Let us now turn to hallucinations properly so called, that is to say, completely developed quasi-percepts.  These commonly assume the form of visual or auditory hallucinations.  Like the incomplete hallucinations, they may have their starting-point either in some disturbance in the peripheral regions of the nervous system or in the automatic activity of the central structures:  or, to use the language of Baillarger, we may say that they are either “psycho-sensorial” or purely “psychical.”  A subjective visual sensation, arising from certain conditions in the retina and connected portions of the optic nerve, may by chance resemble a familiar impression, and so be at once interpreted as an effect of a particular external object.  More frequently, however, the automatic activity of the centres must be regarded, either in part or altogether, as the physiological cause of the phenomenon.  This is clearly the case when, on the subjective side, the hallucination answers to a preceding energetic activity of the imagination, as in the case of the visionary and the monomaniac.  Sometimes, however, as we have seen, the hallucinatory percept answers to previous prolonged acts of perception, leaving a kind of reverberation in the structures concerned; and in this case it is obviously impossible to say whether the peripheral or central regions (if either) have most to do with the hallucination.[58]

The classifications of the causes of hallucination to be met with in the works of pathologists, bear out the distinction just drawn.  Griesinger tells us (op. cit., pp. 94, 95) that the general causes of hallucination are:  (1) Local disease of the organ of sense; (2) a state of deep exhaustion either of mind or of body; (3) morbid emotional states, such as fear; (4) outward calm and stillness between sleeping and waking; and (5) the action of certain poisons, as haschisch, opium, belladonna.  The first cause points pretty distinctly to a peripheral origin, whereas the others appear to refer mainly, if not exclusively, to central derangements.  Excessive fatigue appears to predispose the central structures to an abnormal kind of activity, and the same effect may be brought about by emotional agitation and by the action of poisons.  The fourth case mentioned here, absence of external stimulation, would naturally raise the nervous structures to an exceptional pitch of excitability.  Such a condition would, moreover, prove favourable to hallucination by blurring the distinction between mental image and actual impression.

Hallucinations of Normal Life.

In normal life, perfect hallucinations, in the strict sense as distinct from illusions, are comparatively rare.  Fully developed persistent hallucinations, as those of Nicolai, the Berlin bookseller, and of Mrs. A——­, the lady cited by Sir D. Brewster, in his Letters on Natural Magic, point to the presence of incipient nervous disorder.  In healthy life, on the other hand, while everybody