in fluid than in solid parts, because of their looser texture. There may be other reasons also, drawn from the particular unknown composition of each part, solid and fluid, which may subject them to greater or less rarefaction. Thus, I conjecture, that the red blood is more apt to be rarefied than the other fluids, and that it is by this means made a chief instrument in compressing the white medullary substance of the brain, and spinal marrow, in natural and morbid sleep, to both which heat contributes, as has been observed already.
Secondly, If heat be caused by vibrations, we may expect, that those propagated from the hot or cold body should diffuse themselves freely and instantaneously over the whole nervous system, i.e. the whole body; however along the surface of the skin, in an especial manner. This follows from the uniformity both of the whole medullary substance, and of the skin. The first communicates the vibrations which ascend along the nerves affected to the whole body, the last those impressed upon the part of the skin, which touches the hot or cold body, to the other parts of it. Now this is agreeable to experience: for when the whole body is too hot, or too cold, we find that the mere touch of a cold or hot body will give general relief immediately; and in some cases a thrilling or shivering may be felt to run along the skin.
Thirdly, If the skin be contracted by any cause different from the direct impression of cold, as by the pain propagated from a wound, the colic, the irritation of a stone in the bladder, &c. this contraction, first excited by an increase of vibrations in the muscular variously interwoven fibrils of the skin, may be expected afterwards to check and diminish the vibrations there, and thus to occasion the sensation of cold, agreeably to experience. The chilliness arising from matter absorbed, and from the cause of acute distempers, whatever that be, may admit of a like explanation.
The tremors, i.e. sudden, short, alternate contractions of the antagonist muscles, which happen in the foregoing cases, arise probably from an increase of vibrations, not subject to ideas, and the voluntary power, descending from the brain into the whole system of the muscles; and seem to differ from the stronger and larger convulsive motions of hysteric and epileptic disorders, called convulsions emphatically, rather in degree than kind. These tremors generally precede the sense of chilliness, when the contraction of the fibres of the skin does not arise from the direct impression of cold.
We may from hence pass to the sensation of chilliness, and the tremors, which are sometimes occasioned by the passions, fear, anger, surprise, joy, &c. Both the redness and the paleness of the lips, face, and neck, which are observable in these cases, are marks of a contraction in muscular fibrils; in a less degree in the first case, so as to check the return of the venal blood; in a greater in the last, so as to prevent the influx of the arterial.
Fourthly, It is easy to conceive, that heat may occasion pain,
