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In order to understand certain phenomena of fear, we must first study a few peculiarities presented by the excitable portions of the nervous system. If one stimulates the nerve running through the frog’s leg by very slight electric currents which are incapable of producing a contraction of the muscles, the force of the current may be slowly and evenly increased without the leg moving or in any way responding. This experiment shows us that the motor nerves do not respond to the stimulus as such, because the latter may be very strong without producing any visible effect, but that it is the rapid variations and changes which cause the convulsive movements.

Any pain or fear assailing us unexpectedly causes a great disturbance in the organism, but have a less serious effect when slowly developed.

There is always a more energetic response during the first moments of a sensation. This fact is true of all phenomena of the nervous system, and it is therefore unnecessary to give examples of what everyone knows from experience. This depends also upon the fact that the nervous system discharges a part of its energy at every reaction, so that when the animal is very weak it responds no more after the first two or three times.

We now understand why slight, unexpected emotions produce such intense perturbations in the organism, while very serious events for which we are prepared have in proportion much less effect.