Brown-Sequard * thinks that sleep results from inhibition, pure and simple, and supposes that the inhibitory impulses proceed from the neighbourhood of the medulla. He contends that the drowsiness of indigestion is not produced by the formation and accumulation of leuco-maines of abnormal quality, but arises from impulses of gastric origin, affecting the inhibitory centres through the pneumogastric.

There is almost as great a difference between the profound, dreamless, and physiologically perfect sleep of the healthy labouring man who earns his bread by the sweat of his brow and is untroubled by nerves, and the stuporose condition of the overfed and under-exercised dyspeptic, as there is between natural sleep and hypnosis. The reason of this appears to be that healthy physiological sleep is the outcome of all the causes I have enumerated, acting in their proper and just relationship and proportion; whereas in the sleep of indigestion we have one cause acting in excess - the accumulation of waste products and alkaloids in the tissues, as a result of imperfect assimilation and metabolism. The sleep is incomplete and one-sided here just as it is in hypnotism, and in both cases some of the elements of natural sleep are wanting. As in cases of indigestion, with somnolence and disturbed sleep, there are degrees of disturbance, varying from slight abnormality to complete change of type, so in hypnotism we see all degrees of hypnosis, from the condition, such as is seen in Charcot's cases, which has only a distant resemblance to normal sleep, to a state indistinguishable from and passing into it.

The difference in type appears to depend upon the extent to which one factor is producing the condition to the exclusion of the others - whether the state is an auto-narcosis from the accumulation of alkaloids, is the result of inhibition alone, or is a combination of all the causes which go to make up normal sleep.

* Archives de Physiologie Normale et Pathologique, January, 1889. He has demonstrated the fact that pigeons continue to sleep at regular intervals after the sympathetic nerves in the neck have been divided, and that dogs and cats sleep after removal of the superior cervical ganglion of one side, and division of the vago-sympathetic in the other - procedures leading to continued dilatation of the cerebral vessels, and thus proving that sleep does not depend so much on contraction of the vessels with consequent cerebral anaemia as has been supposed. Moreover, he found that pigeons continue to sleep after the removal of the lobes of the brain, from which he concluded that sleep does not result from influences of medullar origin, and is not dependent on the state of the cerebral lobes.

There is ground for believing that natural sleep assumes the hypnotic type much more frequently than is commonly supposed; i.e., the sleeper is susceptible to suggestion from without. It is certain that many persons who are hypnotized for the first time, and the majority of those who by frequent repetition of the process have become good subjects, do not require any special stimulation of one sense for the production of hypnosis; expectant attention and verbal suggestion being sufficient to determine the condition. Those facts are calculated to upset many of the theories of the causation of hypnosis, and to confirm the dictum of Brown-Sequard that hypnosis does not depend upon exhaustion, but is a result of dynamic inhibition of the highest centres.

When Braid's method is employed, an intense strain is put upon the visual apparatus, and an excessive discharge from the cells of the visual centre is followed by its exhaustion and subsequent cessation of its functional activity. But in Bernheim's method there is no such localized stimulation and nervous exhaustion, and the condition induced would be indistinguishable from ordinary drowsiness or sleep, were it not that the subject is receptive of suggestion. The chemical theory would also be explanatory of the phenomena of Braidism, but it quite breaks down when the suggestive method of inducing hypnosis is in question. The increased functional activity of a centre must lead to the increased oxidation of its elements, with increased formation of the products of molecular disintegration (vital alkaloids); and we know that the accumulation of these in the system is a cause of drowsiness and sleep. It is possible that their accumulation in one part of the brain may induce partial or complete inhibition of certain centres, with consequent interference with or abolition of their functions (Tarchanoff).

Healthy, dreamless sleep depends upon temporary abolition of the functions of the highest centres, together with a partial inhibition of those below - the middle and lower levels. In dreamful and disturbed sleep inhibition is less complete, and certain areas continue to discharge nervous energy and so produce dreams. The more intense the discharge, the more vivid the dream; and if the discharge be very intense, the attending process may overstep the limit of mere ideation and take the form of action: there will be movement or sleep-walking from stimulation and discharge of the motor areas. The actions will be unaccompanied by consciousness: for consciousness depends upon the functioning of all the higher centres in orderly relation, and in somnambulism this order is disarranged or destroyed. The actions, being unconscious, are automatic, and tend to partake of the nature of those usually or habitually performed. They will be in keeping with the character and temperament of the sleeper; for the nervous discharges will travel through well-worn and accustomed channels, and will hardly effect new combinations or movements.

The post-epileptic state shows some very important resemblances to hypnotic somnambulism; the researches of Dr. Hughlings Jackson and other observers enable us to understand its phenomena.

The central nervous system may be considered as composed of three divisions or layers: first, the medulla, pons, and basal ganglia, which function the most organized processes of the animal economy, i.e., nutrition, secretion, respiration, circulation; secondly, the motor areas of the cortex, which function muscular movements; and thirdly, the highest and last evolved cortical centres, which control and regulate the actions of those below them, and which produce new associated movements. Furthermore, we must suppose that these highest centres are assigned different grades in the hierarchy of the nervous system. It is the function of the highest of all, which are those last evolved, to control those immediately below them, and through them to exercise a governing influence over the entire nervous mechanism. It is only the functioning of these highest of all centres which is accompanied by psychical processes and is attended with full consciousness.