1122 



THE DIGESTIVE SYSTEM. 



STKUCTURE OF THE TEETH. 



As mentioned above, the teeth are composed of three special tissues, substantia adaman- 

 tina or adamant (O.T. enamel), substantia eburnea or ivory (O.T. dentine), and sub- 

 stantia ossea (O.T. crusta petrosa or cement), in addition to the pulp which occupies 

 the tooth cavity. The chief mass of the tooth is formed of substantia eburnea, which 

 surrounds the tooth cavity and extends from crown to root ; outside this is a covering 

 of substantia adamantina on the crown, and a layer of substantia ossea on the root. 



The substantia adamantina is the dense, white, glistening layer which forms a 

 cap, thickest over the tubercles, for the portion of each tooth projecting above the 



gum (Fig. 888). At the neck it ceases gradu- 

 ally, being here slightly overlapped by the sub- 

 stantia ossea. 



Crow 



cavity 



Gum 



Neck 



Root 



Adamant 



It is composed chiefly of phosphate and carbon- 

 ate of lime (phosphate of calcium, 89'82 per cent, 

 carbonate of calcium, 4-37 per cent, magnesium 

 phosphate, 1/34 per cent., a trace of calcium fluoride, 

 other salts, -88 per cent), and has generally been 

 Tooth considered to contain about 3 '6 per cent of organic 

 substance ; but Tomes has recently shown this to 

 be inaccurate : " That which has heretofore been 

 set down as organic matter is simply water 

 combined with the lime salts. The substantia 

 adamantina is to be regarded as an inorganic 

 substance composed of lime salts, which have 

 been deposited in particular patterns and formed 

 under the influence of organic tissues, which 

 have themselves disappeared during its forma- j 

 tion." 



The adamantine substance consists of calci 

 fied microscopic prisms, prismata adamantina, 

 radiating from the surface of the ivory, on which 

 their inner ends lie, to the surface of the crown, 

 on which they terminate by free ends. These 

 prisms are hexagonal in shape, solid, and oij 

 considerable length, for most of them reach 

 from the ivory to the surface of the crowr 

 without interruption. The prisms, which ar< 

 calcined themselves, are held together by th< 

 smallest possible amount of calcined matrix 

 (Tomes). In old teeth the cap of adamantin< 

 substance is often worn away over the tubercles 

 the ivory is then exposed, and is easily recog 

 nised by its yellowish colour, which contrast 

 strongly with the whiteness of the adamant. 

 Whilst adjacent adamantine prisms are in general parallel to one another, they do no 

 usually take a straight, but rather a wavy course, and in alternate layers they are oftei 

 inclined in opposite directions, thus giving rise to certain radial striations seen b; 

 reflected light (Schreger's lines). Certain other pigmented lines, more or less parallel t 

 the surface, are also seen in the adamant (brown striae of Retzius). They are due to tru 

 pigmentation (Williams), and mark the lines of deposit of the adamant during its develop 

 ment. The adamantine prisms are more or less tubular in certain animals viz., in ai 

 marsupials except the wombat, in the hyrax, certain insectivora, and certain rodents. 



Cuticula dentis (O.T. Nasmyth's membrane) is an extremely thin (SIFOTRT of an inch 

 cuticular layer which covers the adamant of recently-cut teeth, and is very indestructiblt 

 resisting almost all reagents. Two chief views are held as to its origin. One that it i 

 the last formed layer of adamant, which has not yet been calcined, and therefore the fim 

 product of the adamant cells. The other that it is produced by the outer layer of cells c 

 the adamant organ. This latter seems to be the more probable view. 



Substantia eburnea or ivory (O.T. dentine) is the hard and highly elastic substanc< 

 yellowish white in colour, which forms the greater part of the mass of every toot 

 (Fig. 888). Like the adamant it is highly calcined, but it differs from it in containin 



Bone 



Substantia ossea \ 



Alveolar periosteum or root-membrane 



FIG. 888. VERTICAL SECTION OF CANINE TOOTH 

 to illustrate its various parts, and its structure. 



