s 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 is familiar with subjective
sensations such as flying spots, phosphenes, ringing in th