THE DIGESTIVE SYSTEM 237 



just under the enamel, but some continuing over a short distance 

 into the enamel where they lie in the cement between the prisms. 

 In the root the canals have an almost straight direction (without 

 primary curves). Large branches are more numerous and the main 

 canals have a somewhat more irregular arrangement. They probably 

 do not pass over into the cementum, but end at the granular layer. 

 The dentine immediately around a dentinal canal is harder and 

 more dense than elsewhere and forms a sort of sheath for the canal — 

 Neumann^ s dental sheath. Between the dentinal canals is a calcified 

 ground substance, in which are connective-tissue fibres running in a 

 direction parallel to the surface of the pulp, this corresponding, as in 

 bone, to the deposition of the dentine in successive layers. In a longi- 

 tudinal section of the dentine of the crown, lines are seen running 

 parallel to the surface of the pulp. They are known as the lines of 

 Schreger and are probably due to irregularities in deposition of the 

 dentine. 



Spaces which probably represent incomplete calcification of the 

 dentine occur in the peripheral portion of the dentine of the crown. 

 These are known as interglobular spaces (Fig. 141, Jg). They are 

 filled with a substance resembling uncalcified dentine. The inter- 

 globular spaces do not interrupt the dentinal canals which pass through 

 them with no break in their continuity. 



In the outer part of the dentine of the root are similar spaces which 

 are smaller and more closely placed. These form the so-called 

 Tomes' granular layer (Fig. 140, A'). In the root of the tooth this 

 layer is quite thick, separating the cementum from the dentine. Its 

 spaces or lacunas send off tiny canaliculi which run in all directions and 

 anastomose with one another, with the dentinal tubules, and with the 

 lacunae of the cementum. This layer, with its small closely placed 

 spaces and fine irregularly running canaliculi, contrasts sharply on the 

 one side (Fig. 140) with the dentine and its straight parallel tubules, 

 and on the other side, with the cementum and its large and more 

 widely separated lacunae. Over the crown of the tooth this layer is 

 much thinner and as the apex is approached, becomes lost completely, 

 the dentinal tubules extending to, and some of them entering slightly, 

 (see above) the enamel.-^ 



^Distinction is sometimes made between primary and secondary dentine. Primary 

 dentine is that dentine normally formed during the development of a tooth and corre- 

 sponds to the description given. Later under some stimulus, e.g., excessive wear, for- 

 mation of dentine again takes place, in which case the canals are often much more ir- 

 regularly arranged; this is known as secondary dentine. 



