168. To understand this, it is necessary to remember that the stimulation of a nerve does not arise[191] in the _changed state_ of that nerve, but in the _process of change_, i. e. the disturbance of the tension. The duration of the stimulation is that of the changing process, and the intensity increases with the differential of the velocity of change. So that when a nerve which has been excited by a change of state returns to its former state, this return--being another change--is a new excitation. That it is not the changed state, but the change, which is operative, explains the fact noted by Brown Séquard: a frog poisoned by strychnine, when decapitated and all respiration destroyed, will remain motionless for days together, if carefully protected from all external excitation; but its nervous system is in such a state of tension all this time that the first touch produces general convulsions. Freusberg also notes that if a brainless frog be suspended by the lower jaw, and one foot be pinched, the other leg is moved at first, then quickly droops again, and remains at rest until the pincers are removed from the pinched foot, when suddenly all four legs are violently moved by the _stimulation which the simple removal produces_. Let us also add the well-known and significant fact that if a nerve be divided rapidly by a sharp razor, neither sensation nor motion is produced, because the intensity of a stimulus being, to speak mathematically, _the function of the changing process_, the duration of the process is in this case too brief. On the same ground the application of a stimulus will excite no movement, if the force be very slowly increased from zero to an intensity which will destroy the nerve; but at any stage a _sudden_ increase will excite a movement. 169. We may group all the foregoing considerations in this formula: LAW I. Every neural process is due to a sudden disturbance of the molecular tension. The liberated energy is discharged along the lines of least resistance.