TEETH. 



[ 631 ] 



TEETH. 



rizontal. They have distinct walls, about 

 equal in thickness to their calibre, although 



Fig. 734. 



Ivory-tubes of a fang of a human tooth, a, inner 

 surface of the ivory, with few tubes ; b, their branches ; 

 c, their terminations in loops ; d, granular layer, con- 

 sisting of small ivory globules at the boundary of the 

 ivory ; e, lacunse of bone, one anastomosing with an ivory- 

 tube. Magnified 350 diameters. 



in transverse sections (fig. 735) this thick- 

 ness is generally exaggerated, on account of 



Fig. 735. 



Transverse section of the ivory-tubes, a, closely aggre- 

 gated, b, wider apart. Magnified 450 diameters. 



their being obliquely divided. They contain 

 air in the dry state, which may be displaced 

 by liquids. By removing the inorganic salts 

 from a tooth with dilute muriatic acid, and 

 macerating the remaining cartilage with 

 acids or caustic alkalies until it forms a pasty 

 mass, the tubes may be isolated from the 

 basis. 



The ivory not unfrequently exhibits indi- 

 cations of a laminated structure, forming, 

 in longitudinal sections, curved lines more 

 or less parallel to the outline of the crown 

 (fig. 736), appearing as rings in transverse 

 sections, and called the contour-lines. 



Near the enamel (fig. 736) and the cement 

 (fig. 734 d) also, the ivory presents one or 

 more irregular dark patches or bands, often 

 continuous with the ends of the contour- 

 lines, and exhibiting a coarsely cellular ap- 

 pearance. On careful examination, the dark 

 appearance is seen to result from a number 

 of irregular spaces filled with air (fig. 737 a) 

 intervening between certain globules, called 

 ivory-globules, the spaces being termed the 

 interglobular spaces. In the recent tooth, 

 these spaces are filled with the organic basis 

 of the ivory, containing tubes like the rest 

 of that substance, in which, however, the 

 inorganic matter has not been deposited; 

 hence this structure arises from imperfect 

 development. 



Other ill-defined iridescent stripes, running 

 parallel to the pulp-cavity, are sometimes 

 seen ; these correspond to the primary curves 

 of the ivory-tubes. 



The cement or bone of teeth forms the 

 outer coating of the fangs (figs. 731 c & 738), 

 sometimes cementing them together. It 

 commences as a very thin layer at the part 

 where the enamel ceases, increasing in thick- 

 ness towards the ends of the fangs. The 



