148 THE POPULAR SCIENCE MONTHLY 



the current is broken ; i. e., we obtain stimulation when the preexisting 

 polarization of the irritable elements is rapidly diminished — in other 

 words, when there is a depolarization. We may formulate the essen- 

 tial relations thus: stimulation is equivalent to depolarization, i. e., to 

 a rapid decrease of the already existing or physiological polarization of 

 the plasma-membranes. 



Stimulation, however, is also connected with a change in the per- 

 meability of the membrane, as we have seen. We must therefore con- 

 clude — since a sudden change of polarization stimulates — that simple 

 alteration of the electrical polarization alters the permeability of the 

 membrane. Decrease of the potential-difference between the opposite 

 faces of the membrane — i. e., depolarization — apparently increases per- 

 meability, and often to a remarkable degree. Irritability seems, in 

 fact, to be an expression of this peculiar relation. The electric current 

 thus alters the polarization of the semi-permeable membranes of the 

 irritable tissue, and in so doing alters the permeability. This change 

 becomes the condition of the characteristic electrical variation of the 

 tissue; the latter is self-propagating, and thus the effects of the local 

 stimulus are transmitted to other regions of the cell. These appear to 

 be the essential changes in the stimulation-process as such. 



According to this point of view we must conceive of the plasma- 

 membrane of an irritable element as possessing during rest a charac- 

 teristic impermeability or semi-permeability to which corresponds a 

 definite polarization, or potential-difference between its outer and inner 

 surfaces, of the value of (e. g.) one tenth volt. Now the permeability 

 of the membrane is determined by a number of conditions, some of 

 which are, its chemical composition, the temperature, the chemical 

 changes in the protoplasm and the surroundings, and probably the 

 state of- mechanical tension of the membrane. Another factor is, how- 

 ever, of fundamental importance : this is the existing state of electrical 

 polarization of the membrane. Alteration of this polarization alters 

 the permeability; if we decrease it we increase the permeability and 

 stimulation may follow; if we increase it we presumably alter the per- 

 meability in the inverse direction — hence in all probability the lowered 

 irritability at the anode (anelectro tonus) during the passage of a con- 

 stant current through a muscle or nerve. Such a view ascribes peculiar 

 properties to the plasma-membrane, but the facts lead directly to this 

 interpretation. Girard has shown experimentally that changing the 

 electrical polarization of a membrane of bladder or parchment alters 

 the permeability to neutral salts. The electrical state of a membrane 

 may thus determine its permeability. The plasma-membrane of 

 irritable tissues has apparently acquired extreme sensitiveness to 

 changes in its electrical polarization, such that slight electrical dis- 



