HISTOLOGY OF THE HARD DENTAL TISSUES. 117 



dentine in which a number of tnbnli may be seen running, in curv- 

 ing but unbroken lines, from the central cavity of tbe tooth to tbe 

 boundary-line between it and the enamel. Tbe most casual glance 

 through the microscope will reveal distinct and striking differences 

 in the two structures — the enamel dense and crystalline — the den- 

 tine highly organised and permeable throughout its substance. A 

 closer and more attentive inspection will lead us to see that these 

 dentinal tubuli start from the wall of the pulp-cavity, where they 

 are widest, being about 1-10-000 of an inch in diameter, and tra- 

 versing the dentine towards the periphery of the tooth in curves, 

 gradually become smaller, till they vanish in exceeding fine termi- 

 nations below the enamel. If now we revert to what I have said 

 in speaking of the development of the dentine, we shall find our 

 explanation of the nature of these markings. You may remember 

 that I called your attention to a layer of peculiar cells clothing the 

 external surface of the dental pulp, the layer of odontoblasts — ■ 

 the oval bodies had a central fibre passing through the middle of 

 each one — the distal fibre occupying the commencing tube, and the 

 proximal fibre being lost among the meshes of the connective 

 tissue of the pulp. Now, as the growth of the dentine proceeds, 

 the ossification takes place round the distal fibre, and, as the de- 

 velopment increases, each odontoblast becomes converted into the 

 intertubular structure, while fresh odontoblasts forming, take up 

 the work, and carrying it on from without inwards, a compact for- 

 mation of dentine follows them, till a portion of about. J of the 

 dental pulp remains as a soft centre to the tooth. Here the ossi- 

 fying tendency of the odontoblasts appears to remain in abeyance, 

 but ready to be called into action by any exciting cause. 



In examining the dentine in some of its earlier stages of growth, 

 the dentinal tubuli appear of greater width near the dental pulp 

 than they do in the fully-formed tooth, and will be found to corres- 

 pond to the dimensions of the distal fibres of the odontoblasts 

 with which they are in connection. We shall thus see that these 

 dentinal tubuli result from a progressive calcification round the 

 odontoblasts and their distal fibres. 



Occasionally the dentinal tubuli present an appearance of joining 

 the adjacent tubes; this may be observed principally towards their 

 extremities, and is occasioned by some of the odontoblasts having 

 two or more distal fibres which, anastomosing with similar fibres 

 in their neighbourhood, thus form loops with the adjoining tubes. 



