138 TRANSMISSIVITY IN PASSIVE IRON WIRES 



wave emerges from this region into the normal region beyond, it there 

 regains its former velocity and other characteristics. ^^ The conditions 

 in the passive wire suggest that a block resulting from local injury 

 or constriction in a nerve is due mainly to the influence of the local 

 current (or injury current) between the altered and the unaltered 

 regions — ^just as the platinum blocks the activation wave in a passive 

 wire by interposing in its path a local circuit in which the iron is 

 anodal and hence resistant to activation. In the metallic system par- 

 tial compensation of the current of the approaching active-passive 

 circuit by means of the current of the local Pt-Fe circuit is probably 

 also a factor in the extinction of the wave. Analogous conditions 

 may be presumed to exist in the living tissue. 



A fully recovered passive wire exhibits an "all or none" type of 

 behavior, in this respect also resembling a normal nerve; while any 

 region of a passive wire which has only partially recovered is one in 

 which an activation wave is conducted with a decrement. Such a 

 local region of decrement may be produced at any desired position 

 in a wire which is elsewhere completely transmissive by the simple 

 method of touching the wire locally with a piece of zinc at an appro- 

 priate interval after the passage of a normal activation wave. A local 

 region of temporary activity is thus produced, beyond whose limits 

 the wire remains unaffected. This local region continues to conduct 

 with a decrement at a time when the remainder of the wire has com- 

 pletely recovered. For example, in 65 per cent acid the recovery 

 of complete transmissivity usually occupies between 4 and 5 minutes 

 at 20° ; at 2 minutes after a previous complete activation a new wave 

 travels (on an average) for about 4 cm. from the point of contact. 

 If at this time one touches the wire at a central point with zinc, there 

 is formed a sharply defined newly activated region about 8 cm. in 

 length, which 3 minutes later, at a time when the rest of the wire has 

 recovered completely, still conducts with a decrement. An activa- 

 tion wave which is then started at one end of the wire will travel to 

 this region of decrement and penetrate the latter (usually with a 

 visible progressive retardation) for a variable distance of some centi- 

 meters. In a certain proportion of cases, if the time relations are 



^^ Adrian, E. D., /. Physiol, 1913, xlv, 389. 



