David Hartley

“ When we apply the parts of our bodies to each other, particularly our hands to the several parts of the surface of our bodies, we excite vibrations in both parts, viz. both in the hands, and in that part of the surface which we touch. Suppose the hand to pass over the surface gradually, and the first impression will remain the same, while the last alters perpetually, because the vibrations belonging to the last are excited in different nerves, and by consequence enter the brain, or spinal marrow, at different parts. ”

“ Fifthly, The continued impression of heat makes us more sensible of cold. For when heat has rarefied the parts, and adapted them to a peculiar strength and frequency of vibrations, differing from the usual standard, the cold, whose difference from the usual standard lies on the other side, must raise a greater conflict, and produce a greater change, than if the parts had remained at the usual standard. The continued impression of cold must for the same reasons make us more sensible of heat. ”

“ Let us now inquire how far the several effects of heat and cold upon our bodies are agreeable either to the notion of vibrations in general, or to the particular conjectures of the last paragraph.
First, then, we may expect that heat will rarefy the solids and fluids of the body, and the last more than the first, which is agreeable to experience. For the increase of the agitations will make the small particles recede from one another, and that more in fluid than in solid parts, because of their looser texture. ”

“ Thus the picture of a knife, drawn so well as to deceive his eye, would not, when applied to another body, produce the same change of visible impressions, as a real knife does, when it separates the parts of the body through which it passes. ”

“ Secondly, we impress a great variety of sensations ourselves by our hands, according as they are hot or cold, by friction, scratching, &c. and most impressions from foreign bodies will bear some resemblance to some of these. Thirdly, we often see upon what part impressions from foreign bodies are made. Fourthly, when they leave permanent effects, as in wounds, burns, &c. we always examine by feeling, where the impression was made. ”

“ Thus, since moist bodies adhere to the fingers, and so leave a smoothness with their own degree of cold or heat upon them, moisture may be judged of by the touch from this peculiar alteration of vibrations; and dryness from the absence of it. Liquid bodies make no alteration of figure in our fingers, and yield easily to their motions: soft ones do the same in a less degree; hard ones the contrary. Smooth bodies make an equable pressure, and give no resistance to a motion along their surfaces ”

“ Titillation is nearly related to itching. It is excited by slight impressions upon the more sensible parts, frequently renewed; and this may show, that it arises from an increase of vibrations. The impressions must be so slight as not to excite a contraction in the neighbouring muscular fibrils, and also frequently renewed, that the increase of vibrations may diffuse itself farther and farther perpetually; and thus the whole nervous system may, in some cases, be at last put into a pleasurable state, approaching to the limits of pain, and passing within them at times. ”

“ Eighthly, When the parts contiguous to a mortified slough have a sufficient heat in them, excited by the vis vitæ, or warm applications, the vibrations attending this life and heat of the parts ought to help to shake off and separate the mortified slough, i.e. to stop the mortification; which is agreeable to the fact. Hence mortifications from external cold, in bodies otherwise healthy, will come to separate soonest, and most perfectly, as it is frequently seen in cold climates. ”

“ For great cold, by checking the vibrations in the external part, to which it is applied, will alter the situation and distance of the small medullary particles there, and so must excite vigorous vibrations in the ascending nerves, and the corresponding region of the brain, which is just the effect occasioned by heat, applied to the same external part. Hence, if we are touched by a very hot or a very cold body, inadvertently, and without seeing it, it ought to be difficult to distinguish which it is, agreeably to the fact. ”

“ The general feeling extends to all the parts of the body, external and internal: for they are all susceptible of pain from wounds and inflammations, of being put into a pleasurable state, of numbness, and total want of sensation, and of perceiving heat, cold, and pressure. Some writers consider all the sensations of all the senses as so many kinds of feeling ”

“ Eleventhly, When a person goes into cold water leisurely, he is apt to sob, and to respire in a convulsive manner, for a short time. For the impression of the cold upon the lower limbs excites such vigorous vibrations in the abdominal and other muscles of expiration, as being nearer to the seat of the impression, than the muscles of inspiration, that a convulsive continued expiration is first produced, then a sob, or deep inspiration: and lastly, strong convulsive expirations and inspirations for some successions. ”

“ However, if the informations from touch and sight disagree at any time, we are always to depend upon touch, as that which, according to the usual ways of speaking on these subjects, is the true representation of the essential properties, i.e. as the earnest and presage of what other tangible impressions the body under consideration will make upon our feeling in other circumstances ”

“ Seventhly, If a limb, that has been much chilled with cold, be brought to a fire suddenly, it will first be much pained, and then mortify. For the vibrations excited by the fire, though moderate in respect of the usual standard, are yet excessive in respect of those which the cold has introduced, also in respect of that sphere of attraction, which it has now fixed upon the parts: there will arise therefore a violent conflict, solution of continuity, and consequent pain ”

