18 BIOLOGY. [BOOK i. 



vation. Unceasingly it plays, face to face with tlie exterior 

 medium, the part of dialyser, either directly or by the aid of 

 special apparatus. It forms nutritive soluble substances, and 

 rejects waste substances, likewise soluble, at least in the liquids 

 of the organism. 



When, for instance, the residuum of the waste of the living 

 tissues is composed of crystalloid bodies, these bodies can easily 

 and rapidly traverse the colloidal substance of the tissues to be 

 expelled from the organism ; but their expulsion leaves in those 

 same tissues a void which other soluble substances can come to 

 fill by permeability ; and in this fashion the losses undergone 

 by the living machine are repaired without difficulty. 



Finally, the colloidal state is the form the most suitable for 

 the manifestation of the instability, the molecular mobility of 

 the complex bodies which constitute organised beings. Under 

 this form they are really in the dynamical state ; they yield 

 without difficulty to the shock, to the action of incident bodies. 

 They can unmake and remake themselves, become the scene of a 

 perpetual exchange of molecules and of atoms, in a word, of a 

 vital progression and regression. 



