In somnambulism we have a state which is in some respects the reverse of that usual in dreams. The higher centres are, indeed, split off from the lower centres, but it is the former that are asleep and the latter are awake, whereas in ordinary dreaming the higher centres are acting in accordance with their means, while the lower centres are quiescent. Somnambulism is an approximation to a condition found in some diseases of the brain when, as a result of lesion of the higher nervous levels, we have a mental state--the ideatory apraxia of Liepmann--in which the muscular system carries out plans, but the plans are defective because not supervised by the higher centres. In ordinary dreams, on the other hand, we have a state comparable to that produced by brain lesions in what Pick terms motor apraxia, in which the higher centres are acting freely, but their plans are never carried into action owing to failure of the motor centres.
This characteristic of dreaming has seemed puzzling to some writers. They ask why, in our dreams, we should sometimes be so conscious of failure of movement, and why, when we strive to move in dreams, we do not always actually move.[78]
There scarcely seems to me to be any serious difficulty here; still, the question is one of considerable interest and importance. It is necessary to point out in the first place that, however complete the actual absence of movement, there is usually no failure of movement in the dream vision. We dream that we are talking, that we are moving from place to place, that we are performing various actions. We are conscious of no difficulty, even sometimes of a peculiar facility, in executing these movements. And in normal persons, under normal conditions, it would seem that the dream movements take place without even an incipient degree of corresponding actual movement perceptible to an observer. The efferent motor channels, and even to a large extent the afferent sensory channels, are asleep, and the whole representative circuit is completed within the brain, or, as we say, imaginatively.[79] Thus a middle-aged friend, whose habits are by no means athletic, dreams that, desiring to attract some one's attention, he rests one knee on his wife's small work-table, and holding the foot of the other leg in one hand, he whirls rapidly and easily round and round on the pivot of the knee which rests on the table, the dream afterwards continuing without any awakening. A lady, again, who, when awake, is unable to swim, and knows no reason why she should think of swimming, vividly dreams that she jumps from a houseboat into the river, and proceeds to swim on her side with great ease, this dream also continuing without awakening. These dreamers were able to execute triumphantly the muscular feats they planned, because they had not really attempted to execute them at all, and, moreover, no sufficient sensory messages reached the brain to give information that the limbs were not actually obeying the orders of the brain. The dreamers were probably in a somewhat deep state of sleep.[80]