“ And it is from this difference that we call the touch the reality, light the representative: also that a person born blind may foretell with certainty, from his present tangible impressions, what others would follow upon varying the circumstances; whereas if we could suppose a person to be born without feeling, and to arrive at man’s estate, he could not from his present visible impressions judge what others would follow upon varying the circumstances. ”

“ The sensibility in the last is much greater than in the first, because the impressions can more easily penetrate through the soft epithelium, with which the internal cavities are invested, than through the hard cuticle, because the compact fibrous membrane of the true skin does not suffer the vibrations to pass freely up the nerve through its own substance, but rather diffuses them along its surface, and because the moisture of the epithelium dissolves, and thereby renders active all the saline particles, which touch the internal cavities. ”

“ But then, as, according to this definition of heat, all those objects of taste and smell, which excite strong vibratory motions in the organs, ought to excite heat, we must inquire farther into the vibratory motions of bodies termed hot in common language, and into the difference between these and the vibrations excited in the nerves of taste and smell by sapid and odorous bodies. ”

“ When a palsy arises from an internal cause, we may suppose, that the medullary part of the brain, or of the spinal marrow, or the nerve itself, in all which the vessels are extremely fine, and therefore liable to obstructions, especially in old age, become opaque, and unqualified to receive and transmit vibrations freely, according to Sir Isaac Newton’s opinion. ”

“ It is plain that the violent vibrations excited in the injured part will pass up to the brain, and over the whole muscular system, immediately; putting all the muscles into a state of contraction, as much as may be, i.e. making the strongest set of muscles everywhere overpower the weaker, for a certain time, and then give place to them for a certain other time, and so on alternately. Since therefore the muscles of expiration are stronger than those of inspiration, the air will be forced strongly out of the thorax through the larynx, and, by consequence, yield a sound. ”

“ Whence the several miniatures left by particular impressions of the same kind must strengthen one another in the internal parts, at the same time that they obliterate one another in the external ones. ”

“ The same qualities are made also by means of light to impress vibrations upon our eyes, which correspond in great measure to those made on the sense of feeling, so as to vary with their varieties. And as the sense of sight is much more extensive and expedite than feeling, we judge of tangible qualities chiefly by sight ”

“ But if a bubble be heated, and its parts brought to a larger sphere of attraction by the agitations from heat, it will no longer fall to powder when broken at its point. It may also have its parts ground away at pleasure, without falling to powder, because grinding agitates all the contiguous parts with strong vibrations, like heat, and enlarges the sphere of their attractions. ”

“ The corresponding nerves of different animals have probably a general resemblance to each other, just as the corresponding viscera and fluids have. And thus the poison of rabid animals may have a peculiar power of affecting the nerves of the fauces, and muscles of deglutition, so as to produce the hydrophobia. Cold bathing also, and music, whose immediate effects seem confined to the solids, to the exciting vibrations in them, may cure respectively in the bite of a mad dog, and of the tarantula. ”

“ Tenthly, Hot or cold water feels hotter or colder, respectively, when the hand is moved in it, than when it is kept at rest. For the hand, when at rest, has time, a little to check or exalt the vibrations in the contiguous hot or cold water. ”

“ The distortions of the face, which happen previously to crying, and during the course of it, seem to be sufficiently agreeable to the notions here advanced; the muscles, which draw the lips from each other, being much stronger, than those which close them. ”

“ Some effects of concussions of the brain, and perhaps of the spinal marrow, also of being tossed in a ship, of riding backwards in a coach, and of other violent and unusual agitations of the body, seem to bear a relation to the present subject. ”

“ Lastly, we are to conceive, that when two vibrations of different kinds, or frequences, are impressed at the same time, they must reduce one another to some single intermediate one, unless the quicker be so much more numerous than the slower, as to be comprehended within them, so that both may be performed together without opposition or confusion. ”

“ Here it may be observed, first, that the very words, burn, wound, &c. seem even in adults, though not formed into propositions, or heightened by a conjunction of circumstances, to excite, for the most part, a perception of the disagreeable kind ”

“ However, what is here alleged is not at all to be so understood, as if the immediate effects of poisons upon the fluids were not also very considerable. In some cases they may be greater, in others less, than those exerted upon the solids. It seems probable, that the poison is communicated from the fluids immediately affected to those at a distance, chiefly by means of the serous vessels. ”

“ Twelfthly, Bathing in warm water, impregnated with active mineral particles, may, by exciting and increasing vibrations in the white medullary substance, as well as by other means, remove obstructions in its small vessels, and thus be serviceable in many paralytical disorders, as it is found to be in fact. ”

“ Hence it appears, that as soon as children have learnt to cry, or yield a sound, from nascent pains, and from pleasures just passing into pains, titillation may excite short, alternate, nascent cries in them, i.e. ”