NERVOUS CONDUCTION AND EXCITATION 149 



shocks at an interval of 0-0036 second, the response to the single 

 shock soon vanishes, while the response to the successive shocks 

 persists (Fig. 37). At this stage we have therefore a summation 

 of stimuli individually inadequate to elicit a response, and that 

 this summation does not depend upon polarisation at the seat 

 of stimulation is shown by several facts ; in particular, first, 

 that it is obtained by alternating make and break shocks, in 

 which case the effect of the second stimulus must be to prevent 

 the persistence and extent of 

 the polarisation due to the 

 first ; and second, that it is 

 equally well seen when the 

 successive stimuli are applied 

 at different points along the 

 course of the nerve. The 



interval which must elapse yig. 37.— (After Keith Lucas.) 



between successive stimuH if 



summation is to occur was found by Keith Lucas to have 

 about the same range as the supernormal phase for con- 

 duction in the nerve itself. 



Integrative Action o£ the Central Nervous System.— Con- 

 siderations of the kind which have been advanced already 

 open to us, in the words of Keith Lucas, " a whole range of 

 possibilities in the regulation of nervous activity. According 

 as we time impulses in the nervous system to follow one another 

 at a shorter or a longer interval, we can make them less or more 

 capable of being conducted through any regions of decrement 

 which the system may contain . If there is a region of decrement 

 such that normal impulse just cannot pass, then impulses of 

 moderate frequency may pass it successfully, while impulses 

 of high frequency may not only fail to pass it, but may by their 

 frequency prevent other impulses finding their way through." 

 The last sentence offers a possible interpretation of a very 

 important phenomenon on which Sherrington lays emphasis 

 in discussing the integrative action of the central nervous 

 system. The normal organism is subject to an infinite 

 variety of stimuli : at any moment it is under the influ- 

 ence of not one but many stimuli, each adequate under 



