SECTION VI 



THE CONDITIONS WHICH DETERMINE 

 ELECTRICAL STIMULATION 



FOR every tissue traversed by a current there is a minimum rate 

 of change at which the current through the tissue must be increased 

 or diminished in order to cause excitation. If 

 instead of suddenly making and breaking the 

 current passing through an irritable structure we 

 carry out the change gradually, no excitatory 

 effect is produced, even although the current may 

 finally attain a considerable strength. This fact 

 may be demonstrated by the use of an apparatus 

 known as the rheonome. 



A useful form of rheonome is that devised by Lucas 

 (Fig. 120). Two zinc plates D and E, immersed in a 

 saturated solution of zinc sulphate contained in a rect- 

 angular cell, are separated from one another by a vulcanite 

 diaphragm. In the diaphragm is a hole G by which the 

 two sides of the vessels are connected. This hole can be 

 closed at any desired rate by a shutter r. When the 

 hole is closed no current can pass between the plates, 

 and the amount which can pass will depend on the 

 extent to which the shutter has been raised. By giving 

 Dhe hole the right shape it is possible to diminish the 

 resistance of the apparatus regularly. If this rheonome 

 be placed in circuit with a battery and an excitable 

 tissue, such as the nerve of a nerve-muscle preparation, 

 we can make a current or break a current through the 

 tissue at any desired rate. Thus the course of the current 

 through the tissue will be represented, not by a vertical 

 line, but by a sloping line which may be given any 

 desired degree of steepness (Fig. 121). 



If the current be slowly increased through the 

 nerve or be slowly cut off from the nerve, no 

 excitatory effect takes place, while quickly opening or closing the 

 shutter will cause excitation. It might be concluded that the 

 excitatory effect of a current increases with 



1. The intensity of the current. 



2. The rate of change of the current. 



The second of these conditions needs, however, some correction. 



304 





FIG. 120. 

 Rheonome of 

 Keith Lucas. 



