876 



THE ORGANS OF DIGESTION. 



in a distinct tissue, the cementum or crusta petrosa ; both cementum and enamel are 

 thinnest at the neck and thickest upon their distal portions. 



The solid portion of the tooth consists of three distinct structures viz. the 

 proper dental substance, which forms the larger portion of the tooth, the ivory or 



dentine ; a layer which covers the 

 exposed part of the crown, the 

 enamel; and a thin layer, which is 

 disposed 9n the surface of the fang, 

 the cement or crusta petrosa. 



The ivory, or dentine (Fig. 47o), 

 forms the principal mass of a tooth ; 

 in its central part is the cavity en- 

 closing the pulp. It is a modi- 

 fication of osseous tissue, from 

 which it differs, however, in struc- 

 ture. On microscopic examination 

 it is seen to consist of a number 

 of minute wavy and branching 

 tubes having distinct parietes. 

 They are called the dentinal tu- 

 buli, and are imbedded in a dense 

 homogeneous substance, the inter- 

 tubular tissue. 



The dentinal tubuli (Fig. 476) 

 are placed parallel with one an- 

 other, and open at their inner ends 

 into the pulp-cavity. In their 

 course to the periphery they present 

 two or three curves, and are twisted 

 on themselves in a spiral direc- 

 tion. The direction of these tubes 

 varies : they are vertical in the up- 

 per portion of the crown, oblique 

 in the neck and upper part of the root, and toward the lower part of the root 

 they are inclined downward. The tubuli, at their commencement, are about 

 45 1 0o of an inch in diameter; in their course they divide and subdivide dichoto- 

 mously, so as to give to the cut surface of the dentine a striated appearance. 

 From the sides of the tubes, especially in the root, ramifications of extreme 

 minuteness are given off, which join together in loops in the intertubular sub- 

 stance, or terminate in small dilatations, from which branches are given off. 

 Near the periphery of the dentine the finer ramifications of the tubuli terminate 

 in a layer of irregular branched spaces which communicate with each other. 

 These are called the interglobular spaces of Czermak, or the granular layer of 

 Tomes (Fig. 476, J). The dentinal tubuli have comparatively thick walls, and con- 

 tain slender cylindrical prolongations from the processes of the cells of the pulp- 

 tissue already mentioned, and first described by Mr. Tomes and named Tomes's 

 fibres or dentinal fibres. These dentinal fibres are analogous to the soft contents 

 of the canaliculi of bone. Between Tomes's fibres and the ivory around the canals 

 there is a tissue which is markedly resistant to the action of acids the dentinal 

 sheath of Neumann. 



The intertubular substance or tissue is translucent, and contains the chief part 

 of the earthy matter of the dentine. After the earthy matter has been removed 

 by steeping a tooth in weak acid the animal basis remaining may be torn into 

 laminae which run parallel Avith the pulp-cavity across the direction of the tubules. 

 These laminae show the method of growth to be by deposition of successive strata 

 of dentine. Fibrils have been found in the matrix of the intertubular substance, 

 and are probably continuous with the dentinal fibres of Tomes. In a dry tooth a 



FIG. 476. Ground section through the root of a human 

 premolar : D, dentine ; K, cement corpuscles ; O, osteoblasts ; 

 Ep., remains of Hertwig's epithelial sheath, 200 diameters; 

 J, interglobular spaces (Rose). 



