts. 



According to Noyes, "the appearance of striation 

 is the record in the fully formed tissue of the manner 



