XVI INTRODUCTION. 



longer the solvent action of boiling water, and retain- 

 ing some fine particles of the earthy phosphates when 

 all such earth had been extracted from the dentinal 

 tissue. Cuvier also states that the cement is dissolved 

 with more difficulty in acM than the other dental tis- 

 sues. 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 beginning, give the cells the 

 appearance of an irregular star. These tubes form 

 numerous combinations with each other, partly direct 

 and partly by means of fine branches of -j-g-J^ th to 

 -^-o^ro-th of an inch in diameter. The cells vary in 

 size. The average size of the Purkinjean cells in hu- 

 man cement is TgVffth 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 concentric 

 striae, of which some are more clearly and others more 

 faintly visible, as if the cement were deposited in fine 

 and coherent layers.' The layer of cement is found in 



*Prof. Retzius, of the University of Stockholm, informs us 

 that he had been led by the iridescence of the fractured surface 

 of the substance of a tooth to conceive that that appearance was 

 due, as in the crystalline lens, to a fine fibrous structure, and that 

 he communicated his opinions as to the regular arrangement of 

 these fibers to some of his colleagues in 1834. In 1835, having 

 obtained a powerful microscope, he began a series of more exact 

 researches on the intimate structure of the teeth in man and 

 the lower animals, which he communicated to the Royal Acad- 

 emy of Sciences at Stockholm on January 13, 1836, being then 

 unacquainted with the discoveries of Purkinje. Owen. 



