Observations on Man (6th edition)/Part I/Chapter II/Section I

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Observations on Man (6th edition) by David Hartley
Chapter II, Section I


Section I[edit]


Prop. XXIII.—To distinguish the several Kinds of Feeling from each other, and to assign the general Causes of the different Degrees of Exquisiteness in this Sense.

Here we may first distinguish feeling into the general and particular.

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; but I do not here use this word in so extensive an acceptation.

The particular feeling is that more exquisite degree which resides in the insides of the hands, and especially in the ends of the fingers; and by which we distinguish the tangible qualities of bodies, viz. heat, cold, moisture, dryness, softness, hardness, smoothness, roughness, also their motion, rest, distance, and figure, with more accuracy than by any other part. These sensations are, for the most part, adiaphorous ones.

The greater exquisiteness of the particular feeling arises probably from the following causes:

First, The sentient papillæ rise high from the skin (becoming extreme parts thereby), and receive a large proportional quantity of nerves in the ends of the fingers.

Secondly, The ends of the fingers are themselves extreme parts, and consequently receive stronger agitations in their infinitesimal medullary particles, from the stronger vibrations of the contiguous denser æther. For we must suppose, that the vibrations of the rarer æther within the nerves extend themselves a little way into the denser surrounding æther, and even become stronger to a certain distance; after which they become weaker again, and are, at last, quite suppressed by the increase of density in the æther, and by their own diffusion.

Thirdly, It is customary, in endeavouring to feel exquisitely, to rub the ends of the fingers against the tangible object. Now this friction may, by exciting vibrations, and a consequent contraction in certain muscular fibrils belonging to the papillæ, distend and erect these, and thereby increase their sensibility.

Fourthly, There is much here to be ascribed to practice and habit, i.e. to association; and it is chiefly on this account, that the sensations of the ends of the fingers give us so much more precise information concerning the tangible qualities of bodies, than those of the ends of the toes, since the structure of the nervous papillæ is alike in both. It ought to be considered as a part of this reason, that, according to the principles laid down in the last chapter, we may get a voluntary power of erecting the papillæ without friction, or of increasing and fixing the distention during friction, in order to feel with greater exquisiteness and precision.

The sense of feeling may also be distinguished into that of the external surface of the body, and that of the cavities of the nose, mouth, fauces, alimentary duct, pelvis of the kidnies, ureters, bladder of urine, gall-bladder, follicles, and ducts of the glands, &c. 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. In the mouth and nose this sensibility is so great, and attended with such distinguishing circumstances, as to have the names of taste and smell assigned respectively to the sensations impressed upon the papillæ of these two organs. And as the sensations of the alimentary duct have a near relation to, and connexion with, those of the mouth, I shall refer them to the head of taste. But the sensations of the other internal cavities may be comprehended more properly under feeling.

It ought also to be observed here, that the lips, nipples, and external parts of generation, have a more exquisite sensibility than the other external parts; partly from the structure of their papillæ, and partly from the thinness of the cutis, and softness and thinness of the cuticle. The extreme sensibility of the cornea and tunica conjunctiva of the eye may arise from the manner in which the nerves are here exposed, and the tension of these parts.

Prop. XXIV.—To examine how far the Sensations of Heat and Cold are agreeable to the Doctrine of Vibrations.

A body is termed hot, when its heat exceeds that of the part, with which we touch it; cold, when its heat is less than this. The terms hot and cold are therefore relative ones, and the qualities denoted by them run into each other without any precise distinguishing limits. We may consequently refer cold to heat, and, if we admit the doctrine of vibrations, we are to suppose, that the small parts of all bodies are agitated by subtle vibrations; and that when these vibrations exceed those of the part with which we touch them, they are called warm or hot; when they fall short, cold.

This may be regarded as a gross, general position, which presents itself upon the first consideration of this matter. 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.

I conjecture therefore, that the vibrations belonging to heat are in general quicker and shorter, than the peculiar ones excited by tastes, smells, and colours; also that the last, or the vibrations of the rays of light, are quicker than those of tastes and smells. We may conceive farther, that all the vibrations of the small particles of the medullary substance, and interjacent æther, from whatever cause they arise, grow quicker as they grow shorter, i.e. weaker; or, according to the conjecture just made, that in declining they tend to those which impress the sensation of heat. For vibratory motions of different lengths can be isochronous only according to one law, viz. that of the accelerating force being in the simple proportion of the distance from the middle point of the vibration, as when a heavy body vibrates in a cycloid; whereas, if the accelerating force be in any less ratio than this, short vibrations will be quicker than long ones. 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.

