* De Rerun Natura, Lib. iv.

* Dr. CARPENTER's General and Comparative Physiology, § 156.

We now return to the consideration of sleep in man.

There are different degrees of sleep. It is more or less complete both as a whole, and in its separate parts; for you must have gathered from what we have said, that sleep is a complex state, since the name is given to the hushed condition of the five senses, the suspension of voluntary motion, and that peculiar condition of the mind to which we have adverted. An eminent French Physiologist said, - "Le sommeil général est l'ensemble des sommeils particuliers." We have seen that there are very different degrees of intensity in the sleep of one sense at different times; but I have now to call your attention to the fact, that one or more senses or faculties may be wakeful, or nearly so, while the rest are profoundly asleep. Thus the organ of hearing is often sufficiently impressible to convey sensations to the sleeper, which are mingled with the ideas of his dreams and suggest new scenes, and often with the greatest rapidity, - so much so, that in the short time which elapses while the noises are occurring, a long period may seem to be occupied by scenes and actions suggested to the dreaming faculties. I remember once in my sleep, witnessing what I thought a prolonged storm of thunder and lightning, which I was able afterwards to trace to the light of a candle brought suddenly into the dark room where I had fallen asleep, and to the noise made in opening a door, the lock of which was never turned without a good deal of grating and rattling. Sensations derived from the skin may have a similar effect. The touch or grasp of a person arousing the slumberer, may suggest to his mind images of robbers or enemies, with whom he is struggling. A person having a blister applied to his head, fancied he was scalped by a party of Indians. And a friend of Dr. Macnish's "happening to sleep in damp sheets, dreamed he was dragged through a stream. Another friend dreamed he was stroking a kitten, which in consequence purred most lustily. On awaking, he found that the working of the heavy machinery of a neighbouring mill was slightly shaking his bed, and making the joints produce a sound like the purring of a cat."

By this incomplete sleep, this waking of some of the senses, and the consequent production of impressions which are mixed up with the ideas in the mind, we can easily explain the frightful dreams produced by many disorders of the stomach and other digestive organs.

Some of the most interesting, examples of the incompleteness of sleep, are met with in the locomotive system. I do not allude to the restless movements of the limbs, or to the change of position often observed in light sleepers, which are more of the nature of instinctive or involuntary actions, than of movements directed by the will. In ordinary and natural sleep, though we may dream of making great muscular exertions both with hands and feet, we are lying quite inert; but when the sleep is less perfect, some of the muscles, through their nervous connections, may awake, and do the bidding of the mental images. A friend of mine awoke one morning desperately clutching and tugging at the strings of his night cap : he had been dreaming that a viper had fastened upon his throat, and he was doing his best to tear it away. The most common form of this partial sleep is Sleep-talking, in which the muscles of the voice answer to the ideas. A more inconvenient species is the waking of so large a number of the muscles as those which raise the trunk, and enable the person to walk; this is strictly Somnambulism, or Sleep-walking. But there are very different kinds and degrees of it. In the simplest, - that to which I have just alluded, - the sense of sight being still asleep, the person walks straight forward, unconscious of the impediments in his way, and is soon aroused, and very roughly, by coming in contact with obstacles. But in other cases of a more morbid kind, the eyes may be open, objects may be perceived and avoided in the sleeper's perambulations, and yet he may not be awake, for the images in his mind are as vivid as those which he derives from present sensations; and, therefore, the unreal is confounded, with the real, as in the case of the ghost-seer, only that in the latter the morbid condition takes its starting place from the waking state instead of from sleep. As an example of the serious consequences of this condition, I may remark that I know of a gentleman who, in this imperfect sleep, got out of bed, walked to the window, opened it, and let himself fall down from three stories' height, doubtless under the illusion produced by the mixture of the vivid conception of a dream, with the actual perception of some of the objects around him. As the ghost-seer views the phantom walking among the living, so this somnambulist, when he opened the window, might have had a beautiful garden spread before his mental eye upon which he thought to step out, instead of incurring a dreadful fall, that nearly cost him his life.

It is quite impossible to attempt entering at all satisfactorily into this subject on the present occasion; but I may give one further instance of incomplete sleep, or partial waking, in the case of a person who talks in sleep, while at the same time the sense of hearing is awake. I have heard that this is so often the case with one person that, by a little skillful management, long dialogues may be held with her while she sleeps. It is necessary to speak in an under tone, or the impressions on the auditory nerves are so strong as to awake her completely; but with this precaution, and by taking the cue from what she has said in her sleep, questions may be interposed to which she will give answers very unreservedly, so that a dishonourable person might steal from her, in these unguarded moments, the most cherished secrets. On waking, she is quite unconscious of having held these conversations, except as in ordinary dreaming.