A further part of Vogt's theory bears on the origin of sleep, ordinary and hypnotic. He attempts to prove that sleep is caused by the stimulation of certain centres, more especially the reflex centre for shutting the musculus orbicularis occuli, by the action of the neurokym or nerve-wave. But certainly Vogt thinks that a vasomotor reflex centre here plays a more important part, its stimulation causing an increasing anaemia of the brain and thereby drowsiness and sleep.

I shall not enter into any criticism of Vogt's theory, as the same objections apply to it and nearly all other physiological theories, as were raised to Wundt's.

Finally, I mention the theory of Preyer, who puts the matter thus: - An activity of one hemisphere of the brain results in hypnosis; fixed attention causes a rapid accumulation of waste-products in the parts of the brain which are active, and by this a quick local consumption of the oxygen of the blood is caused. In consequence of this, favoured by the failure of the ordinary change of stimulus of the nerves of sense, there is a partial loss of the activity of the cerebral cortex. The partial loss of activity of one region would then explain the increase of activity of the other, because the inhibition would disappear. Bernheim objects to this that it does not explain a rapidly induced hypnosis, for it is hardly conceivable that waste-matter should accumulate so rapidly. Similarly, the sudden termination of hypnosis is not consistent with this. As we have seen, the one word "wake" is enough to end the hypnosis at once. We should be obliged to suppose that the simple idea of waking was able to dissipate the waste-matter or make it of no effect.

I do not think that any of the physiological theories hitherto propounded can be considered in the least degree satisfactory. This does not imply any reproach to physiology in general. But it should be remembered that as far as physiological theories go we have about reached the limit of our knowledge. The connection between mind and body is still purely hypothetical. The fact that stimuli which differ to but a trifling extent here produce such different effects is a favourite objection to materialistic theories and physiological explanations of hypnosis. Ludwig Busse has recently called attention to this in his excellent work, Mind and Body. The owner of a ship who receives a telegram from the captain, "Reached the Cape," would be very differently affected if the telegram were " Beached the Cape," yet the physiological stimulus differs but very slightly in the two cases. We must admit that similar considerations show the value, or rather the valuelessness, of physiological theories. I certainly think that as long as we are totally unable to understand how an idea, roused for example by the word "wake," changes the subject's whole state, we must be very sceptical as regards physiologists' theories.

Even so unbiassed an observer as Lotze has ironically stated that, according to his own private statistical reckoning, the great discoveries of physiology had an average existence of four years. There may be some exaggeration in this, and I do not think it should be applied to all branches of physiology. But the endeavours of some investigators to explain mental processes by means of our present knowledge of the central nervous system point to a disquieting tendency to over-estimate physiology; and I think that Meynert's assertion that cerebral physiology is no longer a problem will cause many to share my doubts. But I think I can best show how devoid of all value physiological theories of hypnosis are, by calling attention to the contradictions between the views of Mendel and Ziemssen. Mendel explains that in hypnosis we have to do with a strong stimulation of the cerebral cortex, while Ziemssen declares that the cerebral cortex is too little stimulated and the subcortical centres too much! Under these circumstances we may, surely, be allowed to hope that in future less will be asserted and more will be proved.

Such contradictions as those between Mendel and Ziemssen would be inconceivable if it were not for the presence in their works of just such speculations as those with which medicine is in the habit of reproaching philosophy.