The most common form of motor hallucination is probably the vocal.  In the social encounters which make up so much of our sleep-experience, we are wont to be very talkative.  Now, perhaps, we find ourselves zealously advocating some cause, now very fierce in denunciation, now very amusing in witty repartee, and so on.  This imagination of ourselves as speaking, as distinguished from that of hearing others talking, must, it is clear, involve the excitation of the structures engaged in the production of the muscular feelings which accompany vocal action, as much as, if not more than, the auditory centres.  And the frequency of this kind of dream-experience may be explained, like that of visual imagery, by the habits of waking life.  The speech impulse is one of the most deeply rooted of all our impulses, and one which has been most frequently exercised in waking life.

Combination of Dream-Elements.

It is commonly said that dreams are a grotesque dissolution of all order, a very chaos and whirl of images without any discoverable connection.  On the other hand, a few writers claim for the mind in sleep a power of arranging and grouping its incongruous elements in definite and even life-like pictures.  Each of these views is correct within certain limits; that is to say, there are dreams in which the strangest disorder seems to prevail, and others in which one detects the action of a central control.  Yet, speaking generally, sequences of dream-images will be found to be determined by certain circumstances and laws, and so far not to be haphazard or wholly chaotic.  We have now to inquire into the laws of these successions; and, first of all, we may ask how far the known laws of association, together with the peculiar conditions of the sleeping state, are able to account for the various modes of dream-combination.  We have already regarded mental association as furnishing a large additional store of dream-imagery; we have now to consider it as explaining the sequences and concatenations of our dream-elements.

Incoherence of Dreams.

First of all, then, let us look at the chaotic and apparently lawless side of dreaming, and see whether any clue is discoverable to the centre of this labyrinth.  In the case of all the less elaborately ordered dreams, in which sights and sounds appear to succeed one another in the wildest dance (which class of dreams probably belongs to the deeper stages of sleep), the mind may with certainty be regarded as purely passive, and the mode of sequence may be referred to the action of association complicated by the ever-recurring introduction of new initial impulses, both peripheral and central.  These are the dreams in which we are conscious of being perfectly passive, either as spectators of a strange pageant, or as borne away by some apparently extraneous force through a series of the most diverse experiences.  The flux of images in these dreams is very much the same as that in certain waking conditions, in which we relax attention, both external and internal, and yield ourselves wholly to the spontaneous play of memory and fancy.