STRUCTURE OF TEETH. ENAMEL. 



285 



of it is afforded, however, by Mr. Tomes' s researches on the development of 

 dentine ( 213). The Dentinal tubuli are far too minute to receive blood; but 

 it may be surmised that, like the canaliculi of bone, they absorb matter from 

 the vascular lining of the pulp-cavity, which aids in the nutrition of the tooth. 

 Although, when once fully formed, the Tooth undergoes little or no change, there 

 is evidence that it possesses a certain power of repairing the effects of disease; 

 a new layer of hard matter being sometimes thrown out on a surface, which has 

 been laid bare by Caries. It has been found, too, that the Dentine is sometimes 

 tinged by coloring matters contained in the blood. This is most evident, when 

 a young animal is fed upon madder, during the period of the formation of the 

 tooth; but even in an adult, some tinge will result from a prolonged use of this 

 substance; and it has been noticed that the teeth of persons, who have long 

 suffered from Jaundice, sometimes acquire a tinge of bile. The pulp-cavity is 

 sometimes the seat of a secondary development of dentinal substance by which 

 its cavity is greatly contracted, or even obliterated. This is seen especially in 

 the teeth of old persons, or in those which have been much worn; and also in 

 those that are the subjects of caries, a layer of "secondary dentine" being 

 formed between the soft pulp, and the spot towards which the disease is advancing. 

 This "secondary" dentine is not so regular in its structure as the "primary," 

 and more resembles that of the lower animals; for it is usually traversed by 

 "vascular canals" proceeding from the pulp-cavity, and the tubuli radiate from 

 these, instead of from one common centre. Moreover, the presence of stellate 

 lacunae, resembling those of bone, is much more common in this substance than 

 in true dentine; so that, both in the presence of the vascular canals which re- 

 present the Haversian, and also in its own texture, this substance may be con- 

 sidered as intermediate between Dentine and Bone. 



276. The Enamel (Fig. 77) is composed of solid prisms or fibres (Fig. 78, B), 

 from about l-5600th to l-3300th 



of an inch in diameter, arranged Fig. 77. 



side by side, and closely adherent 

 to each other; their direction is for 

 the most part vertical to that of the 

 dentinal surface on which they rest, 

 so that their length corresponds 

 with the thickness of the layer 

 which they form ; and the two sur- 

 faces of this layer present the ends 

 of the prisms, which are usually 

 more or less regularly hexagonal 

 (Fig. 78, A). The course of these 

 prisms is generally wavy (Fig. 77), 

 but their curves are for the most 

 part parallel to each other; not 

 unfrequently, however, the curves 

 separate from each other, or even 

 decussate, the intervening spaces 

 being then filled in with shorter 

 fibres. The enamel-prisms are 

 usually marked by transverse stride 

 (Figs. 77, 78, B), the distance of 

 which is about equal to the dia- 

 meter of the fibre; these appear 

 to be indications of the partitions 

 between the longitudinally-joined A> TransTerse section of J&iaMd> ghowing the hexagonal 



Cells, by whose coalescence each form of its prisms ; B, separated prisma. 



Vertical section of the Enamel of the Human Molar Tooth. 



Fig. 78. 



