ENAMEL 55 



junction 0-0031 mm., at the enamel surface 0-0057 mm., 

 giving a proportion of 1 to 1-83. At the cusps the average 

 measurement at the amelo-dentinal junction was 0-0025 mm., 

 at the enamel surface 0-0065 mm. Kolliker gives the 

 diameters of enamel prisms as from 0-0064 to 0-0051 mm., 

 but does not indicate from what part of the enamel his 

 measurements were taken, and there appears to be little 

 doubt that there is a distinct variation in the diameter of 

 the prisms within the substance of the enamel. 



One of the most complete researches hitherto published Leon 

 on the structure of human enamel is that of Leon Williams 

 (21), and his paper, accompanied by excellent original structur 

 photographs, demonstrates very completely the main points 

 of his observations. 



He holds that the enamel consists of two distinct portions, 

 the prisms, or rods as he prefers to call them, and the sub- 

 stance between the rods, and that these are developed in 

 different manners. The calcific matter which builds up 

 the rods is a product of the ameloblast cell. The rods are 

 built up of little blocks of coalesced granules . and are 

 arranged like piles of bricks ; they show a cross -marking 

 where they meet one another, which is the cause of the 

 minute cross-striations of the enamel prisms. He describes 

 the connexion of the rods with one another laterally by 

 calcified processes. Strings or threads in the calcified 

 material are also seen in many places passing along the 

 length of the rods : these are well shown in fig. 14, and must 

 not be confused with other darker strongly marked vertical 

 lines which are due to refraction. In fig. 18 the granular 

 nature of the calcific deposit can be seen in the formed 

 enamel prism. The cross -striation is confined to the rods 

 and does not traverse the interprismatic or cement sub- 

 stance. The blocks of the enamel rods do not alternate 

 in neighbouring rods, but are opposite one another all across 

 the enamel (figs. 14, 19). The cement substance between 

 them is calcified, according to Leon Williams, independently 

 of the rods, cementing them together and forming a compact 

 tissue which in normal enamel is completely calcified and 

 contains no trace of organic matter. Fig. 20, a photo- 

 graph from a ground preparation, shows at a a detached 



