148 



NORMAL HISTOLOGY. 



FIG. 177. 



bundles which cross one another, producing the alternate dark and 

 light radial prism-stripes of Schreger. The additional dark lines 

 extending more or less parallel to the free surface of the tooth the 

 stripes of Retzius are probably due to inequalities in growth and 

 density. At birth, and for a variable time thereafter, the outer sur- 

 face of the enamel is covered by a delicate but resistant cuticle, the 

 membrane of Nasmyth, composed of uncalcified enamel and 

 the remains of the enamel organ. This cuticle is soon worn away 

 after the teeth are actively used. Next the dentine numerous clefts 

 exist for a short distance between the enamel prisms ; they com- 

 municate with the interglobular spaces and thus indirectly with the 

 dentinal tubules. 



The cementum, or crust a petrosa, invests the fang of the tooth 

 and closely resembles in structure ordinary bone ; the lamellae extend 

 parallel to the dentine, as do likewise the 

 long axes of the bone lacunae. Where the 

 cementum reaches a considerable thickness, 

 as at the apex of the root of the tooth, Ha- 

 versian canals may exist, although usually 

 these are wanting ; perforating fibres of 

 Sharpey are also present in small numbers. 

 The lacunae communicate with the dentinal 

 tubules, while the protoplasmic processes 

 of their contained bone-cells may come in 

 contact with the filaments of the odonto- 

 blasts lying within the dentinal tubules. 



The pulp consists of a matrix of soft 

 embryonal connective tissue, in which nu- 

 merous stellate and spindle cells form pro- 

 toplasmic net-works by their anastomosing 

 processes. At the periphery the connective- 

 tissue elements are arranged as layers of 

 elongated cylindrical cells perpendicular to 

 the inner surface of the dentine, in contact 

 with which they lie ; these cells are the 

 odontoblasts, being the representatives of the cells which were 

 actively engaged in producing the dentinal matrix. The protoplasm 

 of many of these cells is prolonged peripherally as delicate threads 

 into the dentinal tubules, the processes becoming modified to form the 

 stiff elastic dentinal fibres ; centrally, the odontoblasts frequently 

 are connected with the stellate connective-tissue cells. 



The pulp is richly supplied with blood-vessels and nerves. The 

 arteries run in the long axis of the tooth, breaking up into capillary 

 net-works which are closest in the periphery. The nerves accom- 



Section of human tooth at the 

 junction of the dentine and the 

 cementum : D, dentine with its 

 tubules, which communicate with 

 interglobular spaces (B) and with 

 lacunae of cementum (C). 



