Here I observe, first, that new-born children sleep almost always. Now this may be accounted for by the doctrine of vibrations, in the following manner:—The fœtus sleeps always, having no sensation from without impressed upon it, and only becomes awake upon its entrance into a new world, viz. by means of the vigorous vibrations which are impressed upon it. It is reasonable therefore to expect, that the new-born child should fall back into its natural state of sleep, as soon as these vibrations cease, and return again to a state of vigilance only from the renewal of vigorous impressions; and so on alternately, agreeably to the fact.
Secondly, Even adults are disposed to sleep, when the impressions of external objects are excluded, and their bodies kept in a state of rest, for the same reasons as those just mentioned in the similar state of young children. However, they incline more to vigilance than children, partly because their solids and fluids are more active, and less compressible, i.e. more susceptible and retentive of vibrations; and partly, because association brings in perpetual trains of ideas, and consequently of vibrations, sufficiently vivid to keep up vigilance in common cases.
Thirdly, Having presented the reader with the two foregoing observations, which are of a very obvious kind, I will now inquire, with more minuteness, into the intimate and precise nature of sleep. It appears, then, that during sleep the blood is accumulated in the veins, and particularly in the venal sinuses which surround the brain and spinal marrow; and also that it is rarefied, at least for the most part. For as the actions of the muscles squeeze the blood out of the veins during vigilance, so their inactivity during sleep suffers the blood to lodge in the veins; and the decumbent posture, which is common to animals in sleep, suffers it to lodge particularly in the venal sinuses of the brain and spinal marrow. And it is agreeable to this, that, in most dissections, the blood is found chiefly in the veins; and in dissections after lethargies, apoplexies, &c. the venal sinuses of the brain, and consequently those of the spinal marrow, which communicate freely with them, are particularly full. As to the rarefaction of the blood, it follows from the warmth of the body, which is an usual attendant upon sleep, and is caused by the rest of the body, the warmth of the place where the person sleeps, the coverings, and the fermentative disposition of the fresh chyle, which then enters the blood. It follows, therefore, that the brain and spinal marrow will be particularly compressed during sleep; since the blood then takes up more space, is particularly accumulated within the cavities of the skull and vertebræ, and the hardness of these bones will not suffer them to yield, or make more room. It follows, also, that the softness of the medullary substance will subject it to the effects of this compression, more than
