588 THE POPULAR SCIENCE MONTHLY 



the proper proportions these substances render an irritable tissue irre- 

 sponsive to stimulation. I have recently found that they also change the 

 properties of the plasma membranes in Arenicola larvae and sea-urchin 

 eggs in such a way as to make them more resistant to increase of per- 

 meability under the influence of salt-solutions. A definite parallelism 

 appears to exist; if we render the membranes more resistant to alteration 

 than formerly, Ave render the tissue less irritable. This influence of 

 anesthetics on plasma-membranes is a very general if not universal char- 

 acteristic of living organisms. Thus the permeability of plant-cells to 

 salts is decreased by these substances, as Lepeschkin and Osterhout have 

 found, and Loewe has recently shown that artificial lipoid-impregnated 

 membranes are similarly affected. These facts explain why anesthetics 

 counteract the effects of stimulating agencies — which cause temporary 

 increase of permeability; and since most anesthetics are lipoid-solvents, 

 we are led to the conclusion that they cause their effects by changing the 

 state of the lipoid components of the membrane; thus the properties of 

 this structure are altered — particularly the readiness with which its per- 

 meability is changed by external conditions acting upon it. 



Irritability would thus appear to depend on a peculiar state of the 

 plasma membranes — one in which under slight variations of external 

 conditions these structures undergo automatically a rapid and pro- 

 nounced increase of permeability. A certain state of physico-chemical 

 instability or lability of the protoplasmic surface-film seems to be the 

 essential condition on which a highly developed irritability depends. 

 Such a membrane appears to retain its properties unaltered only if the 

 external and internal conditions remain approximately constant, espe- 

 cially the state of electrical polarization. If this latter is suddenly 

 changed, as by an external even slight electrical disturbance, some 

 hindrance to interaction seems to be removed, and a chemical process is 

 initiated which instantly alters the character of the membrane and 

 stimulation follows. This, or something closely similar, appears to be 

 the condition in tissues whose irritability is sensitive and rate of response 

 rapid. Apparently all variations occur in the rate at which this change 

 takes place. The electrical variation, whose rate of appearance and sub- 

 sidence is an index of the rate of the surface-change, is highly rapid in 

 some tissues and slow in others. Thus it lasts for about a thousandth of 

 a second in a frog's motor nerve, and for several seconds in a slowly re- 

 sponding tissue like smooth muscle; and all intermediate conditions are 

 known to exist. These variations in the speed and sensitivity of the re- 

 sponse depend primarily on the specific peculiarities of the plasma mem- 

 branes of the different tissues. What determines the differences between 

 different irritable tissues and organisms in these respects is a subject for 

 future investigation. 



These peculiarities of the plasma membranes enable us to understand 



