STRUCTURE OF THE DEXTIXE. 



307 



Fis. 207 



Fig. 207. — Sections of an Incisor 

 AND Molar Tooth. 



The longitudinal sections show the 

 whole of the jmlp-cavity in the incisor 

 and molar teeth, its exten.sion uinvards 

 within the crowu, and its prolongation 

 downwards into the fangs with the 

 small aperture at the point of each ; 

 these and tlic cross section show the 

 relation of the dentine and enamel. 



vascular, and sensitive substance, called the dental pulp. This pulp con- 

 sists of jelly-like connective tissue containing fine lilaments of the white 

 variety, nucleated cells, blood-vessels 

 and nerves. The cells are partly 

 disseminated in the soft mass and 

 partly form a stratum at the surface 

 of the pulp, where they are elongated, 

 somewhat like the cells of columnar 

 epithelium (see fig. 220, c, p. 318). 

 On extraction of the pulp fi-om the 

 cavity of the tooth, this layer usually 

 remains adherent to the hard tissue, 

 and has been named the memhrana 

 f'boris. The oblong cells composing 

 it (pcloniobk(sts) send processes into 

 tubules in the dentine, to be after- 

 wards noticed, of which more than 

 one may come from the same cell ; 

 lateral offsets, according to Waldeyer, 

 proceed to join adjacent cells, and 

 radical processes connect the super- 

 ficial cells with others lying deeper 



in the pulp. The arteries and nerves, which are derived from the 

 internal maxillary and fifth pair respectively, enter by the aperture 

 at the point of each fang. The vessels form a capillary network 

 beneath the superficial cells ; the nerves, as described by Boll in the 

 rabbit's incisor, end in fine non-medullated fibres which aVe distributed 

 abundantly at the surface of the pulp and run up between the superficial 

 cells. Some appear to take the direction of those cell-processes which 

 enter the hard tissue, but they have not with certainty been traced into 

 the dentinal tubules. 



Hard tissues of the teeth. — The hard part of a tooth is composed 

 of three distinct substances, — viz., the proper dental substance, h'ori/ 

 or dentiiip, the enamel, and the cement or crusta petrosa. The dentine 

 constitutes by far the larger portion ; the enamel is found only upon 

 the exposed part or crown ; and the cement covers with a thin layer the 

 surface of the implanted portion or fang. 



The dentine (Owen.) forming the principal mass or foundation 

 of the body and root of a tooth, gives to both of these parts their 

 general form, and immediately encloses the central cavity. It resem- 

 bles very compact bone in its general aspect and chemical relations, but 

 is not identical with it in structure, or in the exact proportions of its 

 earthy and animal constituents. 



According to the analyses of Berzelius and Bibra, the dentine of 

 human teeth is composed of 28 parts per cent, of animal, and 72 of 

 earthy matter. The fonner is resolvable into gelatin by boiliug. 

 The composition of the latter, according to Bibra, is as follows, viz., 

 phosphate of lime GG*7 per cent., carbonate of lime 3*o, phosphate of 

 magnesia and other salts, including a trace of fluoride of calcium, 1*8. 

 Berzelius found 5*3 carbonate of lime. 



The dentine is penetrated thronghout by fine tubes, which being 

 nearly parallel give it a striated aspect (fig. 201). When a thin section 

 is viewed under the microsco ie by transmitted light, the solid sub- 



