THE TEETH. 



239 



Enamel. 



transverse markings which express, however, no structural pecu- 

 liarity, but are due to irregularities in the prisms. They are 

 joined to each other by a cement-substance which is somewhat 

 more resistant than the substance of the prisms themselves. In the 

 adult they are entirely homogeneous, but in embryos and even in 

 the new-born they show a (fibrillar) longitudinal striation. In their 

 course through the thickness of the enamel they change their 

 direction by a series of 

 symmetrical curves, and 

 cross each other in groups 

 in a typical manner. There 

 are also seen in the enamel 

 the parallel lines known as 

 the lines of Rctzins (see 

 Fig. 181), which pass 

 obliquely through the 

 enamel and which are to 

 be regarded as traces of 

 the strata caused by the 

 periodic deposition of lime- 

 salts ; they are very vari- 

 able, as their structure 

 depends on the nutritive 

 condition during the depo- 

 sition of the lime - salts 

 (Berten). Another series 

 of parallel or nearly par- 

 allel stripes or lines, known 

 as Schrdgcr's lines, are 

 also observed. Those in 

 the lateral portions of the 

 enamel have a direction 

 nearly perpendicular to 

 the surface. They are 

 thought to be due to a 

 difference in the refraction 

 of the light, presented by 

 bundles or layers of enamel 

 prisms so disposed as to be \v,vss^ >-^%m / 



W 



ft--- Pulp cavity. 



Cementum. 



Fig. 181. Scheme of a longitudinal section 

 through a human tooth. In the enamel are seen 

 the ''lines of Retzius." 



cut in different directions. 

 The dentin is, next to 

 the enamel, the hardest 

 tissue of the tooth. After 

 decalcification it presents 



a ground substance in which are found numerous very fine fibrils, 

 which do not branch nor anastomose, and are in their behavior 

 toward acids and alkalies like the fibrils of white fibrous (collagenous) 

 tissue. They yield gelatin on boiling. The fibrils are separated by 



