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 appara- 

 tus known as the rheonome. 



A useful form of rheonome is that devised by Lucas (Fig. 119). 

 Two zinc plates D and B, immersed in a saturated solution of 

 zinc sulphate contained in a rectangular 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 F. 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 the 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. 120). 



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. As we 

 increase the rate of change of current, by employing in the case of induced 

 currents more and more rapid alternations, we find that the excitatory effect, 



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