Or, secondly, they may originate in a certain preternatural activity of the sensory centres, or "sensorium," in what has been called by German physiologists an automatic excitation of the central structures, which activity may probably diffuse itself downwards to the peripheral regions of the nerves.

Baillarger would call hallucinations of the former class "psycho-sensorial," those of the latter class purely "psychical," hallucinations.[57] It is often a matter of great difficulty to determine which part of the nervous system is originally concerned in these rudimentary hallucinations.

It is probable that in normal life they are most frequently due to peripheral disturbance.

And it seems reasonable to suppose that where the hallucination remains in this initial stage of a very incompletely interpreted visual or auditory impression, whether in normal or abnormal life, its real physiological source is the periphery.
For the automatic excitation of the centres would pretty certainly issue in the semblance of some definite, familiar variety of sense-impression which, moreover, as a part of a complex state known as a percept, would instantly present itself as a completely formed quasi-percept.

In truth, we may pretty safely argue that if it is the centre which is directly thrown into a state of activity, it will be thrown into the usual complex, that is to say, _perceptional_, mode of activity.
Let us now turn to hallucinations properly so called, that is to say, completely developed quasi-percepts.