THEORY OF FOOD-ABSORPTION. 59 
for water. But the power of a substance in aqueous solution, whether without 
or within the cell-membrane, to permeate the saturated pores, and to mix thoroughly 
there, certainly depends also on the degree of chemical affinity and of adhesion 
existing between the molecules and micelle 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 cell-membrane 
differs from the free diffusion which would occur if the cell-membrane were not 
present. Experiment has proved that 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 organic compound, such as albumen or dextrin, is on one side, and water on 
the other, water transfuses to the organie 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, as 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 may 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 within. 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 obviously does not 
exclude the possibility of a real exchange taking place between substances on either 
side of a cell-membrane, 7.e. between solutions in the soil and those in the cell- 
sap contained in lacun® 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 
