THE TEETH 257 



be compared to the bone corpuscles and canaliculi of bone. At 

 their inner extremity the dentinal tubules are 2 to 4 /A in diameter, 

 but they taper very gradually, especially in the outer portion of 

 their course, where they finally reach a diameter of no more than 

 0.5 to 1 ^ 



Throughout their course the dentinal tubules give off very fine 

 lateral twigs, which at first leave the parent tubule nearly at right 

 angles, but later are slightly inclined outward. At their distal 

 end most of the dentinal tubules divide into a group of terminal 

 branches, some of the arborizations being very extensive, others 

 consisting of but two or three subdivisions. The coarser branches 

 are frequently looped, the distal end of the loop often anasto- 

 mosing with adjacent tubules. In their course through the den- 

 tine those canaliculi which enter the interglobular spaces are con- 

 tinued through these spaces without interruption. 



The walls of the dentinal tubules are formed by extremely 

 dense calcareous dentinal sheaths which are very resistant to the 

 action of acids. The curvatures in the course of the dentinal 

 tubules, occurring with extreme regularity, give rise to certain 

 parallel lines in the substance of the dentine which follow the 

 contour of the dentinal surface. These are known as the incre- 

 mental lines of Schreger. 



The superficial portion of the dentine is formed by the granu- 

 lar layer of Thomes, 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 denti- 

 nal tubules, and on the other with the canaliculi and bony lacunas 

 of the cementum. The canaliculi of the granular layer are readily 

 distinguished from the adjacent 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 becomes much thinner toward the neck. Beneath the enamel 

 it becomes so thin that toward the apex of the tooth it is scarcely 

 demonstrable. At this point, also, occasional dentinal tubules are 

 continued for a short distance into the enamel, though this con- 

 dition is more characteristically developed in some of the lower 

 mammals (e. g., Rodentia) than in man. 



Enamel. The enamel, which covers the exposed crown of the 

 tooth, is the hardest tissue of the body. It consists of many cal- 

 ls 



