138 MICROSCOPIC ANATOMY OF THE TEETH 



arrest the passage of the solute. This definition is inexact, 

 since no membrane permeable to water is absolutely im- 

 permeable to the solutes. All we can say is that certain 

 membranes are more permeable to water than to the sub- 

 stances in solution, and are, moreover, very unequally 

 permeable to the substances in solution. The term " osmotic 

 membrane " should therefore in all cases replace that of 

 "semi -permeable membrane".' 



In the experiment with the copper sulphate above 

 described, it was seen that although the crystals accumulated 

 on the proximal side of the membrane very rapidly, the part 

 of the tube on the distal side remained clear for several 

 hours, when the slow diffusion of the salts through the 

 membrane caused the production of the calcospherites 

 which appeared in this situation, the lime salts contained 

 in the water which had permeated the tube wall diffusing 

 through the septum. We thus see that under purely physical 

 conditions we have a dialysis of salts of lime through these 

 inorganic membranes, and this observation points to the 

 suggestion that the same thing may occur from physical 

 causes in the calcification of the various hard tissues of man 

 and animals. 



How far the membranes found in the living body are the 

 result of purely physical agencies, and to what degree they 

 are affected by physiological and vital conditions, we are 

 unable to determine, but it is seen that very firm membranes 

 do arise in the absence of all organic substances, although, as 

 Professor Philip says (19), ' a purely physical theory of the 

 exchanges which take place across a living membrane is 

 inadequate ; there is a physiological as well as a physical 

 permeability ' . 



The bearing of these observations will be better under- 

 stood after the calcification of enamel and dentine has been 

 considered. 



In the year 1858 Rainey published his work on molecular 

 coalescence (22), showing that lime salts were deposited in 

 colloidal solutions not in a crystalline but in a globular 

 form, the globules having a very definite structure and 

 arrangement. He also showed that in many animal 

 organisms, as Mollusca and Crustacea, similar forms appeared 