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. 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, agreeably to the hypothesis concerning pain, above proposed. For the strong vibrations excited by great heat must put the small parts beyond the spheres of each other’s attraction, and so produce the solution of continuity. But neither does it seem inconceivable, that cold may have a like effect, agreeably to the doctrine of vibrations, though the process be different. 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 conflict between the diminished vibrations, in the external part to which cold is applied, and the previous ones subsisting in the corresponding part of the brain, may exalt these previous ones, as much as heat does, so as to render the first simple impression of cold similar to that of heat.

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. This explication will perhaps suit with other theories of sensation, as well as with that drawn from vibrations. However, the mere consistency of any phænomena with the doctrine of vibrations is worthy of some attention in this inquiry.

Sixthly, When the calf of the leg is affected with the cramp, setting the foot upon a cold marble will afford immediate relief. For the cold may check the violent vibrations in the fibres of the gastrocnemii and soleus, directly and immediately; or it may do it by exciting vigorous vibrations of a different kind, which extend to their antagonist muscles, as well as to the fore-mentioned ones. But I judge the first account to be more probable.

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 the parts will be agitated so much more than their present spheres of attraction will permit, that they cannot return to it any more, but must be entirely disunited, and run into different combinations, i.e. the limb must mortify. But, if the limb be put into cold water, rubbed, and gradually exalted to the usual standard of heat, it may be preserved. Where it is to be observed, that the heat of water, while fluid, is above the freezing point, and, consequently, greater than that of a frozen limb.

It may somewhat confirm this reasoning, to give a similar explication of some of the phænomena of glass bubbles, made by dropping melted glass into water. We may suppose then, that these fall at once into powder, when broken at their points, because the cold water has so far reduced the sphere of attraction, that all the parts of the bubbles are agitated beyond this, by breaking their points. 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.

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. Hence also mortifications happening in the acute distempers of young persons, if they stop at all, stop sooner than those in the extreme parts of old persons.

Ninthly, It is said that cold water, sprinkled upon the distended limbs of malefactors upon the rack, renews and augments their pains. Now, we may here suppose, that the parts had, in some measure, begun to accommodate themselves to their distended state, by getting new and enlarged spheres of action; when therefore the cold water endeavours to contract the parts again, and to narrow the spheres of action, the limbs still continuing distended by the rack, it is evident, that a strong conflict, with violent vibrations, and the solution of continuity, must ensue. Was the limb released first, and then cold water applied, it might contribute, as in sprains, to restore the parts to their former state, without exciting any such violent conflict. The good effects of vinegar, verjuice, spirit of wine, and other contracting liquids, in sprains, are to be explained upon the same principles.

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.

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.

The good effects of cold bathing arise perhaps, in part, from its narrowing the sphere of attraction in the small parts of the muscular fibres, and at the same time making this attraction stronger. Hence it may be prejudicial in some paralytical affections, as it is found to be. For, if the small vessels of the nerves be obstructed, it may, by contracting the solids, increase the obstruction, and consequently, the impediment to the free vibrations necessary to sense and motion.

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. The same reasoning is applicable to the stiffness, insensibility, and impotency, of motion, which the rheumatism often leaves in the limbs.

Cold bathing may in like manner be serviceable in paralytic and rheumatic disorders, by exciting and increasing vibrations; provided the ill effect from the immediate contraction do not preclude this good one.

Thirteenthly, Since frictions, and other impressions upon the skin, increase the vibrations there, it may be expected, that they should increase the heat. And this is the fact. If a person rub his hands together in cold weather, the sensation of heat will be felt to arise in a moment, and to go off again in a moment after he ceases to rub; for the vibrations excited by rubbing may be expected to languish immediately, if not kept up by continuing the friction.

Fourteenthly, Strong tastes may, according to the doctrine of vibrations, be expected to leave a heat upon the tongue, mouth, and fauces, as they are found to do. And, in general, all vivid impressions upon every part of our bodies ought to increase the heat generally or particularly; which perhaps is the case, though we are seldom able to determine this by observation.

Fifteenthly, All strong emotions of mind ought also to increase the heat of the body. This is a matter of common observation, if we except the chilliness of the skin, and coldness of the extremities, which have been explained above, agreeably to the doctrine of vibrations.

The three last articles favour the above delivered conjectures concerning the peculiar nature of the vibrations belonging to heat. The phænomena enumerated in all the fifteen may admit of other explanations, at least in part, but of none, as far as I can judge, that are inconsistent with the doctrine of vibrations.

Prop. XXV.—To examine how far the Phænomena of Wounds, Burns, Bruises, Lacerations, Inflammations, and Ulcers, are agreeable to the Doctrine of Vibrations.

The manifest solution of continuity, which is the very essence of a wound, may occasion pain, agreeably to the doctrine of vibrations, in the manner that has been explained already. This is the immediate pain that attends a wound. The subsequent one is to be referred either to the head of inflammation, or to that of ulcer.

