140 



Mr. Graham Brown. 



It seems best at present, in view of the difficulties met with in assuming 

 an " all or none " activity, at any rate, in certain species of reflex arcs, to 

 hold that the efferent neurones may discharge each with graded intensities. 



If that be the case then it must appear that there is an essential difference 

 between the activity cf efferent nerves aroused by artificial peripheral stimuli 

 and those evoked reflexly through the centres. 



The question must arise whether this grading is one in which the 

 amplitude of the discharge of each neurone may be varied, or whether the 

 grading is produced by different speeds of repetition of discharges, the 

 amplitudes of which are not varied. 



In the latter case an explanation is offered only if the mechanical response 

 varies with variation in the speed of repetition of nerve impulses. 



That this is indeed the case Mines* has recently given some evidence to 

 show. He points out that the ordinary explanations of the greater tension 

 produced during tetanus than in single muscular twitches do not meet the 

 case. He notes, for instance, that the fact that the tension set up in 

 amphibian muscle in response to more rapid stimuli is greater than that set 

 up in response to less rapid stimuli (which yet are sufficiently rapid just to 

 give complete fusion) is not explained on the von Frey hypothesis. 



One more point. The aspect of the problem which here particularly 

 interests us is the question of an " all or none " response of the efferent 

 neurone to graded reflex stimuli. Even in the case of peripheral stimulation 

 there is little or no evidence of an " all or none " character of the response 

 to graded stimuli of the efferent nerve fibre considered as a unit. As 

 Adrianf himself points out, his experiments seem to show that certain 

 longitudinal units of conduction are characterised in their activity by an 

 " all or none " response to graded stimuli, but there is nothing to show that 

 these units are the nerve fibres. 



If they are units of a smaller size than the nerve fibres the efferent 

 neurone may still respond in a graded manner to graded stimuli, although 

 the activity of the elements of the discharge may be distinguished by this 

 " all or none " character. If this be the case the discharge of the efferent 

 neurone might be graded in " steps " from zero to its maximum. 



That either the reflex discharge of the efferent neurone has not the 

 character of an " all or none " response to graded stimuli, or that the longi- 

 tudinal units, the activities of which possess the character of an " all or none " 

 response to graded stimuli, are smaller than the nerve fibre seems to be 

 shown by the experiments here described for one specific reflex type. 



* 1 Journ. Physiol.,' 1913, vol. 46, p. 1. 

 t 'Journ. Physiol.,' 1912, vol. 45, p. 389. 



