On the other hand, sleep controls the manifestations of several diseases, especially those which are of a convulsive or spasmodic character. Thus the paroxysms of chorea cease during sleep, as do likewise the spasms of tetanus and hydrophobia. Headache is also generally relieved by sleep, though occasionally it is aggravated.
CHAPTER IV.
THE STATE OF THE MIND DURING SLEEP.
We have seen that though during sleep the operations of the senses are entirely suspended as regards the effects of ordinary impressions, the purely animal functions of the body continue in action. The heart beats, the lungs respire, the stomach, the intestines, and their accessory organs digest, the skin exhales vapor, and the kidneys secrete urine. With the central nervous system, however, the case is very different; for while some parts retain the property of receiving impressions or developing ideas, others have their actions diminished, exalted, perverted, or altogether arrested.
In the first place, there is, undoubtedly, during sleep, a general torpor of the sensorium, which prevents the appreciation of the ordinary excitations made upon the organs of the special senses. So far as the nerves themselves are concerned, there is no loss of their irritability or conducting power, and the impressions made upon them are, accordingly, perfectly well conveyed to the brain. The suspension of the operations of the senses is not therefore due to any loss of function in the optic nerve, the auditory nerve, the olfactory nerve, the gustatory nerve, or the cranial or spinal nerves concerned in the sense of touch, but solely to the inability of the brain to take cognizance of the impressions conveyed to it. In regard to the cause of this torpor, I have given my views in a previous chapter.
Now it must not be supposed that because mild excitations transmitted by the nerves of the special senses are incapable of making themselves felt, that therefore the brain is in a state of complete repose throughout all its parts. So far from such a condition existing, there are very decided proofs that several faculties are exercised to a degree almost equaling that reached during wakefulness, and we know that if the irritations made upon the senses be sufficiently strong, the brain does appreciate them, and the sleep is broken. This ability to be readily roused through the senses constitutes one of the main differences between sleep and stupor, upon which stress has been already laid.
Relative to the different faculties of the mind as affected by sleep, great variations are observed. It has been thought by some authors that several of them are really exalted above the standard attained during wakefulness, but this is probably a wrong view. The predominance which one or two mental qualities apparently assume is not due to any absolute exaggeration of power, but to the suspension of the action of other faculties, which, when we are not asleep, exercise a governing or modifying influence. Thus, for instance, as regards the imagination,—the faculty of all others which appears to be most increased,—we find, when we carefully study its manifestations in our own persons, that although there is often great brilliancy in its vagaries, that uncontrolled as it is by the judgment, the pictures which it paints upon our minds are usually incongruous and silly in the extreme. Even though the train of ideas excited by this faculty when we are asleep be rational and coherent, we are fully conscious on awaking that we are capable of doing much better by intentionally setting the brain in action and governing it by our will and judgment.
Owing to the fact that these two faculties of the mind are incapable of acting normally during sleep, the imagination is left absolutely without controlling influence. Indeed, we are often cognizant in those dreams which take place when we are half awake, of an inability to direct it. The impressions which it makes upon the mind are, therefore, intense, but of very little durability. Many stories are told of its power—how problems have been worked out, poetry and music composed, and great undertakings planned; but if we could get at the truth, we should probably find that the imagination of sleep had very little to do with the operations mentioned. Indeed, it is doubtful if the mind of a sleeping person can originate ideas. Those which are formed are, as Locke[34] remarks, almost invariably made up of the waking man’s ideas, and are for the most part very oddly put together; and we are all aware how commonly our dreams are composed of ideas, or based upon events which have recently occurred to us.