site of stimulation; a difference of electrical potential is thus set up between one portion of the irritable element and another. The problem thus becomes clearer: how is it possible that, e. g., a slight mechanical pressure, or the action of a chemical substance or ray of light, may have this effect on an irritable tissue—i. e., may cause a negative electrical variation and so stimulate? and why do electrical changes, of all the processes in nature, bear this distinctive relation to stimulation?
The answer to these questions is far from complete at present, and their consideration brings us at once to some of the most fundamental questions of general physiology. All of the evidence indicates that the bioelectric processes are of critical importance in the life of the cell; they are associated with the most various physiological activities, and accompany the process of stimulation in all irritable tissues; and it is clear that we must understand their controlling conditions before we are in a position to answer the above questions. Now there is one very general peculiarity of living cells which is intimately connected with their power of responding to stimuli—namely, the possession by the surface-layer of protoplasm of peculiar properties in relation to the diffusion of dissolved substances. Living protoplasm is an aqueous solution, chemically complex and containing a high proportion (10-20 per cent.) of colloidal substances, chiefly proteins and lipoids. Experiment has shown that not all soluble substances readily enter the protoplasm of living cells; thus neutral salts like , sugars and amino-acids (the chief elementary constituents of proteins) diffuse into unaltered cells with difficulty if at all; the surface-film of the protoplasm typically acts toward such substances as a semi-permeable membrane. It is for this reason that the cells of plants remain during life turgid or distended with water, often under high pressure. Osmotic effects, dependent on the semi-permeability of the protoplasmic membranes, are the direct cause of this turgor. The living cell, in other words, is typically enclosed by a modified protoplasmic surface-film or membrane, the plasma membrane, which allows water to pass readily but not dissolved substances of the above kinds. The presence of this membrane makes it possible for the dissolved substances within and without the cell to be very different in character and concentration, and upon this condition the integrity of the living cell undoubtedly very largely depends. We find in fact that when the cell dies many substances, confined during life within its interior, diffuse out into the surroundings; the plasma membrane loses its osmotip properties; the plant loses turgor, wilts and withers; analogous changes occur in animal cells, the colloids coagulate and the cell disintegrates. Conversely if we alter the plasma membranes by chemical substances (poisons), so as irreversibly to destroy their semipermeability, death inevitably follows. Semipermeability is thus for many if not for all cells an essential condition of continued life.