The immediate pain from burns has likewise been explained agreeably to the doctrine of vibrations, also the separation of the dead or mortified eschar, under the last proposition. The subsequent pain is to be referred to the heads of inflammation and ulcer, as before.

A bruise is supposed, and with the appearance of reason, to be an infinite number of infinitely small wounds. It ought therefore to be attended with a pain resembling that of a large wound, and yet not exactly the same, which is the fact. As large wounds are sometimes healed by the first intention, without any subsequent ulcer, so may bruises. If otherwise, the subsequent pain must again be referred to the heads of inflammation and ulcer.

Lacerations are great wounds attended with bruises, i.e. with an infinite number of infinitely small ones. These are never cured without coming to digestion, i.e. an ulcer, and the requisite previous inflammation.

The heat and distention of the small vessels in inflammations are sufficient to account for the pain attending them, upon principles already laid down.

In ulcers the nerves are exposed defenceless, and therefore are susceptible of the most violent vibrations, and consequent pain from slight impressions: to which it is to be added, that the moisture of ulcers, by dissolving the saline parts of bodies applied, greatly augments their actions upon the naked nerves.

Fomentations and cataplasms seem to afford relief in the foregoing cases, partly by diffusing an equal warmth all around, partly by their aqueous or oily moisture. For the diffusion of warmth prevents that conflict, which would arise between neighbouring parts of different heats; and the moisture, which insinuates itself among the small particles, sets them at greater distances, and consequently lessens their mutual actions. The violent vibrations will therefore be moderated on both accounts. The friction attending embrocations does in like manner diffuse vibrations all around, and the liniment or liquid, with which the embrocation is made, may contribute according to its particular qualities. Hence embrocations are also of use in resolving obstructions.

In all these cases the violent vibrations, which ascend along the nerves of the injured part, must be communicated in a particular manner to the neighbouring branches, and occasion a slight inflammation, i.e. a soreness, there. This soreness is not perceived while the original inflammation subsists, being obscured by it. The vibrations in the neighbouring nerves may also be increased by the cessation of violent ones in the place of original inflammation. Hence the soreness of the neighbouring parts after colics, head-aches, &c. The pain in the external parts of the head, which follows a debauch, i.e. an inflammation of the brain, and its membranes, may be of the same kind.

These hints may serve to shew, that the doctrine of vibrations is as agreeable to the phænomena of wounds, burns, &c. as any other yet proposed, or even more so. But much farther inquiry is requisite.

Prop. XXVI.—To examine how far the Phenomena of Itching and Titillation are agreeable to the Doctrine of Vibrations.

Itching often attends the beginning and ending of inflammations, and particularly the eruption of inflammatory pustules. We may conclude therefore, according to the foregoing account of inflammations, that itching is caused by a moderate increase of vibrations, in a part of small extent.

It is agreeable to this, that the lodgment of the perspirable matter, or other cutaneous secretion, when hardened, occasions itching; for it is easy to conceive, that in these cases, an obstruction and slight inflammation in the small vessels of the skin may arise.

Pressure, which allays itching, may be supposed to do this by checking the vibrations.

Scratching may convert it into a pleasure, by communicating the increased vibrations to the neighbouring parts, in such a degree as falls within the limits of pleasure. And as this freer communication caused by scratching may increase the vibrations in the neighbouring parts, so it may lessen those which subsisted before in the point that itched, thus reducing all to an equality, or nearly so. At the same time it appears, that rude or long-continued scratching must, by the increase of vibrations, which it occasions, also by laceration, increase the heat, inflammation, and itching, and even end in pain.

Friction of the skin, without previous itching, excites a pleasurable sensation; and also ends in inflammation and pain, when carried too far, for the like reasons.

Since extreme parts are more apt to receive an increase of vibrations than others, as has been observed; it appears, that the itching of the nose and anus may be expected to attend worms in the stomach or intestines, and the itching of the glans penis and anus a slight inflammation at the neck of the bladder from a stone. These instances shew, that vibrations run freely along the surfaces of uniform membranes; and this is farther confirmed in the last case, by the check which a pressure made in perinæo, or anywhere upon the urethra, will give to the itching in the glans.

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.

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. laughter, but not before. If the impressions be made upon the chest, the effect will be quicker and stronger, because these impressions have a direct influence upon the muscles concerned in laughter.

If a feather be passed slightly to and fro between the lips, it will excite a titillation there, which will diffuse itself into the neighbouring parts of the upper lip and chin, and pass into an itching in them. The original titillation between the lips may be allayed by pressing them together, and the consequent itching by pressure and friction, as in other cases. All these things suit with the hypothesis of vibrations, and of their free diffusion.

In like manner, the free diffusion of vibrations, and their influence in contracting the neighbouring muscles, may be inferred from the vomitings, which are excited by tickling the fauces with a feather.

