ENAMEL LINES PARALLEL AND WAVY. XXI 



the dentine, and are wedged into the interspaces of 

 the longer fibers. In the teeth of fishes, the calciger- 

 ous tubes or fibers of the enamel, which ramify and 

 subdivide like those of the dentine, have their trunks 

 turned in the opposite direction, or toward the periph- 

 ery of the tooth. So likewise in human teeth the 

 analogous condition may be discerned in the slightly 

 augmented diameter of the enamel-fibers at their pe- 

 ripheral as compared with their central extremities. 

 When the extremities of the human enamel-fibers are 

 examined with a magnifying power of 300 linear 

 dimensions, by reflected light, they are seen to be co- 

 adapted, like the cells of a honey-comb, and, like these, 

 to be, for the most part, hexagonal. 



"The internal surface of the enamel is marked by 

 fine transverse lines or ridges, of which Retzius counted 

 twenty-four in the vertical extent of one-tenth of an 

 English inch of the crown of a human incisor. These 

 lines are parallel and wavy, and, like the analogous 

 markings on the surface of shells, indicate the succes- 

 sive formation of the belts of enamel-fibers that encircle 

 the crown of the tooth. They may be traced around 

 the whole crown, but are very faint upon its inner or 

 posterior surface. Retzius cites Leeuwenhoek as the 

 discoverer of these superficial transverse lines of the 

 enamel, but the older observer supposed them to be 

 indicative of the intervals between the successive move- 

 ments in the cutting of the tooth through the gum. 



" The enamel, by virtue of its physical qualities of 

 density and durability, forms the chief mechanical 

 defense of the tooth, and is consequently limited in 

 most simple teeth to the exterior surface of the exposed 

 portion of the dentine, forming the crown of the tooth. 

 * * * In the herbivorous mammalia, with th 



