584 THE POPULAR SCIENCE MONTHLY 



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 gen- 

 eral 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, chemi- 

 cally 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 NaCl, 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 mem- 

 brane, 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; anal- 

 ogous changes occur in animal cells, the colloids coagulate and the cell 

 disintegrates. Conversely if we alter the plasma membranes by chem- 

 ical substances (poisons), so as irreversibly to destroy their semi- 

 permeability, death inevitably follows. Semipermeability is thus for 

 many if not for all cells an essential condition of continued life. 



