THE MOUTH, ORAL t)R BUCCAL CAVITY 



1213 



the carbonate of calcium, with traces of fluoride of calcium, phosphate of magnesia, and other 



salts. 



The enamel of a recently erupted tooth is covered by a membrane, the thickness of which is 



^oV<r of an inch. It is known as enamel cuticle or Nasmyth's membrane (cuticula dentis). It 



is probably the remains of the enamel organ, though some consider it the continuation of the 



cementum. 



The dentine or ivory (substantia eburnea) (Fig. 927) forms the principal mass of the tooth. 



It represents modified bone, but differs from the latter in that its cells are upon the surface of 



the pulp and not in the substance of the dentine. The important parts are: dentinal tubules, 



dentinal sheaths, matrix, and dentinal fibres. 



The dentinal tubules are minute canals which have a spiral course, more or less perpendicular 



to the pulp cavity, and extending from this cavity to the enamel or to the cementum. The 



diameter at the pulpal end is about ^Vu of an inch, and this diminishes as the tubules branch. 



The tubules usually end blindly near the enamel; some, however, terminate in the interglobular 



spaces or anastomose with other 

 tubules. The dentinal tubules contain 

 the dentinal fibres, which represent 

 the peripheral processes of the odonto- 

 blastic cells, and their branches follow 

 the divisions of the tubules. The 

 tubule branches are most numerous 

 near the enamel or the cementum. 





FIG. 926. Ground section through the root of a human 

 premolar. D. Dentine. K. Cement corpuscles. O. Osteo- 

 blasts. Ep. Remains of Hertwig's epithelial sheath, 200 

 diameters. ./. Interglobular spaces. (Rose.) 



FIG. 927. From a ground section through 

 the parts of a dentine, near the pulp, of a 

 human canine tooth which has been impreg- 

 nated with pigment. The dental canaliculi 

 are cut across and are joined together by 

 side branches. X 400. 



The dentinal sheaths, or Neumann's sheaths, are tubes of markedly resistant dentine sur- 

 rounding and bounding the dentinal tubules. They branch and curve as do the tubules. 



The matrix, or intertubular dentine, is less dense than that of the sheaths. It fills the spaces 

 between the sheaths, and seems to be composed of lamellae that run parallel to the pulp chamber. 

 Fibrils have been found in the matrix. Scattered in the matrix, and especially near the enamel, 

 are numerous spaces filled with a gelatinous substance; these are the interglobular spaces 

 (spatia inter (jlobnlares), representing areas of incomplete calcification. Between the dentine and 

 cementum the spaces are very numerous, but small; these constitute Tomes' granular layer. 



In a dry tooth a section of dentine often displays a series of lines the incremental lines of 

 Salter which are parallel with the laminre above mentioned. These lines are caused by two 

 facts: (1) The imperfect calcification of the dentinal laminae immediately adjacent to the line. 

 (2) The drying process, which reveals these defects in the calcification. These lines are wide 

 or narrow according to the number of lamina? involved, and along their course, in consequence of 

 the imperfection in the calcifying process, little irregular cavities are left, which are the inter- 

 globular spaces already referred to. They have received their name from the fact that they are 

 surrounded by minute nodules or globules of dentine. Other curved lines may be seen parallel 

 to the surface. These are the concentric lines of Schreger, and are due to the optical effect of 

 simultaneous curvature of the dentinal tubules. 



