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may be plainly seen, indicating, probably between 

 the ivory and the enamel, another source for the pro- 

 duction of the substance which the crusta petrosa has 

 been said to secrete. Moreover, if a tooth be boiled 

 for some hours, a very little force will enable any 

 person to pull off the enamel from the ivory ; and 

 this experiment likewise suggests the intervention of 

 some gelatinous membrane between the two struc- 

 tures. The microscope also confirms the opinion I 

 advance, and shows that, in the old tooth, the mem- 

 brane is absorbed, for its remains can only be de- 

 tected in isolated places. I am fully aware that none 

 of the arguments here advanced are, with the excep- 

 tion of the first, at all conclusive. Appearances are 

 deceptive, and results consequent upon artificial pro- 

 cesses by no means to be trusted. After all, how- 

 ever, the necessity for such a secretive membrane is 

 by no means imperative for the production of the 

 enamel. Our views on these points may be too cir- 

 cumscribed, since at the extremities of the bones we 

 see cartilage and osseous structure connected, without 

 any such interposition ; and no one expects such a 

 matrix for the vitreous table of the cranial parietes. 

 The crusta petrosa may have its uses apart from any 



