INTRODUCTION. XV 



upon the perusal of which I abandoned my intention of pubHshing 

 those general observations on the structure of the teeth which I had 

 before deemed to be new, but now found to have been mainly anti- 

 cipated by Purkinje and Retzius. 



I was not, however, discouraged by this disappointment, but, 

 feehng convinced that no work on the Comparative Anatomy of the 

 Teeth would henceforth be regarded as complete without an account 

 of the leading modifications of the dentinal tissue in the different 

 classes of animals, I proceeded to the microscopical investigation of 

 that tissue in many animals in which it had not been previously so 

 examined. The number of characteristic differences which presented 

 themselves, and which are described in the body of the present work, 

 led to the perception of the value of the microscopic structure of 

 the teeth as a test of the affinities of extinct animals, and to the insti- 

 tution of researches into the laws of development of the dental tissues, 

 which, as then accepted and taught, were irreconcileable with the 

 general demonstration of the intimate structure of those tissues w^hich 

 was yielded by the teeth of fishes, reptiles and mammals. (I) 



The prelude to this generalization may be summarily recapitu- 

 lated as follows : the discovery of Leeuwenhoek that the dentine 

 was made up of very minute tubes, which proceeded from the inner 

 to the outer surface of the tooth, was confirmed by Purkinje, so far 

 as regarded their existence ; but Purkinje' added an exact and particular 

 account of the direction of these tubes in the human dentine, and 

 showed that, in addition to them the dentine contained an interme- 



(1) The chief results of these researches have been successively communicated to the 

 British Association at the Newcastle Meeting, August, 1S38, (Transactions of the Association, 

 vol. vii, p. 135 ;) in the Proceedings of the Geological Society for 1838 and 18.*^9, and in the 

 Comptes Rendus de I'Academie des Sciences, December 12, 1839. 



