288 MICROSCOPIC ANATOMY OF THE TEETH 



examination of a large number of teeth found four different 

 conditions present : 



1. The enamel overlaps the cement. 



2. The cement overlaps the enamel. 



3. The two tissues terminate in direct contact. 



4. There is a solution of continuity and an exposed surface 

 of dentine between the enamel and cement (3). 



In the first case, where the enamel overlaps the cement 

 there is a marked difference in the percentage of cases 

 between the teeth of young subjects and those of adults. 

 In the young teeth this arrangement of the tissues was 

 found in 57 per cent., and in adults in 12-5 per cent. In 

 the second case, where the cement was seen to overlap 

 the enamel, the difference between the teeth at different 

 ages was more marked, this condition not being observed 

 in a single case in young teeth ; but in adults 62-5 per cent, 

 of teeth examined showed this overlap. These results are 

 easily understood on considering the process of development. 

 The formation of the cement proceeding after the enamel 

 is fully laid down, the osteoblasts continuing to deposit 

 cement add somewhat to the thickness of the tissue at 

 the neck of the tooth, and an overlap is easily comprehended, 

 and would be only found in adult teeth. 



The conditions 3 and 4 exist according to this investiga- 

 tion in exactly similar proportions in adult teeth. In some 

 few cases a large overlap of cement with numerous lacunae is 

 seen, as shown in fig. 182. This is probably a pathological 

 condition, and is of rare occurrence, produced by an over- 

 activity of the cells of the follicle wall, probably due to 

 some chronic irritation of the tissues surrounding the tooth ; 

 but it is to be noticed that this deposit of cement takes 

 place outside the Nasmyth's membrane, which has been 

 clearly proved to be of an epithelial nature and derived 

 from the ectodermic enamel organ. 



The basis substance or matrix of the cement appears 

 almost structureless or very faintly granular, and its collagen 

 foundation retains its form and structure after decalcifica- 

 tion by acids. Like bone, cement has a lamellar structure 

 and encloses lacunae and canaliculi, the lacunae enclosing 

 a nucleated cell (an included osteoblast), whose processes 



