CHAP, i.] OF NUTRITION. 77 



that in some, every element living, ultimate, isolated, or associated 

 to other analogous elements is only a small mass of colloidal 

 substance. But like all colloidal bodies, this substance is 

 capable of imbibing water and aqueous solutions ; it may be said 

 greedily to seek them ; thus there is established in the midst of 

 its molecules an incessant aqueous current, which conveys to 

 them soluble substances, modified crystalloids or albuminoids, 

 and which at the same time seizes back other substances, usually 

 crystalloidal, which have become unfitted to form a part of the 

 living body. 



The phenomena of diffusion however are not by any means 

 peculiar to the liquid state. In a gaseous medium the diffusion 

 of liquids is simply replaced by gaseous diffusion direct or 

 indirect. We have seen besides that analogous phenomena 

 are produced even in the midst of living liquids, in the blood 

 and the lymph of the superior animals. 



But if the physical condition of nutrition is simple diffusion 

 in amorphous beings not yet composed of cells or of fibres, it is 

 a little more complex in the others, if we proceed from the 

 monocellular organisms to the superior mammif ers, constituted by 

 fibres and cells cemented or grouped in tissues. Here the diffusion 

 is accompanied by the passage of the liquids through a membrane, 

 that is to say, that there is osmosis, and naturally osmosis with 

 a double current from without to within, and from within to 

 without, endosmosis and exosmosis. 



We must needs do for osmosis what we have done for diffusion, 

 that is to say, briefly recall the principal facts appertaining 

 thereto. Osmosis, discovered by Dutrochet, 1 then studied 

 especially by Graham, who gave it the name of dialysis, is, as is 

 well known, the blending of two unequal densities, separated by 

 a membrane. Definitively it is diffusion in special mechanical 

 conditions, which permit the superposition of the liquids, whereby 



1 J.-B. Dutrochet, De I'Endosmose, in Mimoires pour servir d VHistoire 

 anatomique des Vegetaux ct des Animaux, t I. Paris, 1837. 



