240 



THE DIGESTIVE ORGANS. 



an interfibrillar substance, in which the mineral salts are deposited. 

 The course of the fibrils is, in the main, parallel to the surface of 

 the dentin. They are often grouped in small bundles (v. Ebner). 

 The dentin is permeated by a system of canals having usually a 

 transverse direction, the so-called dentinal tubules, which are from 

 1.3 ju. to 4.5 fjL in diameter. These originate in the pulp cavity, and 

 during their course become slightly curved, like the letter S- The 

 dentinal tubules usually present several dichotomous divisions near 

 their origin, then pass to the outer third of the dentin without 

 conspicuous divisions ; here they again branch, becoming constantly 

 smaller. In their course they give off numerous side twigs which 



Enamel. 



Branching of the 

 dentinal tubules. 



Dentinal tubules. 



Interglobular 

 space. 



Fig. 182. A portion of a ground tooth from man, showing enamel and dentin ; X I 7- 



anastomose with those of neighboring tubules. The general course of 

 these tubules is shown in figure 181. Certain of the tubules pass 

 for a short distance into the enamel, where they are found between 

 the prisms. In the human tooth the majority end just before reach- 

 ing the enamel. In the root of the tooth they end near the surface, 

 or in the interglobular spaces (see below), or, again, they may be 

 joined to form loops. The dentinal tubules possess sheaths, the 

 sheaths of Neumann, which may be isolated, analogous to the 

 sheaths of the canaliculi of bone. They may be regarded as differ- 

 entiated and more resistant ground substance. The dentinal tubules 

 contain throughout their entire length filiform prolongations of cer- 

 tain pulp-cells (odontoblasts), the dentinal fibers. Peculiar, irregu- 



