Unfortunately our knowledge of the physiology of the central nervous system is so incomplete that we cannot expect much from it. In spite of the great progress which physiology has made, we must admit that we know much less about psychical functions of the different elements of the brain than would appear from our physiological text-books. As far as the brain is concerned, Hirschlaff thinks that all we can assume is that it, and especially its cortex, must be intact for mental processes to be carried out. The assumption of some investigators that all conscious processes take place in the cortex and the sub-conscious ones in the subcortical centres, is very justly opposed by others who point out that such an assumption lacks proof. According to Flechsig it is anything but certain that the activity of the secondary consciousness is not due to a minor degree of stimulation; and in both cases any stimulus operates through the same cerebral elements (Lcewenfeld). Considering how very fragmentary our knowledge of the central nervous system is, we cannot expect that assistance from experiments on animals that Heidenhain did; for hypnotism is essentially a psychical process, and to draw conclusions from animals about mental action in men would be very daring.

The investigation of mental processes may, as we have seen, be undertaken in two ways - (1) by observing individuals, and (2) by calling the subject's memory to our aid. This last could not be done in the case of animals. But any observations of animals must be very elementary, for we can only obtain a glimmering of the processes of their consciousness from external signs. Further, many physiologists make the grievous error of assuming that the successful stimulation of any portion of the brain proves that an act of will originates in that spot. Heidenhain and Bubnoff have made numerous experiments in electrical stimulation of the cortex of the brain on dogs poisoned by morphia. But the conclusions which these authors attempted to draw about the action of will in men from such experiments must be pronounced mistaken till it is proved that the impulse of the will is an electrical stimulation. For the above reasons I consider Heidenhain's endeavours to draw conclusions on hypnosis in man from experiments on dogs too hazardous.

In spite of all these weighty doubts many attempts have been made to give a physiological explanation of hypnosis. Heidenhain must here be mentioned first. He supposes that the cause of the hypnotic states is an inhibition of the action of the ganglion cells of the cerebral cortex, induced by continuous weak stimulation of certain nerves, and he thinks that this inhibition is analogous to reflex paralyses, as in these also the functions of the ganglion cells are impaired by peripheral stimuli.

But even if we take the inhibition of the action of the ganglion cells for granted, Heidenhain's theory does not explain the connection between this and the means used to induce hypnosis. For (1) Fixation unaccompanied by mental effort does not lead to hypnosis. Braid and Berger considered that there must be concentrated attention as well; (2) in any case there would be no causal connection here between the purely psychical methods and hypnosis.

Besides this, Heidenhain starts from a mistaken premise when he supposes an inhibition of the ganglion cells. He concludes this inhibition from the lowered state of consciousness during hypnosis. But consciousness expresses itself in many ways during hypnosis. The processes of consciousness seem merely to be concentrated on one point, which is chosen by the experimenter and is removable at his pleasure. Heidenhain maintains, like Despine, that the subject is not conscious of the external stimuli. Heidenhain was led to this erroneous view, which has lately been taken by Landois and brain. Salvioli and Bouchut have, on the contrary, found cerebral hyperemia during hypnosis. Krarup finds a narrowing of the internal carotids, an enlargement of the external carotids and of the vertebral arteries during hypnosis. Regnier studied the carotid pulse with the sphygmograph during and after hypnosis. He concludes from the changes in the carotid pulse that there is retardation of the circulation and congestion of the cerebral capillaries during hypnosis.

He thinks that his view is confirmed by the ophthalmoscopic investigations of Luys and Bacchi who found hyperemia of the back of the eye Laker concluded that there is a change in the amount of blood in the cerebral cortex during hypnosis, because he once observed oedema of the face after hypnotic sleep. This he assumed to be an analogous phenomenon to the facial oedema observed in the angioparalytic forms of hemicrania. But he was more careful in his other conclusions. Tamburini, Seppilli, and Kaan also investigated the circulation of the blood during hypnosis, but only in connection with Charcot's stages. In the same connection Meynert investigated circulation in hypnosis; he speaks of a strong cramp of the musculature of the vessels in hypnosis. The three other last-named investigators used several methods: - (1) Mosso's method, which determines the volume of an extremity, and concludes from a decrease in the mass of blood contained in it, an increase in the mass contained in the brain. (2) The action of cold and hot compresses on the head (Kaan), which cause anaemia or hyperemia.

From the resulting changes, i.e., from the cessation or modification of the hypnosis, a conclusion is drawn between this and the mass of blood in the brain. (3) Ophthalmoscopic investigation of the vessels of the retina. I do not enter into details of the different experiments, because they are valid for the stages of Charcot alone, and therefore only have a historical interest. Apart from this, these are quite untrustworthy methods for ascertaining the mass of blood in the brain. Brodmann made a series of excellent investigations on a subject who had been trepanned, and proved that there is no antagonism between the circulation in the brain and in the arm, either in sleep or in the waking state. Mosso's method may, therefore, be dismissed as of no account But Hirschlaff points out that apart from this, Brodmann has shown that the relative conditions of the circulation of the blood in the brain are as yet of no general use in discussing the theory of sleep, and of course we must be just as cautious in regard to hypnosis.