ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 131 
they be surrounded by a solution of the same osmotic pressure as 
their own contents. The fact that whatever be the chemical nature 
of the solute, so long as it does not injure the cell, its osmotic pres- 
sure must be of the same definite value, unless either swelling or 
contraction of the cell is to occur, is in itself proof of the semi- 
permeability of the cell membrane. The facts are so simply ex- 
plained on this view that it is somewhat puzzling to understand why 
elaborate theories should be invented to explain the phenomena 
otherwise. The question naturally arises, nevertheless, how do 
organisms like amceba, living in water, avoid swelling up and 
disintegration on account of the osmotic pressure of their contents ? 
We have no accurate knowledge of the value of this osmotic pressure, 
but we know that it must be higher than that of the extremely 
dilute solution in which they live. It is pointed out by Stempell 
(1914) that the well-known pulsating vacuole of protozoa is the 
means of removing excess of water taken in by osmosis. A minute 
drop of water makes its appearance at a particular place in the 
protoplasm, gradually increases in size until it touches the outer 
surface of the organism and bursts to the exterior. The process. is 
continually repeated. 
The taking. up of solid particles and small organisms, such as 
alga: and bacteria, by living cells, as in the process of ‘ phagocytosis,’ 
seems at first sight to be difficult to understand. If molecules of 
sodium chloride are unable to pass through, how do such large 
masses manage to do so? The writer has suggested in another 
place (‘General Physiology,’ p. 144) that the membrane is actually 
perforated in the latter process, as when a needle is dropped through 
a soap film, the film closing up again as the object passes through. 
As we have seen, the cell-membrane is not a fixed structure, and the 
difference between the impermeability to salts and the permeability 
to large particles is that in the former case the molecules would 
have to pass through pores which are too small for them; in the 
latter case they break the film mechanically, but it is formed again 
behind them. 
It will be of some interest, in conclusion, to refer briefly to some 
typical physiological phenomena in which membranes of variable 
permeability are believed to play an essential part. A limited 
selection is all that is possible, since, as pointed out above, the 
properties intervene in nearly all vital processes. 
I. The Stimulation of Nerve. Nernst (1899) was the first to 
suggest that the electrical stimulation of nerves is conditioned by the 
concentration ef ions of acertain sign of charge in the neighbourhood 
of a semi-permeable membrane. He made no statement as to the 
situation of such a membrane, nor did he claim that his theory was 
more than an approximation. A more complete extension of this 
theory was made by A. V. Hill (1910) and found by Keith Lucas 
(1910) to satisfy experimental results. here is no evidence of the 
existence of membranes within the nerve fibre itself, the contents of 
which, so far as can be made out, are liquid. Hence the membrane 
in question must be that on the exterior of the central core, that is, 
the cell-membrane. The relation of increased permeability to the 
condition of excitation has been emphasized by Lillie (1914) and the 
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