ON THE DEVELOPMENT OF THE TEETH 235 



Since then this " Nasmyth's membrane " is identical, on the one 

 hand, with the persistent capsule which lies external to both enamel 

 and cement, and, upon the other hand, with the preformative membrane 

 of Raschkow, or otherwise with the basement membrane of the pulp ; 

 it is clear that all the tissues of the tooth are formed beneath the base- 

 ment membrane of the pulp ; in other words, they are all true dermic 

 structures — none epidermic.^ 



The third problem was, the relation of the histological elements of 

 the soft parts (that is, as we now see, of the pulp) to the Dentine, 

 Enamel, and Cement. 



Three theories have been prevalent as to the mode of development 

 of the dentine. The first, the old excretion theory, need not be con- 

 sidered here, as it has been given up on all sides. The second, the 

 Conversion theory, consists essentially in the supposition that the 

 dentine is the "ossified pulp ;" that the histological elements of the 

 pulp become calcified and converted directly into the dentine — the 

 arrangement of the elements of the dentine depending upon that of 

 the elements of the pulp. This is the doctrine niaintained by Blake, 

 Schwann, Xasmyth, Owen, Tomes, Henle, Todd and Bowman, and, 

 more or less doubtfully, by Kolliker and Hildebrandt.^ The third 

 theory is that contained in the remarkable phrase of Raschkow. 



" Postquam . . fibrarum dentalium stratum depositum est (quoted 

 by Schwann) idem processus continuo ab externa regione internam 

 versus progreditur germinis dentalis parenchymate viaterimn suppedi- 

 tante . . . Conversse fibrarum dentalium flexura; qua; juxta latitudinis 

 dimensionem crescunt, dum ab externa regione internam versus 

 procedunt sibi invicem apposita; continuos canaliculos effingunt, qui ad 

 substantiae dentalis peripheriam exorsi multis parvis anfractibus ad 

 pulpam dentalem cavumque ipsius tendunt, ibique aperti finiuntur 

 novis ibi quamdiu substantiae dentalis formatio durat fibris dentalibus 

 aggregandis inservientes." 



1 That the enamel is not formed directly from the enamel pulp might have been 

 concluded from Professor Goodsir's observations (/. c, p. 25). He says, "The absorption 

 (in the granular matter) goes on increasing as the tooth substance is deposited, and when 

 the latter reaches the base of the pulp, the former disappears, and the interior of the 

 dental sac assumes the villous vascular appearance of = mucous membrane. This change is 

 nearly completed about the seventh or eighth month." It will not be said, however, that 

 the growth of the enamel ceases at the seventh or eighth month. 



"- Dr. Sharpey, on the other hand, with characteristic caution, after citing the statements 

 of some of the advocates of the Conversion Theory, adds, "We must confess that, after a 

 careful examination of the human teeth, we have been unable to discover any of the abo\-e- 

 mentioned changes, except the enlargement of the more superficial cells of the pulp, and their 

 elongations in the immediate vicinity of the dentine." — Qiiain and Sharpey, p. 988. 