Prop. XXVII.—To examine how far the Sensations attending Pressure, and muscular Contraction, are agreeable to the Doctrine of Vibrations.

As friction and titillation agree with heat in increasing vibrations, so pressure agrees with cold in checking them. Thus pressure abates the uneasiness from itching, as mentioned above; and binding, or otherwise gently compressing parts in pain, or inflamed, i.e. parts in which the vibrations are excessive, will, for the most part, afford relief; whereas friction would increase the pain to a great degree. But the pain usually becomes more violent the instant the compression is removed. For the mere elasticity of the parts alters their figure, i.e. produces internal motions, with an increase of vibrations. Pressure may also increase vibrations in the internal parts, contiguous to those where it checks them; or even in the external ones, if it be so great as to occasion any considerable distention there. And thus there may be a variety of vibrations occasioned by the several kinds and degrees of pressure, sufficient to correspond to all the variety of sensations excited thereby.

Muscular contraction most commonly attends and is attended by pressure, as in the common motions of handling and walking, whereby we overcome the vis inertiæ of our own bodies, and of those which we have occasion to move or stop. Hence all the sensations which we receive from the vis inertiæ of matter, must be derived from these two sources of muscular contraction and pressure.

Now it has been observed already, that muscular contraction checks the vibrations in the contracted fibres, and increases them in the neighbouring parts. And it is easy to conceive, that the sensation corresponding to this alteration of vibrations may sometimes fall within the limits of pleasure, sometimes go beyond them. In young animals, also after sleep and rest in all, it is usually pleasant; after much labour, or sprains, and in inflammations, painful; and this, whether the disordered muscle itself, or its antagonist, be contracted. For there must be an increase of vibrations in the disordered muscle both before it can be itself contracted, and also in consequence of the contraction of its antagonist; as has been shewn before.

Prop. XXVIII.—To examine how far the Phænomena of Numbnesses, and paralytical Insensibilities, are agreeable to the Doctrine of Vibrations.

Numbness, being a diminution of sensibility, ought, according to the doctrine of vibrations, to proceed from such causes, as either indispose the parts for the reception of vivid vibrations, or hinder their free ascent to the brain. Agreeably to this, a compression made upon the nerve, which leads to any part, will occasion a numbness in that part, the nerve below the compression being unfitted thereby to receive vibrations freely, and the nerve above incapable of transmitting freely such as are excited. A compression of a blood-vessel may have a like effect, because it must lessen that heat, and intestine motion, which a free circulation would communicate to the part. The compression usually made upon the skin, when we press a nerve or blood-vessel, will also contribute. And external cold will hasten the effect, when joined to the just-named causes; or produce it alone, if intense, or long continued. In like manner, numbness, from a compression made upon the nerves or blood-vessels, is much favoured by sleep, because the parts are then indisposed both to receive and to transmit vibrations.

The benumbed limb feels larger, because any gross body, which encompasses and presses upon a limb by its weight or stricture, deadens the vibrations in it; and therefore conversely, when the vibrations are so deadened from a different cause, the idea of a gross encompassing body, or, which is almost the same thing, of the enlargement of the limb, will be suggested to the mind. But this circumstance must be referred to the head of association.

When the benumbed part begins to recover its feeling, violent prickings are often perceived. Now these seem to take place in the points where the natural vibrations first return; suppose at the ends of the nervous papillæ, and to arise from the conflict between the natural vibrations in these points, and the languid ones in the neighbouring parts. However, they come to an equality at last, by their mutual influences, as well as by the return of the natural vibrations to all the parts; which may serve to shew how itching ceases at last of itself. Friction helps to disperse and remove these prickings, and to restore the lost sensibility, which is very suitable to the notion of vibrations, and to the effect which it has in itchings.

If the hand be held down, and shaken, its muscles being first relaxed by a voluntary power, a numbness will be occasioned, in which the fingers feel large, for the reason given above. This numbness seems to arise from the irregular agitations, or vibrations, excited in the small parts; which, being different from the natural ones, or those in which sensibility consists, must check them; just as the agitations of water from the wind hinder the free propagation of regular undulations from a stone cast into it; or as any commotion of the air checks the free and distinct communication of a sound. It seems also, that those irregular and dissonant vibrations, which shaking the hand causes in the small medullary particles of its nerves, may pass on from part to part, though not so freely as regular ones.

From hence we may proceed to consider the numbness occasioned by the stroke of the torpedo. For the oscillations of this fish’s back may neither be isochronous in themselves, nor suitable to those which existed previously in the hand; and yet they may be so strong as not only to check and overpower those in the part which touches the fish, but also to propagate themselves along the skin, and up the nerves, to the brachial ganglion, and even to the spinal marrow and brain; whence the person would first feel the stupefaction ascend along the arm to the shoulder, and then fall into a giddiness, and general confusion, as is affirmed to happen sometimes. 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. But it would be too minute to pursue these things.

