IVORY OF THE TEETH. 101 



the growth of the other unorganized tissues treated of in the 

 previous class. If we suppose, for example, that the little 

 cylinders (columnar epithelium) of the mucous membranes 

 (which, according to Henle, are constantly being thrown off) 

 could become ossified at the moment when they separated from 

 the surface of the mucous membrane, we should obtain a cover- 

 ing to the membrane, consisting of little calcareous cylinders, 

 each of which, however, would still have its organic basis like 

 the enamel-fibres. Beneath this covering would be other 

 cylinders not as yet ossified, which, when they in like manner 

 became calcified would add to its thickness, whilst new cvlinders 

 grew forth from the mucous membrane. The quantity of the 

 organic basis is extremely small in the teeth of adults which 

 have been exposed for a considerable time to the action of the 

 saliva, a circumstance which I suppose to be referrible to its 

 undergoing a chemical solution in that fluid. 



b. The ivory. 



This is known to consist of a structureless 1 substance, 

 traversed by a great many minute canals. These canals 

 (tubes) have for the most part a radiate course from the 

 cavity of the tooth towards its external surface, and, accord- 

 ing to Retzius, often give off branches as they proceed. 

 Their peripheral terminations are extremely minute ; they are 

 thicker towards the dental cavity, and, when the pulp is removed, 

 open freely into it. Midler observed that the tubes projected 

 beyond the intermediate substance from the fractured surface 

 of thinly-ground lamina?, and of lamella? which had been 

 macerated in hvdrochloric acid, and were surrounded therefore 



«/ J 



by a special membrane ; Retzius also remarked the same 

 upon a transverse section. Purkinje and Midler noticed that 

 when teeth are placed in ink, the fluid penetrates into the 

 tubes ; they must therefore be hollow. If any of them contain 

 calcareous matter, it must be only the most minute ones. 

 According to Retzius, many teeth present corpuscles which 

 resemble those of bone, and like them send forth minute 

 radiating canaliculi. 



1 See note to page .'>!•. 



