TEETH. 



[ 632 ] 



TEETH. 



cement does not differ from bone in struc- 

 ture, except in rarely containing Haversian 



Fig. 736. 



Perpendicular section of the apex of a human incisor 

 tooth, a, pulp cavity ; b, ivory ; c, curved contour lines 

 with interglobular spaces ; d, cement ; e, enamel, with 

 indications of the course of the fibres in various directions ; 

 /, coloured stripes of the enamel. 



Magnified 7 diameters. 



Fig. 737. 



canals. In the molar teeth of old persons, 

 these are, however, met with (fig. 738 e). 



Fig. 738. 



Portion of the ivory, with ivory globules and interglo- 

 bular spaces filled with air. Magnified 350 diameters. 



Cement and ivory of the fang of a tooth of an old 

 person, a, cavity ; b, ivory ; c, cement with lacunae ; 

 e, Haversian canal. Magnified 30 diameters. 



The lacunse are frequently absent from the 

 thinner portion of the cement, and it some- 

 times contains tubes like those of the ivory. 

 The interlacunar substance is sometimes 

 striated, and exhibits a laminated struc- 

 ture. 



The enamel (fig. 731 a) covers the ivory of 

 the crown of the teeth. It is thickest at the 

 opposing surface, decreasing towards the 

 neck, where it terminates. It is covered by 

 a very thin membrane, separable after the 

 action of muriatic acid, and containing cal- 

 careous matter ; this has been regarded as a 

 continuation of the cement. The enamel 

 has a fibrous aspect, and appears of a bluish 

 white colour by reflected light, and of a 

 greyish brown Ijy transmitted light. It is 

 very brittle, and so hard as to strike fire with 

 steel. It consists of numerous solid fibres 

 or prisms (fig. 739), about 1-6000 to 1-5000" 

 in breadth, mostly six-sided, more or less 

 wavy, slightly varicose, and transversely 

 striped. These usually extend throughout 

 the thickness of the enamel, and are placed 

 in a direction generally perpendicular to the 

 surface of the portions of the ivoiy which 

 they cover (figs. 731, 736). The form of* 

 the fibres is best seen by viewing their 

 ends or a transverse section (fig. 740). The 



