STRUCTURE OF THE DEXTIXE. 309 



ing together in loops, or in the interglobular cavities shortly to be 

 described. 



The tubules arc described as having each a proper wall, independent 

 of the intertubular matrix, hut intimately adhering to, and under ordi- 

 nary circumstances indistinguishable from it. This wall, named the 

 dental or dentinal sJieafJi, is formed of a calcified membranous tube.* 

 By steeping sections of decalcified dentine in strong hydrochloric acid, 

 the matrix is destroyed, and the membranous tubes, which consist of a 

 more resisting material (probably elastic substance), remain behind. 

 In sections of hard dentine made across the tubules (fig. 209), the walls 

 of these often ajipcar as a distinct thin border : this may, however, be 

 due to an optical effect. 



In properly prepared sections of softened teeth fine processes may be 

 seen passing into the tubules from the surface-cells of the pulp before- 

 mentioned ; and it is suggested by Tomes that these are not only 

 subservient to the nutrition of the dentine, but probably also confer on. 

 it a certain degree of sensibility. It has been ^ 



noticed, indeed, that the dentine is more ^^' " 



sensitive near the surface than deeper in its 

 substance, — a fact not easily intelligible on 

 the supposition that the sentient tissue is 

 confined to the pulp-cavity. But the sensibility 

 of the teeth may not improbably be in part 

 dependent on the nerves in the dental 

 periosteum (Salter). 



T ,, , 1 , . . ^, Fig. 209. — Sections op Den- 

 Li the tempoi-aiy, and sometimes even m the per- %^^^^ Tubules (after 



manent teeth, the tubules are constncted at short FraenkeU 



intervals, so as to present a monilifonn character. 



Tlae terminal branches of tubules are occasionally ,. "' ,^^^* ''^^^^"f^ '\ \^^^^ ,. 

 „ „ .^ .,,, J.T---U hquely. (About 300 dia- 



seen to pass on into the cement which covers g- .*\ 



the fang, and to communicate with canaliculi 



proceeding' from the characteristic lacmiaj found in that osseous la3'er. Tu- 

 bules have likewise been observed by Tomes passing into the enamel in the 

 teeth of marsupial animals, and in a less marked degree in human teeth. 



The mferliihular suhstanre is translucent. The animal matter which 

 remains in it, after the earth has been removed by an acid, exhibits 

 a tendency to tear in the direction of the tubules, "but is in reality a 

 homogeneous substance, deposited in a laminated manner. This was 

 shown by Sharpey, who observed that in the softened teeth of the 

 cachalot or sperm-whale the animal substance was readily torn into fine 

 lamella}, parallel with the internal surface of the pulp-cavity, and there- 

 fore across the direction of the tubules. In these lamella the sections 

 of the tubules appear as round or oval apertures, the lamelte having 

 the same relation to the tubules as those of true bone to the canaliculi. 

 The same tendency to lamination is exhibited by boiling a longi- 

 tudinal section of tooth with caustic potash, after which it presents 

 closely set, short, and regular fissures, lying at right angles to the 

 tubules, throughout the extent of the dentine (Cleland) ; moreover, 

 a thin cross-section of a tooth may be broken up, after decalcification, 

 into concentric riugs like the year-rings of wood (Salter). 



* Some authorities maintain that the sheatlis are not calcified. 



