VI INTRODUCTION. 



analysis : the late improvements in mechanical optics have led 

 to a resumption of the microscopical observations originally 

 commenced by Malpighi and Leeuwenhoek ; and the consequent 

 acquisition of more exact knowledge of the mode in which the 

 particles of the phosphate of lime and other salts are arranged in 

 the animal basis or matrix of bone and tooth. 



As regards the teeth, the principle of chief import to the physio- 

 logist arises out of the fact, which has been established by microscopic 

 investigations, that the earthy particles of dentine are not confusedly 

 blended with the animal basis, and the substance arranged in superim- 

 posed layers; but that these particles are built up, with the animal basis 

 as a cement, in the form of tubes or hollow columns, in the predeter- 

 mined arrangement of which there may be discerned the same relation 

 to the acquisition of strength and power of resistance in the due 

 direction, as in the disposition of the columns and beams of a work 

 of human architecture. 



The disposition of the calcareous particles of bone in the 

 parietes of the Haversian canals, Purkinjian cells and of the fine 

 tubes which radiate from these cavities, was ascertained before the ana- 

 logous conditions of the intimate structure of dentine were discovered. 



Until a recent period the analogy of dentine to bone was 

 supposed to be confined to their chemical constitution, and the 

 nature of the hardening material ; while the arrangement, as well 

 as the mode of deposition of the firm tissue, were considered to be 

 wholly different from that of bone, and the dentine to agree in its 

 general nature and mode of growth with hair and other extravas- 

 cular horny parts, with which most teeth closely correspond in their 

 vital properties. 



The structure of a tooth, in fact, was regarded as simply 



