account of the compression here asserted; also, that they become contiguous at the instant of sleep, excluding the denser æther, mentioned in the foregoing proposition, thereby? By this means, the power of sensation would receive a remarkable diminution at the instant of falling asleep, as it seems to do. There might also, in certain circumstances, arise a very vivid exertion of the perceptive and motive faculties at that instant, from the compression of the æther previous to its rarefaction, such as would account for the sudden terrors and startings which happen at the instant of going to sleep, in some morbid cases.
Fourthly, It is observed, that vigilance continued, fatigue, and pain, all dispose strongly to sleep. For all vigorous or long-continued vibrations must both generate heat, whereby the blood and juices will be rarefied, so as to compress the medullary substance, and also exhaust this substance of its fluid and active particles, so as to render it more easily compressible, and less susceptible and retentive of vibrations. Great degrees of heat seem to produce an extraordinary propensity to sleep, in nearly the same way.
And when persons exposed to extreme cold are overcome by a pleasing but fatal sleep, it seems as if the internal parts were affected with a preternatural warmth, from the vigorous sensations and concomitant vibrations impressed on the external parts by the cold, and thence ascending to the brain. It agrees with the hypothesis here proposed, that these uneasy sensations decline by degrees, till they fall within the limits of pleasure, and at last end in insensibility. This sleep may prove fatal, from the great difference between the internal and external parts, in respect of heat; also from the cold’s penetrating farther and farther. Muscular motion may prevent it, and its ill effects, partly as the veins are emptied by this, partly as it warms the external parts, and cools the internal, from the return of the cool blood into the course of the circulation. If we suppose the circulation to cease entirely at the surface of the body, from the cold, then will warm blood circulate through the internal parts alone; and these parts will continue to be defended from the cold by the external ones, for a time. And thus the body will approach to the common state of a person going to sleep.
It is easy to see, from the method of reasoning here used, how persons recovering from long illnesses should be much disposed to sleep, viz. from the exhaustion of the medullary substance, their almost constant rest, their being kept warm, and the frequent taking sustenance, so as to beget great quantities of fresh chyle, and consequently, an extraordinary degree of a fermentative heat.
Fifthly, The manner in which opiates produce sleep, may be thus explained, agreeably to the doctrine of vibrations. Opiates evidently excite grateful sensations in the stomach and bowels. This appears from the short time in which liquid opiates take
