INTRODUCTION. XIX 



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, as has been already 

 observed, in the simple teeth of man and carnivorous animals. The 

 cement is most conspicuous where it invests the fang of the tooth, 

 and increases in thickness as it approaches the apex of the fang. 

 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 

 consistent than the animal basis of the dentine, but resisting longer 

 the solvent action of boiling water, and retaining some fine particles 

 of the earthy phosphates when all such earth had been extracted from 

 the dentinal tissue. Cuvier, likewise, states that the cement is dis- 

 solved with more difficulty in acid than the other dental tissues. 

 Retzius, however, states that the earth is sooner extracted by acid 

 from the cement than from the dentine of the teeth of the horse. 



In recent mammalian cement the radiated cells, like the 

 dentinal tubes, owe their whiteness and opacity to the earth which 

 they contain. According to Retzius, " numerous tubes radiate from 

 the cells, which, being dilated at their point of commencement, give 

 the cell the appearance of an irregular star. These tubes form 

 numerous combinations with each other, partly direct and partly 

 by means of fine branches, of io;uooth to som of an inch in diameter. 



" The cells often vary in size, and some put on the appearance 

 of a canal or tube ; this is especially seen in recently formed 

 cement. The average size of the Purkinjian cells in human cement 

 is riuoth of an inch. In sections made transversely to the axis of the 

 tooth it is clearly seen that these cells are arranged in parallel or 



&2 



