THEORY OF FOUD-ABSURPTION. 59 



for water. But the power of a substance iu aqueous solutiou, whether without 

 or withiu the cell-membrane, to permeate the saturated pores, and to mix thoroughlj' 

 there, certainly depends also on the degree of chemical affinity and of adhesion 

 existinof between the molecules and micellae of the cell-membrane on the one hand, 

 and these infiltrating substances on the other. A very complex interaction of 

 forces takes place which we cannot here investigate any further, as it would take 

 us much too far afield. 



Returning to the explanation of food-absorption, attention must be drawn to 

 the fact that the mixing or diffusion which takes place through the eell-menibrane 

 differs from the free diftusion which would occur if the cell-membrane were not 

 present. Experiment has proved tliat if one side of a cell-membrane is steeped 

 in a saline solution and the other in an equal volume of pure water, the number 

 of saline particles which pass through into the water are many fewer than the 

 number of water-particles which pass into the solution of salt; and, moreover, if 

 an oi'ganic compound, such as albumen or dextrin, is on one side, and water on 

 the other, water transfuses to the organic compound, whereas no trace of the 

 albumen or dextrin (as the case may be) passes through to the water. Now this 

 phenomenon, which is called "osmosis" (" endosmosis and exosmosis"), is of great 

 importance for the conception we have to form of food-absorption. It is clear that, 

 whilst water and substances dissolved in water are brought under the control of 

 the protoplast within a cell through the cell-membrane, an a consequence of the 

 action of albuminous and other compounds constituting the body of the protoplast, 

 and of the salts dissolved in the so-called cell-sap in the vacuoles, there is no 

 necessity for any part of the cell-content to pass out through the cell-membrane. 

 Thus the protoplasm is able to exercise an absorptive action on aqueous solutions 

 outside the cell-membrane, and to continue to absorb until the cell is filled. Indeed, 

 the chemical affinity for water possessed by the substances in a cell maj- occasion 

 so great an absorption of water that, in consequence, the volume of the cell is 

 enlarged and the cell-membrane is subjected to pressure from withiu. The cell- 

 membrane is able to yield to this pressure to the extent permitted by its elasticity; 

 but excessive stretching of the cell-membrane is at length counteracted by cohesion, 

 and thus a condition is attained in which the cell-contents and the cell-membrane 

 are subjected to mutual pressure, a state which is called "turgidity." 



The process just described, of the absorption of water in large quantities into 

 the precincts of the protoplasm without any simultaneous transmission of matter to 

 the outside, is certainly in no respect an exchange. But it obviouslj^ does not 

 exclude the possibility of a real exchange taking place between substances on either 

 side of a cell-membrane, i.e. between solutions in the soil and those in the cell- 

 sap contained in lacunae of the protoplasm. Certain phenomena in fact put it 

 beyond doubt that on occasion a real exchange of this kind does occur. But it 

 is complicated by the circumstance that substances in process of being exchanged 

 have to pass not only through the cell-membrane but also through the primordial 

 utricle: and the primordial utricle consists of molecules of a kind other than 



