INTRODUCTION. V 



capibara, the cement, which forms nearly half the mass of the 

 tooth, wears down sooner than the dentine. 



The ' enamel' is the hardest constituent of a tooth, and conse- 

 quently the hardest of animal tissues ; but it consists, like the 

 other dental substances, of earthy matter arranged by organic forces 

 in an animal matrix. Here, however, the earth is mainly contained 

 in the canals of the animal membrane, and, in mammals and reptiles, 

 completely fills those canals, which are comparatively wide, whilst 

 their parietes are of extreme tenuity. The hardening salts of 

 the enamel are not only present in far greater proportion than in the 

 other dental tissues, but, in some animals, are peculiarly distinguished 

 by the presence of fluat of lime. 



The absolute and relative size, form, disposition, direction and 

 intercommunication of the cellular and tubular cavities characterizing 

 the several tissues of the teeth will be the subjects of the special 

 descriptions of these organs in the different classes and species of 

 animals ; but a brief notice of the leading steps to the present 

 knowledge of the structure peculiar to each tissue may be appro- 

 priately given in this place(l). 



In a retrospect of the history of the science of the orga- 

 nization of animal bodies, anatomy may always be perceived 

 to have made a marked advance in connection with the progress 

 of some collateral science. With regard to the hard parts of our 

 frame in particular, our knowledge of the elementary constitution 

 of the earthy salts has been due to the refinement of chemical 



(1) The review here given of the discovery of the tubular structure of the dentine is 

 essentially the same as that prefixed to my own observations on the subject communicated to 

 the British Association in August, 1838.— See Trans, of the Brit. Assoc. 1838, p. 135. 



