"The senses," says Johannes Mueller, "inform us of the various conditions of our body by the special sensations transmitted through the sensory nerves. They also enable us to recognize the qualities and the changes of the bodies which surround us, in so far as these determine the particular state of the nerves." l The senses, in other words, are the means through which we obtain the knowledge of our own bodies and of the external world.

For the exercise of their function are necessary: (1) the reception of an internal or an external impression by a peripheral organ; (2) the transmission of this impression to the brain; (3) its elaboration in the brain, which transforms it into a phenomenon of consciousness: first sensation, then perception. Only the latter operation is of interest to the psychiatrist.

We shall study successively:

I. Insufficiency of perception;

II. Illusions (inaccurate perceptions);

III. Hallucinations (imaginary perceptions). Hallucinations and illusions are often classed together under the name of psychosensory disorders.

Sec. 1. Insufficiency Of Perception

Insufficiency of perception in its slightest degree may be met with in states of depression, at the onset of confusional states, etc. All external impressions are vague, uncertain, and strange. The patients complain that everything has changed in them and around them: objects and persons have no more their usual aspect; the sound of their own voice startles them.

1 Johannes Mueller. Handbuch der Physiologie.

In a more marked degree of insufficiency external impressions no longer convey to the mind of the subject any clear or precise idea; questions are either not understood at all, or understood only when they are very simple, brief, energetically put, and repeated several times. External stimulation, even the strongest, is but vaguely perceived and often causes no reaction proportionate to its intensity or appropriate to its nature.

Finally, complete paralysis of one or several forms of psychosensory activity is observed in connection with profound disorders of consciousness, as in mental confusion of the stuporous form.

Insufficiency of perception constitutes an important element of clouding of consciousness, which will be considered later on.

Its pathogenesis is closely connected with disorders of ideation. The normal act of perception really consists of two elements: (1) a sensory impression; (2) a series of associations of ideas which enables the mind to recognize the impression and which almost always completes it and renders it more definite. If the associations of ideas are not formed in sufficient numbers the perception can only be vague and ill defined.