§ 7. The general conclusion from the foregoing paragraphs is plain - that hallucinations of the senses may be spontaneously initiated by the brain; that they are often a pure projection of the brain from within outwards. The hypnotic "subject " will smack his lips over the sweetness of sugar when there is nothing in his mouth - will sniff with delight at a piece of wood when told it is a rose. When the brain does for sight and hearing what it there does for taste and smell, we have a percept which differs from an ordinary external percept only in lacking the objective basis which it suggests. And looking back from this point, we can completely account for the fact noticed in the preceding chapter, that hallucinations occur with disproportionate frequency to people who are in bed. For it is only natural that images should assume the unwonted vividness of sensations especially at those moments when the external organs of sense are not occupied with other sensations. We know that the sort of day-dream which comes nearest to hallucinations is favoured by repose of the sense-organs; that when we want to call up the vivid image of a scene, to make it as real - as sensorial - as possible, we close our eyes.

One step farther, and we realise the complete continuity of the waking and the sleeping phenomena. Dreams are by far the most familiar instances of the projection by the mind of images that are mistaken for realities: indeed, it is just because they are so familiar, and waking hallucinations comparatively so rare, that there is a danger of overlooking the psychological identity of the two classes. We might call dreams the normal form of hallucination, or waking hallucinations the pathological form of dreaming; and we might present the waking-dreams of hashish-poisoning and of starvation1 as a sort of intermediate link. The normal dream disappears when sleep departs; having been able to impose its images as realities only because in sleep our sensory faculties are to a great extent benumbed, and images cannot therefore be compared with actual presentations. Thus the normal dream cannot survive the corrective which the contact of the waking-senses with the external world supplies; it fades like a candle at sunrise; and its images, if they survive, survive as images and nothing more, memories of a vanished world.

The hallucination, or pathological dream, on the other hand, does not require to be thus guarded from comparison with real presentations; its " images " are able to resist the normal corrective, for they are often as fully charged with sensory quality as the external realities which compete with them.

1 See the interesting case of Mr. Everts, in Scribner's Magazine for Nov., 1871. Dagonet considers that fasting meditation, and solitude favour hallucinations by "diminishing the impressionability of the senses" to external stimuli, (Op. cit., p. 93).

§ 8. I may now proceed to an altogether different question - namely, at what part or parts of the brain the constructive process takes place, and in what it can be conceived to consist. The distinction that has so long occupied us, between central and peripheral initiation, may henceforth be dismissed; for wherever initiated, hallucinations are assuredly constructed by the brain from its own resources. An initiating stimulus may probably come from any point on the line from the external organ to the central terminus, along which a nervous current passes in our normal perception of objects. But that stimulus will clearly not determine what the imaginary object shall be, or invest it with any of its qualities: it will merely set the constructive machinery in motion; and the same stimulus - the same inflammation of the eye or ear - may set the machinery in motion a hundred times, and each time evoke a different hallucination. Where then, and what, is this constructive machinery? It would be out of place here to attempt any minute account of the various theories, which have for the most part rested on anatomical observation; and the more so, that their details are still sub judice.

But in a more general way the problem can be stated, and even, I think, to some extent determined.

There can be no doubt that certain sensory centres are connected in a special way with hallucinations. This follows, as soon as the full sensory character of the phenomena is recognised; for that character can only be the psychical expression of physical changes at the "sensory centres" - the spots where (in the ordinary crude but convenient language) impressions are transformed into sensations. As to the exact locality of these spots, there is a conflict of views which may be to some extent reconciled if we regard the process as taking place in several stages. Some (Luys, Ritti, Fournie) believe the principal scene of action to be the large midway masses called the optic thalami; others (Schroder van der Kolk, Meynert, Kahlbaum, Kandinsky) would place it lower down - the centre for vision, for instance, in the corpora quadrigemina; while others again (Hitzig, Ferrier, Tamburini) locate it higher up, in the cortex itself. But the authorities are generally agreed in connecting the several forms of sense with several limited areas, distinct from the larger tracts associated with the most highly developed phenomena of intelligence.

A diagram may make the relation of the parts clearer.

Let A represent the retina of the eye, which in itself has no more power of seeing than a mirror has. Let B represent the group of cells in the brain which constitutes the sensory centre - say the "visualising centre" - and which is excited into activity whenever sight takes place. And let C represent the cortical or exterior substance of the hemispheres of the brain, part of which is excited into activity whenever any of the higher psychical faculties - intelligent perception, imagination, comparison, memory, volition - are called into play. A is connected with B by the fibres of the optic nerve, and B is connected with C by other nerve-fibres. Now any disturbance of the cells at B which reaches a certain intensity will be accompanied by the sensation of sight; and when this disturbance is propagated onwards in the natural course from B to C, this sensation will develop into a complete percept - an object for the mind - which can be reflected on, compared with other objects, and remembered. But the indispensable event - the disturbance or discharge at B - may itself originate in several different ways.

It may be excited (1) normally, from A, by the stimulus of external rays of light, which makes us see surrounding objects; (2) accidentally, from A, by the stimulus of a blow on the eye, which makes us see sparks; (3) pathologically, or by morbid irritation at A, or on the line of nerve A B; and (4) pathologically or abnormally, but spontaneously, at B itself.

Now for one view of the construction of hallucinations these data are sufficient. We have only to suppose that in cases (3) and (4) the agitation at the sensory centre falls readily into certain lines and combinations; so as not only to produce a large variety of sensations - colours, if it be the visual centre, sounds, if it be the auditory one - but to arrange these elements in various definite groups. Everything will now proceed precisely as if these effects had been due to the presence of a real object. The excitation will pursue its ordinary upward course to the highest parts of the brain, and will lead to intelligent perception of the sensory group as an object; while, in the most complete or "external" form of hallucinations, it is possible that by yet a further process a refluent current passes downwards to the external organ,1 to which the perception is referred, just as though its object were really acting on the eye or ear from outside. There then is the full-fledged hallucination; and its creative machinery, according to this view, lies wholly in the sensory centre.