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. Hence a diminution or entire loss of sense or motion, or both, may follow, according to the degree and extent of the obstruction and opacity. The voluntary power of motion is soon lost, as being an acquired faculty, and depending upon associated circumstances, and memory. But if there be any degree of inflammation in the fine vessels of the motory nerves, or of the corresponding parts of the brain, this may occasion convulsive motions; and, for the same reason, an inflammation in the sensory nerves, or their origins in the brain, may occasion pain. Now it is reasonable to expect such inflammations in many cases as a consequence of the obstructions, and both convulsive motions and pains are often found to attend paralytic affections.

Prop. XXIX.—To examine how far the Phænomena attending on venomous Bites and Stings are agreeable to the Doctrine of Vibrations.

That the active liquors, infused by venomous animals after they bite or sting, operate, in part, by the violent vibrations which they excite in the living parts immediately affected, and which are thence propagated along the nerves up to the brain, and also along the surface of the body, by means of the continuity and uniformity of the skin, may appear from the following reasons:

First, As the solids and fluids seem, in general, equally concerned in all the natural functions, and morbid deviations from them, it is most reasonable to refer part of the effects of venomous bites and stings to each. But it is difficult to conceive how these poisons should have any immediate effect upon the solid nervous capillaments, but by agitating their parts.

Secondly, The active particles of these poisons, which are able, in so small a quantity, to produce such violent disorders, and sometimes in a very short space of time, may well be conceived able also to agitate the nervous parts with strong vibrations.

Thirdly, If we suppose their first and most immediate effect to be upon the fluids, yet this may, or must, be agitations, that will afterwards be communicated to the solids.

Fourthly, The vibrations of the medullary particles, mentioned in this theory, seem peculiarly suited to answer the several quick and surprising effects of these poisons. The pain, swelling, redness, and lividness, all around the part affected, may easily be derived from the vibrations propagated all around. Oils and fats, rubbed upon the part, may, by damping these, prevent the ill effects. Vibrations, propagated either along the skin, through the mouth and nose, or up to the brain, and thence along the eighth pair of nerves, or, which is most probable, both ways, to that very sensible part the stomach, may produce sickness and vomitings. And if the gall-duct be contracted from the same cause during the vomitings, a sudden jaundice will follow from the violence with which the gall is forced back into the blood by the action of vomiting. Joy, sorrow, fear, melancholy, may easily follow according to the respective natures of the poisons, because these, according to this theory, all arise from, and are attended by, corresponding vibrations in the white medullary substance of the brain. And a like account may be given of the aversion to black, and the delight in glaring objects, and strong colours. 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.

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. For these, having numerous immediate communications with each other, will transmit it freely, and yet so as that all the neighbouring parts may be affected somewhat in proportion to their nearness to the seat of the injury, as they are found in fact to be; whereas, were the diffusion of the poison to be made by the circulation of the fluids alone, all the parts would have an equal chance. But the propagation of the poison along the solid capillaments of the nerves is also a principal reason why the neighbouring parts are more affected than the distant ones. The effects of inoculation bear a great resemblance to those of venomous bites and stings; and the same may be said of venereal and other infections.

Prop. XXX.—To examine how far the tangible Qualities of Bodies admit of an Explanation agreeable to the Doctrine of Vibrations.

Hitherto we have considered only the more vigorous sensations of feeling, such as may be called the pleasures and pains of this sense. We come now to the feeble and adiaphorous sensations. These are moisture, dryness, softness, with fluidity, hardness, smoothness, roughness, motion, rest, distance, and figure. Now it seems very easy to conceive, that these, with their several varieties, may impress corresponding varieties of vibrations upon the nerves of feeling; also, that these last varieties will be chiefly compositions of the vibrations arising from pressure, and muscular contraction, i.e. from the vis inertiæ of matter.

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; rough ones the contrary. The motions of our own bodies are attended by the vibrations peculiar to pressure, and muscular contraction; of other bodies which touch our own, by those from pressure. We judge of rest by the absence of these. Distance is judged of by the quantity of motion, and figure by the relative quantity of distance. And thus it appears, that all degrees and kinds of these tangible qualities may impress corresponding vibrations upon those regions of white medullary substance of the brain, and spinal marrow, which correspond to the skin and muscles.

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; which therefore may be considered, agreeably to Bishop Berkley’s remark, as a philosophical language for the ideas of feeling; being, for the most part, an adequate representative of them, and a language common to all mankind, and in which they all agree very nearly, after a moderate degree of experience.

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; also what changes it will produce in other bodies; of which again we are to determine by our feeling, if the visual language should not happen to correspond to it exactly. 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. 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. But the touch is not liable to these deceptions. As it is therefore the fundamental source of information in respect of the essential properties of matter, it may be considered as our first and principal key to the knowledge of the external world.

