xii INTRODUCTION. 



which characterize the structure of true bone : and he observed in 

 one instance that this bone-Uke substance was continued upon the 

 enamel of the crown of a human incisor. 



This fact I have confirmed(l) as regards the human teeth and 

 the simple teeth of many mammals and reptiles. The layer of 

 coronal cement varies in thickness ; its tenuity is extreme in the teeth 

 of man and the quadrumana. 



Purkinje also found that the third substance, crusta petrosa or 

 cement of compound teeth, as those of the horse and ox, was 

 in like manner characterized by the presence of numerous bone- 

 corpuscules or cells ; and thus proved that the difference between 

 the so called simple and compound teeth depended, not on the pre- 

 sence of a third and additional substance in the latter, but on its 

 greater abundance and different disposition in the tooth. 



At the time that these observations^ were being made at Breslau 

 and Berlin, it appears that similar investigations had been set on foot 

 at Stockholm. Professor Retzius of the University in that city 

 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 fibres to some of his colleagues in 1834; and that the Uni- 

 versity having obtained, in the summer of 1835, a powerful micros- 

 cope, by Plessl of Vienna, he commenced a series of more exact 

 researches on the intimate structure of the teeth in man and the 

 lower animals. He operated on thin sections of teeth both before, and 

 after, the removal of the earthy matter by means of acid, and atten- 



(1) Trans. Brit. Assoc. 1838, vol. vii, p. 130. 



