TEETH 105 



When the tooth comes out through the gum, or erupts, the enamel is 

 covered with a "persistent capsular investment" described by Nasmyth 

 (1849) an d called "Nasmyth's membrane" (cuticula dentis). Huxley 

 studied this structure as it covers the teeth in an embryo of the seventh 

 month (Trans. Micr. Soc. London, 1853, v l- I > PP- I 49~ I ^4)- He found 

 that the inner enamel cells could be easily removed, leaving the surface 

 of the enamel covered with a finely wrinkled or reticulated structureless 

 membrane. Upon adding strong acetic acid the membrane became 

 voluminous and transparent, and was thrown into coarse folds. The 

 ends of the enamel prisms could be seen through it. This dental cuticula 

 is now generally considered to be composed of the last-formed uncalcified 

 ends of the enamel prisms, which are composed of horny material. After 

 the eruption of the tooth it is gradually worn away, remaining longest in 

 the depressions of the enamel. 



The fully developed enamel is the hardest substance in the body. 

 Several analyses have shown that it contains less than 5% of organic 

 matter. No cells or protoplasmic structures are found within it, but it 

 exhibits various markings, shown in Fig. 94. The outer surface of the 

 enamel of the permanent teeth, especially on the sides of the crown and 

 on young teeth, presents a succession of circular ridges and depressions, 

 which may be seen with a hand lens. These were discovered by Leeuwen- 

 hoek (1687), whose figure of them is reproduced in Fig. 94, A. He con- 

 sidered that they marked the intervals during the eruption of the tooth, 

 and wrote, "For example, let us assume that the tooth has fifty circles 

 or ridges; if this is so, the tooth has been pushed through the gum during 

 fifty successive days or months." This explanation is not supported by 

 any evidence. 



The enamel, as seen in ground sections passing lengthwise through 

 the tooth, shows numerous brownish bands which are broadest and most 

 distinct toward the free surface (Fig. 94, B). These are the contour lines 

 or lines of Retzius, first described in Miiller's Archiv, 1837 (pp. 486-566). 

 The coarsest of them may be seen with the naked eye, but upon magni- 

 fication these are resolved into a number of finer lines, and many new 

 lines appear. Their direction is shown in the figure; they arch over the 

 apex of the crown, and on its sides tend to be parallel with the long axis 

 of the tooth. Thus they cross the enamel prisms, and are not the lines 

 along which the enamel most readily fractures. Apparently they indi- 

 cate the shape of the entire enamel at successive stages in its development, 

 and for this reason they are called contour lines. When Leeuwenhoek's 

 ridges are present, the lines of Retzius end in the furrows between them. 

 It was once supposed that their brown color was due to pigment, and it is 

 well known that the enamel of certain teeth in rodents is deeply pigmented 

 and brown. But when the lines are highly magnified, no pigment granules 



