The tendency of nerve habits to be formed more readily in the hypnotic than in the waking condition is shown by the fact that the subject is extremely apt to assume spontaneously, on subsequent occasions, the attitude he was made to adopt when first hypnotized. Thus, if the arm was raised at right angles to the body at the moment hypnosis was complete, it is very probable that this action becomes in the mind an essential part of the procedure, and will be repeated on every subsequent occasion unless checked by suggestion.

The subject of inhibition has been closely studied by Sir Lauder Brunton, and his researches enable us to get a little nearer the comprehension of some hypnotic phenomena. He founds his theory on the analogy afforded in physics by the interference of rays of light and sound with one another when they meet in certain relationship.* When two rays of light are combined so that the crests of the waves of which they consist coincide, the light becomes brighter; but if they are thrown together so that the crests of the waves in the first ray correspond with the hollows of those in the second, mutual interference is the result, and they so neutralize one another that darkness is then produced. Applied to the nervous system, the hypothesis implies that nervous impulses travel like waves along the nerve tracts, and as long as they coincide - apex to apex, and hollow to hollow - sensation or movement is the result of the impulse; but if the coincidence be interfered with, we get more or less complete neutralization of the impulse and inhibition.

The way in which waves of light may be made to mutually interfere is by causing them to pass through channels of different lengths, so that when they meet, one may be half a length behind the other, the crest of the first corresponding with the fundus of the second. In the nervous system it is a matter of constant occurrence that the impulse waves of nerve energy are travelling towards the centres through channels of different lengths, and it follows, ex hypothesi, that they are interfering with each other in different degrees. The whole nervous mechanism is subjected in its normal state to a mutual check system, so that a balance is automatically maintained betweeen sensory and motor nerves, and they are influenced to a greater or less degree by impulses arriving from the higher centres - i.e., those concerned in volition, etc.

* Loc. cit.

Lauder Brunton illustrates this point by taking the case of tickling. Here convulsive muscular movements are set up by gentle continuous irritation over a sensory surface. An impulse made up of waves is promulgated to the sensory centres, and reflected from them down the motor tracts. The stimulation being monotonous, continuous, and consisting of currents of the same intensity, there is no wave interference, and the motor movements resultant are reflex. But let the pressure be increased, so that, instead of tickling, pain is produced: then, in place of a weak current travelling up one nervous channel, we have a strong, irregular impulse disseminated into channels of different lengths; when it arrives at the centres which have been subjected to interference, a different condition will result, and the reaction will cease to be merely reflex.

Up to a certain point the action of the will may be called forth to check the convulsive movements, and this will operate by interference; the waves constituting the impulse from the highest centres so impinging upon the excito-motor waves as to cause interference and inhibition.

Inhibition, therefore, according to Lauder Brunton, is not a special function of certain cells and nerve fibres, but may be produced through any sensory or motor cell, and depends not on the properties of the cell, but on its relationship to other cells or fibres. 'Motion, sensation, inhibition, or stimulation, are not positive, but simply relative terms, and stimulating or inhibitory functions may be exercised by the same cell, according to the relation which subsists between the wave-lengths of the impulses travelling to or from it, the distance over which they travel, and the rapidity with which they are propagated.' * Showing the effect of sufficient inducement to overcome what we may almost call reflex action, I may mention that a physician of my acquaintance found his little daughter extraordinarily ticklish, and used to tease her by tickling her; but he called up such a strong inhibitory action by giving her sixpence when she bore it without wincing that she soon became quite callous to it.

Applying the theory of interference to the induction of hypnosis, we find that it serves to explain several points. Take hypnotization by the method of fixation, for example. An intense and unusual stimulus is applied to the optic nerve, and by it carried to the optic centres, in the form of an afferent current of abnormal form and intensity. The effect of such strong stimulation is not confined to the receiving centre, but overflows it and acts upon neighbouring and associated ones. The nerve impulse thus sent through the communicating nerve fibres is composed of waves which meet the normal currents traversing these channels in such a way as to interfere with and neutralize them, and hence we arrive at inhibition, either complete or in part, of the functions of many or a few of the cortical centres. The condition once induced, its reproduction is rendered easy by the setting up of a nerve habit. Psychical processes, such as auto or outside suggestion, may be supposed to cause hypnosis by originating a nerve impulse, starting from the ideational centres; but as to the nature of that nerve impulse it is impossible to dogmatize. Myers says that psychical processes can never be explained by physiological theories, but the physiologist will not readily admit that proposition.

Inhibition, therefore, is interference, not abolition of function, and its distinguishing characteristic is its immediate production and removability. By suggestion we may be supposed to start an impulse from the higher centres, the waves of which are propagated to the centres it is sought to influence, and which either coincide with and strengthen the efferent waves proceeding from these (dynamogenesis), or by interfering with them cause inhibition.

* Lauder Brunton, discussing the action of atropia on frogs, shows how the animal gradually loses, first, the power of voluntary motion, next the power of directing its movements, then the power of springing at all, and finally reflex action, and argues that the drug lessens the functional activity of nerve cells and fibres. The impulses are retarded, and thus the length of nervous connection between the cells of the spinal cord which is calculated to keep them in proper relation in the normal animal, just suffices at a certain stage to throw the impulses half a wave's length behind the. other, and thus to cause complete inhibition and apparent paralysis. The paralysis is only apparent, for after a time the animal will be found to respond to slight stimulation in the same manner as if it had been poisoned with strychnia, i.e., in an exaggerated degree. He explains this by assuming that the retardation of the impulses proceeds as the poison continues to act, so that the waves after a time interfere less and less with one another, and finally again coincide. The effect of the coincidence is increased excitability on stimulation.

This is on the theory that impulses reach the centres by different routes of varying length, so contrived that in normal life a constant relationship is maintained between the position of waves, and a healthy balance is maintained.