MICROSCOPIC STRUCTURE. 



467 



cular, more rarely fusiform : they average about asoo^h of an inch 

 in the long diameter : their contour is broken by the numerous 

 fine tubes which radiate from them, with wide angular beginnings, 

 but quickly contracting to the minute size of opuoth of an inch. 

 In transverse sections of the fang, the cells are generally seen to 

 be arranged in parallel concentric lines ; their long axis being in 

 the direction of the lines, which vary in clearness of definition, 

 like the concentric striae of enamel ; and, like these, indicate 

 stages of formation in the cement. The radiating tubuli anasto- 

 mose together either directly or by their numerous fine branches ; 

 which latter also communicate with the terminal ramifications of 

 the dentinal tubes, either directly or through the medium of the 

 fine granular cells dispersed through the boundary line between 

 the dentine and cement. In the deciduous teeth the cement is 

 relatively thinner and the cells fewer and less regularly arranged. 

 The thickened cement near the end of the fang of old permanent 

 teeth is occasionally perforated by a vascular canal, conveying 

 capillary vessels to the osteo-dentine and the small remnant of 

 the pulp. An increase beyond the usual thickness is usually ac- 

 companied by the formation of vascular canals in the cement ; and 

 it is the existence of this most highly organized of the dental 

 tissues which explains the * possibility of engrafting teeth upon 

 vascular parts. (1) 



In my Report to the British Association in 1838, which 

 contains the first announcement of some of the observations des- 

 cribed in detail in the present Work, I stated, with respect to the 

 component structures of a tooth, " that in addition to those 

 usually described and admitted, there were other substances en- 

 tering into the composition of teeth and presenting microscopic 

 characters equally distinct both from ivory, enamel and cement, 



Nov. 20th. 1840), succeeded in injecting them with Canada balsam, and I have generally 

 found in fossil teeth that the mineral matter had penetrated into the cells and tubes of 

 the cement, as well as into the tubes of the dentine. 



(1) Retzius rightly points out the cement as the seat of the abnormal growths called 

 'exostosis,' to which the fangs of teeth loosened by the scurvy or mercurial medicines are 

 more particularly liable. One of his figures of the thickened cement on the fang of the 

 tooth of an aged person is given in PI. 122, fig. 8. 



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