III. _Application of the law of intermission of action to the theory of sleep._ General sleep is the sleep of all the parts. It follows from that law, which with respect to the functions of the animal life, enchains intermission with periods of action, from that law, by which this life is particularly distinguished from the organic life. Very numerous varieties are remarked in this periodical state, to which all animals are subject. The most complete sleep is that in which the outward life is entirely suspended. The least perfect sleep is that which affects one organ only; it is that of which we have just been speaking. Between these two extremes there are many intermediate states. At times perception, locomotion, and the voice only are suspended; the imagination, the memory, and the judgment remaining in action. At other times, to the exercise of the latter faculties are added those of the locomotive organs and the voice.--Such is the sleep, in which we dream, for dreams are nothing more than a portion of the animal life escaped from the torpor, in which the other portion of it is plunged. Sometimes but very few of the senses have ceased their communication with external objects. Such is that species of somnambulism, in which to the action of the brain, the muscles, and the larynx, are added the very distinct actions of the ear and the sense of touch.[13] Sleep then cannot be considered as a constant and invariable state with regard to its phenomena.--Scarcely ever do we sleep in the same manner twice together. A number of causes modify in applying to a greater or less portion of the animal life the laws of intermission of action. Its different degrees should be marked by the different functions, which these intermissions affect. But the principle of it is every where the same from the simple relaxation of a muscle to the entire suspension of the whole of the animal life. Its application, however, to the different external functions, varies without end. These ideas on sleep are different, no doubt, from that narrow system, where its cause exclusively placed in the brain, in the heart, in the large vessels, or in the stomach, presents an isolated and frequently an illusory phenomenon, as the base of one of the great modifications of life.