STIMULATION OF LIVING ORGANISMS 585 



Semipermeability also forms the condition of another fundamentally 

 important property of the living cell, namely, its possession of highly 

 characteristic electrical properties. Physical chemists find in general 

 that if a solid film or other partition consisting of any sufficiently im- 

 permeable material — e. g., glass, an organic membrane or a precipitation 

 membrane of copper ferrocyanide or similar material — is interposed be- 

 tween two different solutions containing electrolytes which are thus pre- 

 vented from mixing, a permanent difference of electrical potential arises 

 between the two solutions. The same appears also to be true of the 

 protoplasmic surface-films or membranes. Apparently, so long as the 

 plasma membrane preserves its normal semi-permeability there exists a 

 considerable difference of electrical potential between its external and 

 internal surfaces — i. e., between the exterior and the interior of the cell — 

 dependent on the difference in composition between the protoplasm and 

 its surroundings. Thus the exterior of a resting muscle cell or nerve 

 fibre is always found positive relatively to its interior. But with the 

 loss of semipermeability at death this potential difference — or demarca- 

 tion-current potential — also disappears. It thus evidently depends upon 

 the semipermeability of the plasma membrane ; and since this electrically 

 polarized condition of the membrane is undoubtedly a factor of prime 

 importance in many cell activities, including stimulation, we see again 

 how physiologically essential a property this semipermeability of the 

 plasma membrane may be. 2 



Further, there is little doubt that this property is one of the essential 

 conditions on which the possibility of stimulation depends. Nernst has 

 shown that an electric current stimulates by changing the concentration 

 of ions at the semipermeable membranes of the irritable tissue; this is 

 equivalent to producing a potential-difference or electrical polarization 

 between the outer and inner surfaces of the membrane. The normal pre- 

 existing or physiological potential difference is thus altered — in stimu- 

 lation is typically diminished — and when this change is sufficiently ex- 

 tensive and rapid the tissue gives its characteristic response. Now these 

 polarization-effects depend on semipermeability, since if the membrane 

 allowed all ions to pass freely the differences of concentration in which 

 the polarization depends evidently could not arise. "We find in fact that 

 the cell whose membranes have lost their semipermeability does not re- 

 spond to stimulation. Such a cell is "dead"; this however need not 

 mean that all vital manifestations have ceased ; many metabolic processes 

 may in fact continue in dead cells and lead to far-reaching chemical 

 transformation of the cell-constituents; such changes are called "auto- 

 lytic." What is lost is the power of responding to stimulation; hence 

 the automatic regulation of the vital processes ceases, and presently these 



2 Cf. my article on the role of membranes in cell processes in The Popular 

 Science Monthly for February, 1913. 



VOL. LXXXIV. — 40. 



