io6 



HISTOLOGY 



are found. It then appears that the lines are due to imperfect calcification 

 of the enamel cement, which is often vacuolated where a line crosses it. 



Another set of lines crosses the enamel radially, taking the shortest 

 course from the dentine to the free surface. These radial lines are due 

 to the arrangement of the enamel prisms, and fractures of the enamel 

 tend to follow them. As seen in reflected light, under low magnification, 

 they appear as alternating light and dark bands, often called Schreger's 

 lines. The prisms in crossing the enamel are bent in such a way that they 

 are cut in alternating zones of cross and longitudinal sections, respectively 

 (Fig. 94, C). These zones vary in shape and sometimes the prisms in 

 cross section form an island surrounded by longitudinal sections. Since 

 an entire prism cannot be isolated or included within the limits of a single 

 section, the course which they take is difficult to determine. There is no 



F 



FIG. 94. THE'MARKINGS OF THE ENAMEL IN ADULT TEETH. 



A, Leeuwenhoek's figure showing ridges encircling the enamel. B, Longitudinal ground section of a canine 

 tooth; c, cement; c. 1., contour lines (lines of Retzius); d. c., dentinal canals; i. s., interglobular spaces. 

 C, Longitudinal section of the enamel of an incisor tooth, the dentinal surface being toward the left. 

 The enamel shows zones of transverse and longitudinal sections of enamel prisms. D, Fragment of 

 enamel showing prisms in longitudinal view, slightly affected by hydrochloric acid. X 350 (Koelliker). 

 E, Cross section of the decalcified enamel of a canine tooth from a child of three years. X 350 (Koel- 

 liker). F, Cross section of enamel prisms of a permanent molar from a child of about eight years. 

 (Smreker.) 



evidence that they branch, and the greater surface which they cover at 

 the periphery of the enamel, as compared with the dentinal surface, has 

 been explained by an increase in the diameter of the prisms as they pass 

 outward. Such an enlargement is not well marked, however, and is partly 

 offset by an outward thinning of the interprismatic cement. Apparently 

 there is an increase in the number of ameloblasts as the tooth becomes 

 larger, and there may be some late-formed enamel prisms which do not 

 reach the dentinal surface. The plan according to which the prisms bend 

 is discussed in Koelliker's Gewebelehre (6th ed.) but it has never been 

 fully explained. 



The individual enamel prisms, when seen lengthwise, exhibit trans- 

 verse markings. These may be made out in ground sections, but they 

 become more evident after the prisms have been treated with acid (Figs. 

 94, D and 99). They have been regarded as artificial products, but prob- 

 ably they indicate successive stages in the elongation of the prism. Fre- 



