134 



Mr. Graham Brown. 



decrement regains its original size when it emerges into normal tissue. He 

 points out that this favours the supposition that the relation between the 

 disturbance and the strength of the evoking stimulus is of an ''all or 

 nothing" character. 



Quite recently Mines* has given a description of experiments which suggest 

 either that there is no gradation in the response of efferent amphibian nerve 

 fibres to graded stimuli consisting of single induction shocks, or that the 

 smallest excitation is capable of exciting any neuro-muscular synapse which 

 can be excited by single impulses. 



These various experiments certainly seem to point to the conclusion that 

 the response both of the peripheral efferent nerve fibres and of the muscle 

 fibres of the skeletal muscles is of an " all or nothing " character when the 

 exciting stimulus is an artificial electrical one. And there is a temptation 

 to argue from this that the activity of the same efferent nerve fibres and 

 skeletal muscle fibres in the less artificial reflex excitation is also of an 

 " all or nothing " character. 



If this view be taken we must suppose the efferent neurone to discharge 

 maximally or not at all. We must look at the reflex mechanism as one 

 split longitudinally into units (as indeed we do look at it), which are each 

 either maximally active or inactive, but never of intermediate activity. We 

 must suppose that the grading of the muscular response is due to the differing 

 proportions of its component units which at any one time are in action. 

 But we must even then admit that a certain sort of grading of activity may 

 occur even in one efferent neurone — for it might be supposed that the 

 discharges proceeding from it might vary in frequency. 



A subsidiary question is that of the possibility of a similar " all or nothing " 

 character in the activity of the afferent neurones. 



III. Objections to " All or Nothingness." 



At first sight it might seem that a strong objection to the " all or none " 

 character of afferent activities is before our eyes on any clear and moonless 

 night. The stars appear to be of very different brightness, although the 

 size of their images upon the retina is almost infinitely small and, theoreti- 

 cally at any rate, must be looked upon as stimulating only one retinal 

 element each. It might seem that it is hardly possible to explain the 

 number of distinguishable brightnesses as due to different numbers of retinal 

 elements stimulated — for instance, are as many as ten stimulated in the 

 case of a bright star, and as few as one in a star on the limit of visibility ? 



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



