108 TRANSMISSIVITY IN PASSIVE IRON WIRES 



characteristic; after a wire has been activated in the usual manner and 

 has reverted to the passive state it is found impossible to reactivate it 

 as a whole by a single contact until a certain interval has elapsed, 

 the duration of which varies with the concentration of acid and with 

 the temperature in a manner to be described later. At first, touch- 

 ing with zinc causes only a brief local reaction which is confined to 

 an area of 1 or 2 cm. from the contact; a second trial made somewhat 

 later gives a reaction which spreads more rapidly and through a 

 greater distance; and with further successive trials the distance 

 through which the activation wave travels, as well as its speed, in- 

 creases by degrees until eventually rapid transmission through an 

 indefinite distance becomes again possible. 



This failure of complete activation and transmission for a certain 

 period succeeding the passage of an activation wave may be com- 

 pared with the similarly inexcitable and non-conductive interval or 

 "refractory period" of irritable living systems. It is well known that 

 a decline or disappearance of irritability and conductivity during a 

 certain interval succeeding the response to stimulation is a constant 

 feature of the excitation process in all irritable tissues. ^ The duration 

 of this interval varies with the nature of the tissue ; the most notable 

 correlation is that it is brief in tissues with rapid rate of response 

 (i.e. brief chronaxie), such as voluntary muscle and nerve, and rela- 

 tively prolonged in slowly responding tissues like the heart or invol- 

 untary muscle. In certain cases, as in the photoreceptors of mol- 

 lusks, the period of diminished sensitivity following the response may 

 last for several minutes;^ such a phenomenon suggests a fatigue effect, 

 and there are various significant resemblances between the refractory 

 period and cases of brief or evanescent fatigue; thus the period is 

 lengthened by conditions which delay recovery from fatigue, such as 

 lack of oxygen,^ and also by repeated stimulation, as shown in the 



* The relation of the refractory period to the excitation process in general is 

 weU discussed in the Croonian lecture of Lucas, K., Proc. Roy. Soc. London, 

 Series B, 1912, Ixxxv, 495. 



' For a description of this phenomenon cf. Hecht, S., /. Gen. Physiol., 

 1918-19, i, 545. 



^Verworn, M., Allgemeine Physiologic, Jena, 4th edition, 1903, 559. Froh- 

 lich, F. W., Z. Physiol., 1904, iii, 468. 