Prop. XXXI.—To explain in what Manner we are enabled to judge of the Seat of Impressions made on the external Surface of our Bodies.

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. And this difference in the last impression or its vibrations, corresponding always to the part on which the impression is made, will at last enable us to determine immediately what part of our bodies we touch; i.e. what is the distance of the part touched from the mouth, nose, shoulder, elbow, or other remarkable part, considered as a fixed point. For by passing frequently from the mouth, nose, &c. to the part under consideration, children learn this very early, even without attending to it at all explicitly.

Sight also helps us to judge of this distance in the parts, which are frequently exposed to view, and this in proportion to that frequency.

Let us suppose then, that we are able to determine at once what external part of our bodies we touch, i.e. to determine how it is situated in respect of the other parts, and to shew the corresponding part in the body of another person; it will follow, that if a like impression be made not by our own hand, but by that of another, or by any foreign body, we shall know at once the part on which it is made. We shall also, supposing us arrived at a sufficient degree of voluntary power over the muscles, be able at once to put our hand upon the part on which the impression is made.

By degrees we shall learn to distinguish the part, not only when an impression like the gentle ones of our hands is made upon it, but also when a vivid, rude, or painful one is. For, first, all impressions made upon the same part agree in this, whatever be their differences as to kind and degree, that they enter by the same nerves, and at the same part of the brain, and spinal marrow. 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.

Now from all these things laid together it follows, that in itchings from an internal cause, and in impressions where neither our hand or eye give us any information, we shall, however, be able to determine at once with tolerable accuracy what external part is affected, and to put our hand upon it, so as to confirm our present judgment, and render our future judgment, and voluntary power, more certain and ready. We shall also do this most readily in those parts which we see and feel most frequently, the hands for instance; less so, cæteris paribus, in those we seldom see or feel; and least so, where we never see the part, and seldom touch it. At least this seems to result from the theory. But it is to be observed, that the fact ought to be tried chiefly in children. For in adults the several degrees approach more to perfection, i.e. to an equality among themselves.

Prop. XXXII.—To explain in what Manner, and to what Degree, we are enabled to judge of the Seat of Internal Pains.

Here we may observe, first, that as we never see or feel the internal parts, such as the lungs, heart, stomach, intestines, liver, kidneys, bladder, &c. we can have no direct information in the manner explained under the foregoing proposition.

Secondly, Since all pains diffuse an increase of vibrations into the neighbouring parts, the increased vibrations in the external parts, arising from internal pains, will be a gross general direction, so as to determine the seat of the pain within gross limits, in respect of superior and inferior, anterior and posterior, right and left.

Thirdly, Pressing the external parts, so as to augment or alleviate the internal pain, must contribute also.

Fourthly, Since all the internal parts in the thorax and abdomen receive branches from the intercostal nerve, which communicates with each vertebral pair, it follows that the internal pains will send vibrations up to the spinal marrow, which will enter in at the same parts of it, as the vibrations from external pains in the neighbourhood. At the same time it appears from the many ganglions, plexuses, and communications of nerves, in the thorax and abdomen, also from the origin and distribution of the nerves of the cauda equina, that this can be no more than a gross general direction; and that the great number of sympathetic influences from these causes, also from the running of vibrations along membranes, and from their fixing particularly in nervous parts or extreme ones, will give occasion to many deceptions here, and in certain cases make the pain be felt, i.e. appear to be, in parts at a considerable distance from the seat of the disorder.

Fifthly, Suppose the patient to shew by the external parts whereabouts his pain is felt internally, then the physician may, from his knowledge of the situation of the internal parts in respect of the external, guess pretty nearly what internal part is affected.

Sixthly, The symptoms attending the pain, its cause and consequences, compared with the natural functions of the parts, with the history of diseases, and morbid dissections, will enable the physician to determine with great precision in some cases, and help a little in most.

Seventhly, When the patient has had long experience of the same kind of internal pains, or of different ones, he describes more exactly, and also gets certain fixed points, to which he refers his pains.

Eighthly, Anatomists and physicians may sometimes judge with great exactness in their own cases, having both a knowledge of the parts, and their functions, and also their own feelings to guide them.

This subject deserves a particular and accurate examination, it being of great consequence to be able to discover the seat and causa proxima of the distemper, from the complaints of the patient, and from the previous, concomitant, and consequent circumstances. I hope these two propositions may cast some light upon it.

