WHAT THE TUBES COtfTAIK. XV 



jecting here and there from the surfaces. They were 

 white and opaque, stiff, straight, and apparently not 

 flexible. This appearance is well represented in the 

 old figure by Leeuwenhoek. If the lamellse had been 

 previously acted upon by acid, the projecting tubes 

 were flexible and transparent, and often very long. 

 Hence Miiller inferred that the tubes have distinct 

 walls, consisting of an animal tissue, and that, besides 

 containing earthy matter in their interior, their tissue 

 is, in the natural state, impregnated with calcareous 

 salts."* 



THE CEMENT. 



" The organized structure and microscopic character 

 of the cement were first determined by Purkinje and 

 Faenkel, and the acquisition of these facts led to the 

 detection of the tissue in the simple teeth of man and 

 carnivorous animals. The cement is most conspicuous 

 where it invests the root of the tooth, and increases in 

 thickness as it approaches the apex of the root. The 

 animal constituent of this part of the cement had been 

 recognized by Berzelius as a distinct investment of the 

 dentine long before the tissue of which it formed the 

 basis was clearly recognized in simple teeth. Berzelius 

 describes the cemental membrane as being less consist- 

 ent than the animal basis of the dentine, but resisting 



* If Lord Bacon's theory is correct, tlie probability is tliat these 

 tubes contain something besides earthy matter and calcareous 

 salts, to wit, spirit. In "Novum Organ um" he says (B. Mon- 

 tagu, vol. xiv, p. 417): "All things abhor a solution of their 

 continuity, but yet in proportion to their rarity. The more rare 

 the bodies be, the more they suffer themselves to be thrust into 

 small and narrow passages ; for water will go into a passage 

 which dust will not go into, air which water will not go into, 

 and flame and spirit which air will not go into." 



