THE TEETH 



327 



tubules divide into a group of terminal branches, some of the arboriza- 

 tions being very extensive, others consisting of but two or three sub- 

 divisions. The coarser branches are frequently looped, the distal end of 

 the loop often anastomosing with adjacent tubules. In their course 

 through the dentin those canaliculi which enter the larger iiiterglobular 

 spaces are continued through these spaces without interruption. 



The walls of the dentinal tubules are 

 formed by extremely dense calcareous den- 

 tinal sheaths (of Neumann) which are very 

 resistant to the action of acids. The curva- 

 tures in the course of the dentinal tubules 

 are of two types ; the longer primary curves 

 and the shorter spiral secondary curves. 

 They occur with extreme regularity and as 

 a result give rise to certain parallel lines 

 in the substance of the dentin which follow 

 the contour of the dentinal surface. These 

 are known as the incremental lines of 

 Schreger. 



A second system of dentinal striae, vis- 

 ible in ground sections of tooth under low 

 magnification, are the contour lines of 

 Owen (arched incremental lines of Salter). 

 They run nearly parallel to the lines of 

 Schreger in the crown and toward the tip 

 of the root, but elsewhere cut these lines at 

 wide angles (Fig. 303). They represent 

 lines of defective calcification between suc- 

 cessively deposited layers of dentin. 



The superficial portion of the dentin is formed by the granular layer 

 of Tomes, in which there are no dentinal tubules, but instead there are 

 in this layer numerous small interglobular spaces from which minute 

 canaliculi radiate in various directions. Many of these canaliculi are 

 connected, on the one hand with the dentinal tubules, and on the 

 other with the canaliculi and bony lacunae of the cementum. The 

 canaliculi of the granular layer are readily distinguished from the ad- 

 jacent dentinal tubules by the extreme irregularity of their course, which 

 contrasts sharply with the straight or regularly curved course of the 

 dentinal tubules. 



The granular layer is relatively thick in the root of the tooth, but 



FIG. 307. DENTIN FROM A 

 GROUND SECTION OF A HU- 

 MAN MOLAR, SHOWING THE 

 DENTINAL TUBULES CUT 

 ACROSS. 



The tubules appear as dark 

 round or oval areas in the den- 

 tinal matrix. Each tubule is 

 surrounded by a narrow lighter 

 halo, corresponding to the 

 sheath of the tubule, perhaps 

 an optical effect. X 750. 



