420 PRINCIPLES OF GENERAL PHYSIOLOGY 



must, therefore, devote a few words to it. It is a familiar fact that a wave motion 

 can be annulled by another similar wave motion of the same period, if the phase 

 difference is of half a wave length. In this case, the hollows produced by the one 

 are exactly filled up by the crests of the other. But it is essential to remember that 

 these wave motions are, both of them, movements to alternate positions at an 

 equal distance on opposite sides of a mean position, and that it is to this mean 

 position that the process is reduced when the two wave motions mutually 

 counteract one another. 



For example, take an alternating electric current, as perhaps most analogous to a nerve 

 impulse. This raises the potential of a point on a conductor alternately, say, to 100 volts 

 above the potential of the earth and 100 volts below this value. Another similar current, 

 with a phase difference of half a wave length, reduces the potential to a constant one of 

 that of the earth, that is to zero. 



Now, in a series of nerve impulses, the state of excitation is merely raised 

 periodically from zero to one of a certain potential, and by no possibility could 

 another similar set of disturbances reduce this potential to zero. In the nm-t 

 favourable case, it could be reduced only to one-half of its value. Inhibition, as 

 Sherrington points out (1906, p. 99), is complete, and we see the fact in Figs. 107, 

 113, and 118. This could only happen, on the assumption of a process analogous 

 to physical interference, if we could apply a nerve process of the opposite nature 

 to that of excitation. This is just the fact which has to be explained. We may, 

 then, dismiss the hypothesis as inadequate. 



It will be clear that if a nerve is stimulated at the same moment at two points 



i A A i 



A C D B 



A and B, at equal distances on opposite sides of the two leading off electrodes, C and D, 

 the waves arriving at C and D simultaneously will produce an equal and opposite electrical 

 potential at the two electrodes, so that no external change will be obvious. It is incorrec-t, 

 however, to speak of this as an "interference," as is sometimes done. It is also unnecessary 

 to point out that the non effect of a stimulus falling in the refractory period, or the disappear- 

 ance of a disturbance in a region of decrement, cannot be spoken of as interference in any 

 physical sense. 



In a certain sense, the refractory period after an excitation is itself an 

 inhibition, since the excitable tissue is incapable of entering into activity ; so that 

 there is a certain superficial resemblance to inhibition when a stimulus applied in 

 this period is ineffective. But it does not quell a state of excitation already 

 present, as a true inhibition does. 



At the same time, the phenomenon known as the " Wedensky inhibition" requires a little 

 consideration. Although this is a somewhat special case, it is of interest as showing the 

 complex possibilities involved in joint action of refractory period and "all-or-nothing" law, as 

 worked out by Adrian (1913). At a certain stage in narcosis or fatigue of a nerve-muscle 

 preparation, a rapid series of strong stimuli applied to the nerve produces a small initial 

 twitch only, whereas a similar series of weak stimuli produces a continued tetanus. After the 

 first stimulus, in either case, a refractory period is present ; if the next succeeding stimulus 

 is strong, it will set up a propagated disturbance early in the relative refractory state, but 

 this will only be a very small one and consequently unable to pass the region of decrement 

 between the excited spot and the muscle. This region of decrement may be in the place of 

 synapse between the nerve and the muscle, or in a narcotised area of the nerve itself. Although 

 it is a small disturbance, it will leave behind it a refractory period, which will have the 

 corresponding effect on the next succeeding stimulus and so on ; thus no excitation will reach 

 the muscle. If the stimuli are weak, on the contrary, no propagated disturbance can l>e set up 

 by any one until the refractory period is practicall} 1 at an end, and then the disturbance, 

 although set up by a weak stimulus, will be of the full normal magnitude and able to pass the 

 region of decrement and excite the muscle. Further details will be found in the original 

 paper ; those given above will suffice for our present purpose. 



The experiments of von Frey (1876), previously referred to, were undertaken 

 to test the hypothesis of an interference process, in which the excitation and 

 inhibition were supposed to act on the same cell mechanism. It was found that, 

 if the vaso constrictor and vaso-dilator nerves to the submaxillary gland were 

 stimulated simultaneously, the former obtained the victory during the period of 

 stimulation, but the latter showed their effect afterwards. Now, although this 

 result is inconsistent with a purely physical interference process, which should 



