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fact of the ,Unterschiedschwelle’. In what manner, relying on 
the neuron-theory, can we understand the process of the origin- 
ating of a sensation? In the first place a stimulation of the 
direct sensible neuron is needed. This causes a potential wave 
to travel along the nerve and to act on a series of secondary 
neura, in case it has reached a certain magnitude. As soon 
as here too the stimulation-effect has passed a certain threshold- 
value, tertiary neura are excited. In the same way the stimulus 
is transmitted to neura of still higher order. In what now may 
consist a difference of sensation? 
Firstly in the fact that a neuron of higher order is alternately 
more or less strongly stimulated. This view is highly improbable, 
as it would postulate a localisation for every elementary sensation; 
each neuron, each nervecell would serve for one particular sen- 
sation. Still another view may be taken however, one that appears 
to me far more probable. We know that a whole system of 
neura of a higher order is excited, whenever an externa] stimulus 
is applied. The excitation of this complex of neura occasions a 
sensation; to intensify this sensation it is necessary that the number 
of excited neura should be augmented. And this will be the 
case only then, when the external stimulus is increased to such 
an amount, that the threshold-value of those several following neura 
is passed. 
The Unterschiedschwelle in a sensation is formed by the threshold- 
value of a following system or number of neura. 
About the manner in which by a stronger stimulus a higher 
system of neura or perhaps more neura may be excited, dif- 
ferent views may of course be taken into consideration. One of 
these views I think especially plausible: viz. that the potential wave, 
in proportion to its centripetal extension along the primary neuron, 
decreases in amplitude. Consequently this stimulus will become 
feebler in centripetal direction, and operate an ever diminishing 
effect on the successive secondary neura, until at last the ampli- 
tude has become so small, that it remains under the threshold-value 
of a following secondary neuron. 
In accepting this view we suppose the conditions for the originating 
of a more or less intensive sensation to be determined already in 
the spinal chord; the facts known to us about the skin-reflexes 
make this supposition very allowable. 
What is the relation existing between our law and the psy- 