Here we may add an observation deducible from the doctrine of association; viz. As we learn by degrees, from impressions made on the surfaces of our bodies, to attend particularly to the sensations impressed on, or existing in, each part, at pleasure, i.e. to magnify the vibrations which take place in it; so after disorders in the internal parts, the associated circumstances seem often to renew the painful vibrations there, and to occasion either the return of the like disorder, or some other; at least to have a considerable share in these effects, when produced by their causes in an inferior degree. Thus disorders in the bowels, caused at first by acrid impressions, lay the foundation for a return of like disorders on less occasions. Thus women that have often miscarried, seem to irritate the muscular fibres of the uterus by the recollection of the associated circumstances, and so to dispose themselves to miscarry more than according to the mere bodily tendency; fear and concern having also a great influence here. All this will be farther illustrated by what follows under the next proposition.

Prop. XXXIII.—To explain in what Manner, and to what Degree, the Pleasures and Pains of Feeling contribute, according to the Doctrine of Association, to the Formation of our intellectual Pleasures and Pains.

It follows from the foregoing account of the power of leaving traces and of association, that all the pains from intense heat and cold, wounds, inflammations, &c. will leave a disposition in the nervous system to run into miniature vibrations of the same kind, and that these miniature vibrations will be excited chiefly by the associated circumstances. That is to say, the appearance of the fire, or of a knife, especially in circumstances like to those in which the child was burnt or cut, will raise up in the child’s nervous system painful vibrations of the same kind with, but less in degree than, those which the actual burn or wound occasioned.

By degrees these miniature pains will be transferred upon the words, and other symbols, which denote these and such-like objects and circumstances: however, as the diffusion is greater, the pain transferred from a single cause must become less. But then, since a great variety of particular miniatures are transferred upon each word, since also the words expressing the several pains of feeling, affect each other by various associations, and each of them transfers a miniature of its own miniature upon more general words, &c. it comes to pass at last, that the various verbal and other symbols of the pains of feeling, also of other pains bodily and mental, excite a compound vibration formed from a variety of miniatures, which exceeds ordinary actual pains in strength. These compound vibrations will also have a general resemblance, and particular differences in respect of each other.

It follows therefore a priori, as one may say, and by a synthetic kind of demonstration, that admitting the powers of leaving traces, and of association, compound or mental pains will arise from simple bodily ones by means of words, symbols, and associated circumstances. And they seem to me to answer in kind and degree to the facts in general. If, farther, we admit the doctrine of vibrations, then these compound mental pains will arise from, or be attended by, violent vibrations in the nervous system, and particularly in the brain.

Agreeably to this account, we may observe that the mere words denoting bodily pains, though not formed into propositions or threatenings, affect children. However, since there happen daily associations of the mere words with freedom and security, and of propositions and threatenings with sufferings, children learn by degrees to confine their fear, sorrow, &c. to those things which are esteemed the genuine signs, reasons, causes, &c. of sufferings. This is the case in general; but there are great particular differences both in children and adults; which yet, if accurately pursued, would probably not only be consistent with, but even confirm and illustrate, the doctrine of association.

And we may conclude upon the whole, since the pains of feeling are far more numerous and violent than those of all the other senses put together, that the greatest part of our intellectual pains are deducible from them.

In like manner the pleasures of an agreeable warmth, and refreshing coolness, when we are cold or hot respectively, of gentle friction and titillation, leave traces of themselves, which by association are made to depend upon words, and other symbols. But these pleasures, being faint and rare in comparison of others, particularly of those of taste, have but a small share in forming the intellectual pleasures. Titillation may perhaps be excepted. For laughter, which arises from it, is a principal pleasure in young children, and a principal source of the other pleasures, particularly of those of sociality and benevolence. Farther, since the miniatures left by the pains of feeling must in some cases be faint originally, in others decline from the diffusion, the faintness of the association, &c. these miniature pains will often fall within the limits of pleasure, and consequently become sources of intellectual pleasure; as in recollecting certain pains, in seeing battles, storms, wild beasts, or their pictures, or reading descriptions of them.

Prop. XXXIV.—To give an Account of the Ideas generated by tangible Impressions.

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, so faint in degree, that it may be reckoned amongst the number of ideas, agreeably to the definitions given in the Introduction.

Secondly, The words expressing the pleasures of this sense are probably attended with perceptions, though still fainter in degree. These perceptions may therefore be called the ideas belonging to those words.

Thirdly, The words moist, dry, soft, hard, smooth, rough, can scarce be attended with any distinguishable vibrations in the fingers, or parts of the brain corresponding thereto, on account of the faintness of the original impressions, and the great varieties of them; however, analogy leads us to think, that something of this kind must happen in a low degree. But when the qualities themselves are felt, and the appropriated vibrations raised, they lead by association to the words expressing them; and thus we can distinguish the several tangible qualities from each other by the differences of their vibrations, and declare in words what each is.

