OF VITAL PHENOMENA 123 



phenomena by saying that the current of injury has been ob- 

 served in all living tissue where it has been looked for, but that 

 action currents have been observed in the leaves of land and 

 fresh water plants, and in animal nerve (and retina), muscle 

 . and glands and electric organs derived from them, as well as in 

 the eggs of fish. These electric phenomena are due to changes 

 in permeability to ions. The excited state (increased permeabil- 

 ity) remains but a few thousandths of a second in muscle or 

 nerve but may remain fifteen minutes in plants. 



The most wonderful thing about the electrical variation of 

 muscle, nerve or sensitive plant is that it is self propagated. 

 The results of study of electrical stimulation have made the self 

 propagation of the electrical variation a necessary consequence 

 of excitability. It is probably the negative variation of nerve 

 that stimulates another nerve or the motor end plate of the 

 muscle. The medullated nerve fiber is surrounded by an insu- 

 lating sheath, broken only at the occasional nodes of Ranvier. 

 When a minimal current is sent through a medullated nerve cross- 

 wise no stimulation takes place, apparently because of the insula- 

 tion of the fibers. But when it is sent lengthwise through the 

 nerve it may enter at one node and leave the fiber at another. 

 (The author found non-medullated nerves of Limulus to be as 

 sensitive to a current sent crosswise as to a longitudinal current.) 

 The nerve is stimulated nearest the cathode or apparently where 

 the current leaves the fibers at the nodes nearest the cathode. In 

 other words, the approach of a cathode to the plasma membrane 

 stimulates the nerve fiber. But the stimulated region becomes 

 electronegative and hence a little cathode. It stimulates regions 

 adjacent to it, and the stimulated area spreads like a fire. The 

 reason for this can only be learned when we discover the cause 

 of the increase in permeability of the plasma membrane. H. N. 

 Morse (1914) states that copper ferrocyanide membranes are 

 colloidal and that a crystallization, resulting in increase of the 

 size of the particles, increases the permeability of the membrane. 

 But the size of colloidal particles may be considerably increased 

 without crystallization (by electrical disturbances). Perhaps the 

 plasma membrane is colloidal and the sudden electrical disturb- 

 ance, due to the approach of a cathode, causes the particles to 

 aggregate into groups. If the plasma membrane is composed of 



