1() 1)11. nEALK, ON THE TISSUES. 



material^ and the impregnation of this formed material Avitli 

 calcareous matter^ rapidly at first,, but more slowly as the 

 quantity of calcareous matter increases. In the adult, the 

 remains of the germinal matter slowly undergo conversion 

 into formed material^ and this slowly becomes impregnated 

 with calcareous salts. In old age^ although the pulp is very 

 much reducecb this conversion is not complete, and a certain 

 amount of germinal matter still remains in the tubes and 

 in the pulp cavity from vrhich dentine might have been pro- 

 duced. 



Stellate tissue on surface of crusta peirosa. — Upon the sur- 

 face of the fang of the tooth in contact with the crusta 

 petrosa is a tissue of a very interesting structure, Avhich takes 

 part in the formation of the cementum. It is composed en- 

 tirely of what may be described as branching cells (elementary 

 parts), the processes of which anastomose ' freely with each 

 other. It is from this tissue that the crusta petrosa is formed; 

 but the lecturer alluded to it because it was a most perfect 

 example of a tissue consisting entirely of cells, the cavities of 

 Avhich communicate ■with each other by tubes. The stellate 

 cells are here as distinct as they are in the pith of a rush. 

 But do these cells and tubes merely constitute an elaborate 

 system of channels for the distribution of nutrient material 

 to the tissue which intervenes between them ? This tissue it 

 may be remarked, grows very slowly ; it is a very low simple 

 form of tissue, and probably requires but very little nutrient 

 matter. If the above view is adopted it must be admitted 

 that the means for nourishing the structure are far more 

 elaborate than would be expected, supposing the conclusion 

 is accepted that there ought to be a constant relation between 

 the activity of change in a tissue and the mechanism for 

 bringing new matter to the elementary parts and carrying off 

 the effete material from them. 



Neither does it appear that all these bodies become lacunae 

 of the cementum. The stellate cells just described for the 

 most part have not more than from ten to twelve processes 

 or tubes projecting from them, while many of the lacunae of 

 the cementum have as many as thirty or forty, hence these 

 tubes are certainly not an early stage of the canaliculi, and 

 the cells cannot become lacunte simply by the deposition of 

 calcareous matter in the intervening matrix or intercellular 

 substance. 



This stellate tissue on the surface of the fang nevertheless 

 undergoes calcification. The process of calcification may be 

 seen to take place in one of the specimens. The processes of 

 the stellate masses become narrower and narrower, until the 



