I3 2 NORMAL HISTOLOGY. 



as interwoven. In thin, accurately transverse sections, enamel presents 

 a mosaic of minute hexagons, which are the ends of the cut individual 

 prisms. Each prism consists of a darker central and a lighter peripheral 

 zone, which depend upon variations of density. The lighter peripheral 

 zone probably represents a film of less completely calcified substance and 

 is often interpreted as cement material holding the prisms together. After 

 decalcification and staining, the true cement substance may be distinguished 

 as delicate lines defining the prisms. Particularly, but not necessarily, 

 after the action of acids, the enamel-prisms in longitudinal sections exhibit 

 alternate light and dark transverse markings with seemingly beaded or 

 varicose outlines. These appearances are probably optical and depend 

 upon the wavy contour of the central denser substance of the prisms. 

 The true outlines of the prisms are straight, the opposed surfaces of 

 the adjoining columns being separated by an uniform thin layer of the 

 cement-substance. 



When an axial longitudinal section of a ground tooth is examined by 

 reflected, not transmitted, light, the enamel exhibits a series of alternate 

 dark and light bands, known as the prism- stripes of Schreger. These 

 markings (Fig. 171), which are comparatively coarse 

 and generally vertical to the surface of the tooth, 

 depend upon the relation of the ranges of enamel- 

 prisms to the axes of the rays of light. Rotation of 

 the illuminating pencil through 180 changes the dark 

 stripes to light ones and vice "versa. Each stripe 

 includes from ten to twenty enamel-prisms and is 

 ^\"' invisible by transmitted light. In addition to the 

 foregoing markings, the enamel often presents, in 

 radial longitudinal sections, brownish parallel lines, 

 the stripes of Retzius, which correspond in their 

 general direction with the contour of the tooth, but 

 run at an angle of from 15 to 30 with the free 

 surface. In cross sections, these stripes are repre- 

 sented by a series of concentric lines encircling the 



FIG. 172. Ground section crown, parallel to and near the surface; in the middle 



eL e me^pris S ms Wi x g 5 r ^ ngei E and deeper parts of the enamel they are much less 



evident or entirely wanting. The significance of the 



stripes of Retzius is still disputed, but it is probable, since they surely do 

 not depend upon pigment, that they are due to local imperfections of the 

 calcification of the enamel-prisms during certain periods of the growth of 

 the enamel. 



The enamel- cuticle, or membrane of Nasmyth, is a delicate envelope that 

 completely invests the crown of the newly-erupted tooth. In the course of 

 time it disappears from the areas exposed to wear, but over the protected 

 surfaces it may persist throughout life. The membrane (from 24 /* in 

 thickness) is transparent, structureless and resistent to the action of acids, 

 less so to alkalies, and affords protection to the subjacent enamel. Since 

 the cuticle is not only continuous with the cortical substance of the enamel- 

 prisms, but also agrees with it in optical and chemical properties, the origin 

 of the membrane may be referred to the enamel-producing elements, the 

 epithelial cells forming the inner layer of the enamel-organ (page 140). 

 After the completion of their work in producing the enamel-prisms, they 

 produce a continuous envelope, which never undergoes calcification and 

 remains as the enamel-cuticle. 



