38 SHARKS. 



pulp, and in the ratio of the number, minuteness and aggregated 

 disposition of the cavities containing that earthy matter, is the fa- 

 cility of detaching the ossified from the unossified part of the tooth 

 increased. But this facility of separation is quite inadequate to prove 

 an absence of organic connection between the separated parts, and 

 a formation of the calcified layer by transudation from a free secreting 

 surface. The calcigerous tubes of the mammiferous tooth have 

 distinct parietes in both the calcified and uncalcified portions of the 

 pulp ; these parietes are rendered brittle by the deposition therein of 

 earthy particles in the calcified part of the pulp, and separate readily 

 from the uncalcified continuation of the tubes in the remaining pulp ; 

 while the minuteness of the ruptured tubes renders the irregularity 

 of the denuded surface of the pulp invisible to the naked eye ; but 

 the appearance thus presented of a naturally free transuding surface is 

 deceptive. (1) 



(]) Since these facts and the general conclusions as to the nature of dentification deduced 

 rom them were mentioned in my Lectures at the College of Surgeons (May 1839), and subse- 

 quently communicated with more detail to the French Academy, I have perused the work by 

 Dr. Schwann, entitled " Untersuchungen ueber Einstimmung der Struktur und Wachsthura 

 der Pflanze und Thiere," 8vo., 1839 : or, " Observations on the Correspondence between Plants 

 and Animals in their Structure and Growth." Not anticipating, from the title of this work, that 

 it contained observations bearing immediately on dental anatomy, I have to regret that some 

 months elapsed after its publication before I ordered it from my bookseller. Dr. Schwann 

 describes the results of microscopic observations, which he instituted on the development of the 

 dentine in mammalian teeth, and arrives at the conclusion that the process-like ossification, is one 

 of intus-susception. Thus the theory of dentification, which I apphed analogically from observation 

 of the process in the shark, to the same process in the higher vertebrate animals, is established ex 

 visu by one of the most accurate and experienced micrographers of the present day. A full analysis 

 of his observations will be given in the general introduction to the present work. Cuvier, after stat- 

 ing that the teeth of fishes grow Uke those of quadrupeds, by layers, adds in the first edition of the 

 Lemons d'Anatomie Comparee, iii, p. 112, "Mais les dents qui ne tiennent qu'a la gencive 

 seulement, comme celles des Squales, croissent a la maniere des epiphyses des os, c'est-a-dire 

 que toute leur substance est d'abord tendre et poreuse, et qu'elle se durcit uniformement, et finit 

 par devenir entierement dure comme de I'ivoire." I have never seen an instance of such uni- 

 form hardening — if by this is meant a uniform deposition of the earthy particles through the 

 whole substance of the pulp— in the tooth of any shark or other animal ; and, were such actually 

 the case, it would not be like the ossification of an epiphysis. It seems that Cuvier himself had 

 withdrawn his confidence in the observation and conclusion above quoted, as the passage is sup- 

 pressed in the second edition of the Le5ons, by M. Duvernoy, in 1835. It is true, however, that 

 the teeth, not only of sharks^ but of the higher animals, are developed like bones ; the harden- 

 ing salts in both cases are deposited in preformed cavities organized in a pre-existing mould or 



