STRUCTURE OF THE TEETH. 1549 



The enamel — the product of epithelial cells, the ameloblasts — consists of an aggre- 

 gation of five- or six-sided columnar elements, the enamel -prisms, which measure 

 from .0035-. 0045 mm. in diameter and from 3-5 mm. in length. Their general 

 disposition is at right angles to the surface of the dentine upon which they rest, on 

 the one hand, and to the exterior of the crown on the 



other. They usually extend the entire thickness of the Fig. 1307. 



enamel, and are of slightly larger diameter at the surface  1 



of the tooth than next the dentine, in this manner com- 

 pensating for the increase in the external circumference 

 of the crown. The assumption that additional prisms 

 are intercalated at the periphery is not supported by the 

 manner of the production of the enamel-columns. The 



latter run for a short distance almost at right angles to .; 



the surface of the dentine, then bend laterally for a vj 



considerable part of their course, but assume a vertical , '1 



disposition on approaching the external surface. In j 



addition to these general curves, the ranges of enamel- J 



columns possess a spiral arrangement, in consequence ! 



of which the parallelism of the prisms, as seen in ground- ^1 



sections, is disturbed and their bundles are apparently 

 interwoven. ^ j , ^ 



... , . , GrouiiQ-sectioii ot enamel, showing 



In thm accurately transverse sections enamel pre- ranges of enamei-prisms. -. 500. 

 sents a mosaic in which the hexagonal areas represent 



the ends of the individual prisms. Critically examined, the areas consist of a darker 

 central portion surrounded by a narrow lighter peripheral zone. The interpreta- 

 tion of the latter has been various, many observers regarding such lines as cement- 

 substance holding together the prisms. According to Walkhoff,^ however, what is 

 usually regarded as cement-substance is a cortical, apparently homogeneous layer of 

 less thoroughly calcified material which encloses the denser central portion of the 

 prism and acts as a cushion, thereby reducing the effect of pressure. After the 

 decalcifying action of acids, the prisms may be outlined by stains which color the 

 very meagre amount of true cement-substance which exists between the enamel- 

 columns and appears as delicate lines defining the prisms. 



Under favorable conditions, especially, but not only, after the action of acids, the 

 enamel-prisms exhibit alternate light and dark transverse markings. The true rela- 

 tions of these bands are to be appreciated only by accurate focusing in thin sections 

 passing exactly parallel to the axes of the prisms ; otherwise the obliquity of section 

 produces the optical distortions often represented in the assumed wavy contour of the 

 enamel-rods. The varicose appearances commonly seen depend upon the beaded 

 form and consequently scalloped border of the denser central portion of the prisms, 

 which give a corresponding arrangement to the lighter cortical substance which fills 

 the minute inequalities of that portion ; the true outline of the enamel-prism, how- 

 ever, is smooth and straight, and not varicose, as the optical impressions lead one 

 to believe and as usually pictured. According to Williams, the apparent varicosities 

 depend upon the spherical form of the enamel-globules of which the prisms are built up. 



When an axial longitudinal section of a tooth is examined by reflected light, the 

 enamel displays a series of alternate dark and light bands, — the. prism-stripes 0/ 

 Schreger. These markings extend generally vertical to the surface of the enamel, 

 and depend upon the relation of the ranges of the enamel-prisms to the axes of the 

 light-rays. Rotation of the illuminating pencil through 180° effects the change of 

 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 longi- 

  tudinal sections, brownish parallel lines, the stripes of Retzitcs, which run in the 

 general direction of the contour of the tooth, but at an angle of from 15° to 30° with 

 the free surface. Seen in sections cut at right angles to the tooth-axis, these stripes 

 appear as a series of concentric lines encircling the crown parallel to and near the 

 surface ; in the middle and deeper parts of the enamel they are less evident or entirely 



^ Normale Histologie mensch. Zahne, 1901. 



