The excito-neural centres watch over the nutrition of the brain cells, and are, in fact, trophic centres for the cortex. The neuro-regulatory centres are kept au courant with the condition of each organ, and its wants and capabilities, through thin connecting fibres; and they act accordingly, either exciting or inhibiting their representative centres in the brain. When the blood-supply is cut off from the cortical centres the functions of higher cerebration are inhibited altogether or in part, and unconsciousness or altered conditions of consciousness results. Some conditions of the organism cause a response from the neuro-regulatory centres in the form of an inhibitory impulse which puts the organ or function represented out of action, and others, on the contrary, convey a dynamo-genic impulse to the regulatory centres. The influences which cause excito-neural action are stimuli having a great degree of intensity - e.g., pain, noise, mental anxiety. The influences which produce neuro-inhibitory action, with consequent decreased functioning of the cortical cells, are muscular and mental fatigue (with the coincident accumulation of waste products), and monotonous continuous stimulation of a sensory organ.

This stimulation causes an impulse resembling that due to local fatigue to be sent to the neuro-regulatory centres, and these respond by inhibiting the activity of the cortical cells.

* Op. cit., p. 12.

Preyer is inclined to consider hypnosis as a cortical neurosis, dependent on disturbance of the functions of the grey matter, due either to its inhibition or excessive activity. He supposes that hypnotizability indicates an abnormal amount of instability of the cells of the highest centres, and that in hypnosis the nutrition of the cortex is altered by the accumulation in its cells of oxidizable waste product (lactates). Unless we can speak of normal sleep as a neurosis, I should object to the term being applied to its hypnotic analogue as confusing, and as somewhat committing us to a theory.

Dr. Lehmann, lecturer on Experimental Psychology at the University of Copenhagen, has published the lectures he gave in 1889, and his book is one of the best which have been written on the subject. He works out very carefully the causation of natural sleep and the points wherein it differs from hypnosis. He considers that hypnosis closely resembles natural sleep, but differs from it in that the process is arrested or cut short before complete unconsciousness is reached. Sensation depends upon attention, and attention implies increased supply of nutrition to the part of the sensorium affected. In hypnotism parts of the sensorium are maintained in a state of activity by the attention of the subject being directed to them by the suggestions of the operator. There is increased nutrition of certain centres, with corresponding innutrition of others, and the phenomena produced are the outcome of these changes in cortical nutrition. In the waking state there is continuous stimulation of the vaso-motor centres proceeding from the sensorium, and cessation of mental activity from fatigue or other cause brings about a discontinuance of the stimulation, with resulting relaxation and dilatation of the cerebral vessels and blood stasis.

Lehmann points out that functional activity depends on the amount of blood passing through an organ, and not on the amount present at a given time. In hypnosis he supposes there is a condition of tetanic contraction affecting the bloodvessels of the active part of the brain, and this tonic condition leads to increased rapidity of the flow of blood through them, increased nutrition of the part, and increased functional activity. The result of blood stasis from withdrawal of the stimulating influences arriving at the vaso-motor centres from the sensorium is drowsiness and sleep, and in ordinary sleep this condition affects equally the whole of the hemispheres. In hypnosis it is partial, and can be modified by the suggestions of the hypnotist, and the subject under his direction dreams a series of dreams. In the transformation of the natural into the hypnotic sleep the subject is so far awakened by the voice of the hypnotist as to be enabled to act upon his suggestions, but not sufficiently to regain the use of all the faculties.

See also review in Mind, July, 1891.

Forel goes deeply into the physiology and psychology of hypnotism, and quotes largely from the writings of Oscar Vogt. The student is referred to these for a full exposition of the theories advanced. They explain fairly satisfactorily nearly all the most striking phenomena of hypnosis, including catalepsy, sensory anaesthesia and hyperesthesia, paralysis by suggestion, etc., on the physical side, and monoideism, amnesia, suggested hallucinations and negative hallucinations, etc., on the psychical side. It is interesting to note that Forel's ideas are generally in tune with those of Liebeault, though probably they are arrived at by quite different processes of thought. Forel calls the nerve waves or activities neurokymes. Consciousness is the subjective side of concentrated cerebral activities. Attention is the maximum of our psychical activity, and depends upon concentration of the neurokymes in one direction with corresponding deflexion from other centres, for they tend towards the centre in which the greatest metabolic activity prevails. He likens attention to a shifting functional macula lutea in the brain, and it has also been compared to the changing of a kaleidoscope.

Stored-up impressions (memories) and hereditary instincts render certain centres in the individual more excitable than others, and the arrival at these of comparatively few or weak neurokymes is sufficient to cause them to function. Forel holds with Vogt that all psychical phenomena, as well as movements, are produced by peripheral stimuli, and that no movements originate in the brain. He supposes there is a subcortical sleep centre, and that this is stimulated by the arrival and accumulation there of neurokymes, set in motion by suggestion, hypnotic or otherwise.