30 

 had been in active employment for twelve months) 

 still incomplete, and not enveloped by crusta petrosa. 

 The above w^ood-cut, the accuracy of v^^hich is not 

 to be disputed, shows a comparatively large portion 

 of the assumed secretive membrane to be absent. 

 The deficient part of the outer covering, I may here 

 state, was removed by a natural process, as I myself 

 extracted the tooth. Nevertheless where the crusta 

 petrosa is deficient, the laminated external surface of 

 the enamel is exposed in perfect and normal con- 

 dition. This evidence appears pretty conclusive, for 

 where the secretive substance is absent, the secretion 

 would hardly appear. On the other hand, if the 

 secretion of the enamel were the special function of 

 the crusta petrosa, then wherever the one was found, 

 the other would also be present. Let the reader, 

 however, refer to the wood-cut inserted at page 2 1 . 

 In that diagram, the crusta petrosa is faithfully repre- 

 sented as thickest at the root ; but at this very point 

 the enamel is seen to be entirely wanting. The con- 

 clusion towards which such facts point, is almost 

 decisive ; but, nevertheless, there are many infer- 

 ences which help to support it. If a longitudinal 

 section of a tooth be made, a very thin white line 



