360 FORMATION OF DENTAL ENAMEL. 



chemical action are more or less changed into a disc-like shape as 

 they take their place in the building of the enamel rods. Thus, 

 for the first time, we have a clear explanation of the varicose 

 appearance of the rods or prisms in completely-formed enamel. 

 Each varicosity of the rods represent one of these globules or 

 disc-like bodies. In the inner portion of enamel next to the 

 dentine the enamel globules usually melt together so completely 

 that their individual form is lost, but as we approach the outer 

 surface of the enamel the rods are clearly seen to be composed of 

 globular or flattened bodies, not completely united, but still con- 

 nected by calcified plasmic strings. These plasmic strings, of 

 which there are many to each enamel rod, are often seen under a 

 power of from 800 to 1000 diameters to be made up of granules, 

 and these granules, so far as our present powers of observation are 

 concerned, constitute the ultimate structure of enamel. 



The Cross Stri^ and Retzius Bands. 



My observations on these points have led me to totally different 

 conclusions from those reached by Professor von Ebner of Vienna. 

 Professor von Ebner regards the striae of the enamel rods as an 

 artificial appearance produced by the action of acids, and the 

 Retzius bands as due to ground-off or artificial prism ends, and also 

 to imprisoned air or gas contained in minute tubes or canals. 



I have clearly demonstrated that the cross striae are due to the 

 varicose enlargements of the enamel rods, and that these varicosi- 

 ties are produced by the successive deposit of globular masses 

 from the enamel cells. The deposit of these bodies is simulta- 

 neously over the entire surface of the forming enamel, hence the 

 regularity of the lines of striation and increment. I have further 

 shown that the Retzius bands cannot be due to air or gas in minute 

 canals, because there are no such canals in normal human enamel. 

 Neither are these bands due to the ground-off ends of enamel 

 rods, as may be clearly seen from Fig. 8, where the rods are seen 

 to pass through several bands without break in their continuity. 

 The Eetzius bands are caused by pigmentary matter in the granules 

 which constitute the' ultimate structure of enamel. Completely 

 formed normal human enamel contains no living matter in the 

 sense in which this term is generally understood. There are con- 

 sequently no nutritive changes in enamel after it is once formed. 