Fourthly, The vibrations excited in the sense of feeling by motion, distance, and figure, are so faint, and so various, that neither these words, nor any related expressions, can be supposed to excite any miniature vibrations in this sense. Yet still upon feeling motions and figures, and passing over distances, the differences of vibrations from pressure and muscular contraction, i.e. from the vis inertiæ of our own bodies, or of foreign matter, suggest to us the words expressing these, with their varieties, by association.

Fifthly, The great extent of the sense of feeling tends to make the miniatures fainter, especially as far as the external parts are concerned; and would probably have so powerful an effect upon the miniatures raised in the internal parts, as to make them by opposing destroy one another, did not all the impressions of the same nature, viz. all those from heat, from cold, from friction, &c. by whatever external part they enter, produce nearly the same effect upon the brain. 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. However, where a person has suffered much by a particular wound, ulcer, &c. it seems, according to the theory, that an idea of it should be left in the part affected, or corresponding region of the brain, or spinal marrow.

Sixthly, The visible ideas of the bodies which impress the several sensations of feeling upon us, are, like all other visible ones, so vivid and definite, that they mix themselves with, and somewhat obscure, the most vivid ideas of feeling, and quite overpower the faint ones. Sight communicates to us at once the size, shape, and colour of objects; feeling cannot do the last at all, and the two first only in a tedious way; and is scarce ever employed for that purpose by those who see. Hence persons born blind must have far more vivid and definite ideas of feeling than others. An inquiry into their real experiences would greatly contribute to correct, illustrate, and improve, the theory of ideas, and their associations.

Prop. XXXV.—To explain the automatic Motions, which arise from tangible Impressions.

The principal of these is the action of crying, which is in all animals, but especially young ones, the natural and necessary consequence of pain. I have already given some account of this action; but will here enter into a more particular detail of the circumstances, and their agreement with the foregoing theory.

Let us suppose then a young child to have a very painful impression made upon the skin, as by a burn. 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. It contributes to this, that the muscles of the os hyoides and larynx, acting all together, and drawing different ways, must suspend the cartilages of the larynx, so as both to narrow the passage of the air, and also render these cartilages more susceptible of vibrations. As to the muscles which contract and dilate the larynx, they are perhaps about equal in strength to each other, and therefore may, by opposing each other, keep the chink in a state intermediate between its least and greatest dilatation.

That the strongest set of antagonist muscles overpower the weaker, during the great effort in crying, may farther appear from the action of the extensors of the neck, and flexors of the hand (both which sets are stronger than their antagonists), at that time.

If it be objected here, that the elevators of the lower jaw, being stronger than the depressors, ought to keep the mouth shut during the action of crying, according to the foregoing reasoning, whereas the contrary always happens; I answer, first, that when both these sets of muscles act at the same time, in proportion to their natural strength, the mouth ought to be a little opened; secondly, that the vibrations which take place in the cartilages of the larynx seem to impart a peculiar degree of force to all the neighbouring muscles, i.e. to the depressors of the lower jaw; and, thirdly, that the muscles which pass from the larynx and os hyoides to the lower jaw act to an advantage in drawing it down, in the present case, because the os hyoides is at this time fixed by its other muscles.

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.

The manner in which titillation occasions laughter in its automatic state, has been already explained. We may add here, that touching the cheeks of young children gently will excite smiling.

Friction also occasions many automatic motions in young children, as may be observed when their naked bodies, or hairy scalps, are rubbed by the nurse’s hand; the motion being determined in these cases, as appears, by the strength and vicinity of the muscles.

The contraction of the hand in young children, which has been taken notice of already, may be excited by titillation, friction, and almost any impression on the palm; and is to be deduced partly from the superior strength of the flexors here, partly from the exquisite sensibility of the palm. The contraction of the foot from impressions made on the sole is analogous to that of the hand. It may not perhaps be amiss to add here, that the cellular substance intervenes less between the skin and subjacent muscles in the scalp, palm, and sole, than in other parts, as appears both from anatomical inspection and emphysemas; and that this may increase the influence of the impressions on these parts over the subjacent muscles.

Prop. XXXVI.—To explain the Manner in which the automatic Motions, mentioned in the last Proposition, are converted into voluntary and semi-voluntary ones.

This has been done, in some measure, already, in respect of the actions of crying, and contracting the hand, and their derivatives, speaking and handling; and will be done more completely hereafter in a proposition appropriated to the distinct consideration of the motions that are perfectly voluntary. I have therefore inserted the present proposition chiefly for the sake of regularity, and that the reader might have in one short view, from the propositions of this section, all the principal heads of inquiry relating to the sense of feeling.

It may not, however, be improper here to observe, that the great variety of frictions, flexures, and positions, which nurses give to young children, make a proportional variety of combinations of muscles which act together; and that these, by opposing the natural ones from juxtaposition, derivation of nerves, &c. to a certain degree, prepare the way for such voluntary combinations as are requisite in the future incidents of life.