THE CONNECTIVE TISSUES. 



r CH. V. 



which give the dentine a certain faint sensitiveness under ordi- 

 nary circumstances and, without doubt, have to do also with 

 its nutrition. These prolongations from the tooth-pulp are 

 processes of the dentine-cells or odontoblasts which are columnar 

 cells lining the pulp-cavity ; the relation of these processes to the 

 tubules in which they lie is precisely 

 similar to that of the processes of the bone- 

 corpuscles to the canaliculi of bone. The 

 outer portion of the dentine, underlying 

 the cement, and the enamel to a much 

 lesser degree, forms a more or less distinct 

 layer termed the granular or interglobular 

 layer (fig. 95). It is characterised by the 

 presence of a number of irregular minute 

 cell-like cavities. The explanation of these 

 will be seen when we study the develop- 

 ment of a tooth. 



Enamel. 



Chemical composition. The enamel, 

 which is by far the hardest tissue in the 

 body, is composed of the same inorganic 

 compounds that enter into the composition 

 of dentine and bone. Its animal matter, 

 however, amounts only to about 2 or 3 

 per cent., and does not yield gelatin on 

 boiling. According to Tomes it contains 

 no animal matter at all. Gelatin is a 

 characteristic product of connective tissue, 

 and enamel is not a connective tissue, but 

 is epithelial in origin. 



Examined under the microscope, enamel 

 is found composed of six-sided prisms (figs. 

 96, 97) 50*00 f au mcn in diameter, 

 which are set on end on the surface of the 

 dentine, and fit into corresponding depres- 

 sions in the same. 



They radiate in such a manner from the 



dentine that at the top of the tooth they are more or less vertical, 

 while towards the sides they tend to the horizontal direction. 

 Like the dentine tubules, they are not straight, but disposed in 

 wavy and parallel curves. The prisms are marked by transverse 

 lines and are solid. 



The enamel prisms are connected together by a very minute 



Fig. 97. Thin section of the 

 enamel, and a part of the 

 dentine. a, cuticular 

 pellicle of the enamel 

 (Nasmyth's membrane) ; 

 &, enamel columns with 

 fissures between them 

 and cross striae ; c, larger 

 cavities in the enamel, 

 communicating with the 

 extremities of some of 

 the dentinal tubules (d) . 

 x 350. (Kolliker.) 



