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

THE DOCTRINES OF VIBRATIONS AND ASSOCIATION IN GENERAL.
CHAP. I.
THE GENERAL LAWS ACCORDING TO WHICH THE SENSATIONS AND MOTIONS ARE PERFORMED, AND OUR IDEAS GENERATED.
MY chief design in the following chapter is briefly to explain, establish, and apply the doctrines of vibrations and association. The first of these doctrines is taken from the hints concerning the performance of sensation and motion, which Sir Isaac Newton has given at the end of his Principia, and in the Questions annexed to his Optics; the last, from what Mr. Locke, and other ingenious persons since his time, have delivered concerning the influence of association over our opinions and affections, and its use in explaining those things in an accurate and precise way, which are commonly referred to the power of habit and custom, in a general and indeterminate one.
The doctrine of vibrations may appear at first sight to have no connexion with that of association; however, if these doctrines be found in fact to contain the laws of the bodily and mental powers respectively, they must be related to each other, since the body and mind are. One may expect, that vibrations should infer association as their effect, and association point to vibrations as its cause. I will endeavour, in the present chapter, to trace out this mutual relation.
The proper method of philosophizing seems to be, to discover and establish the general laws of action, affecting the subject under consideration, from certain select, well-defined, and well-attested phænomena, and then to explain and predict the other ​phænomena by these laws. This is the method of analysis and synthesis recommended and followed by Sir Isaac Newton.
I shall not be able to execute, with any accuracy, what the reader might expect of this kind, in respect of the doctrines of vibrations and association, and their general laws, on account of the great intricacy, extensiveness, and novelty of the subject. However, I will attempt a sketch in the best manner I can, for the service of future inquirers.

### Section ISection I

THE DOCTRINE OF VIBRATIONS, AND ITS USE FOR EXPLAINING THE SENSATIONS.
Under the word brain, in these observations, I comprehend all that lies within the cavity of the skull, i.e. the cerebrum, or brain properly so called, the cerebellum, and the medulla oblongata.
This proposition seems to be sufficiently proved in the writings of physicians and anatomists; from the structure and functions of the several organs of the human body; from experiments on living animals; from the symptoms of diseases, and from dissections of morbid bodies. Sensibility, and the power of motion, seem to be conveyed to all the parts, in their natural state, from the brain and spinal marrow, along the nerves. These arise from the medullary, not the cortical part, every where, and are themselves of a white medullary substance. When the nerves of any part are cut, tied, or compressed in any considerable degree, the functions of that part are either entirely destroyed, or much impaired. When the spinal marrow is compressed by a dislocation of the vertebræ of the back, all the parts, whose nerves arise below the place of dislocation, become paralytic. When any considerable injury is done to the medullary substance of the brain, sensation, voluntary motion, memory, and intellect, are either entirely lost, or much impaired; and if the injury be very great, this extends immediately to the vital motions also, viz. to those of the heart, and organs of respiration, so as to occasion death. But this does not hold equally in respect of the cortical substance of the brain; perhaps not at all, unless as far as injuries done to it extend themselves to the medullary substance. In dissections after apoplexies, palsies, epilepsies, and other ​distempers affecting the sensations and motions, it is usual to find some great disorder in the brain, from preternatural tumours, from blood, matter, or serum, lying upon the brain, or in its ventricles, &c. This may suffice as general evidence for the present. The particular reasons of some of these phænomena, with more definite evidences, will offer themselves in the course of these observations.
The evidence for this proposition is also to be taken from the writings of physicians and anatomists; but especially from those parts of these writings which treat of the faculties of memory, attention, imagination, &c. and of mental disorders. It is sufficiently manifest from hence, that the perfection of our mental faculties depends upon the perfection of this substance; that all injuries done to it affect the trains of ideas proportionably; and that these cannot be restored to their natural course till such injuries be repaired. Poisons, spirituous liquors, opiates, fevers, blows upon the head, &c. all plainly affect the mind, by first disordering the medullary substance. And evacuations, rest, medicines, time, &c. as plainly restore the mind to its former state, by reversing the foregoing steps. But there will be more and more definite evidence offered in the course of these observations.
This is very evident in the sensations impressed on the eye. Thus, to use Sir Isaac Newton’s words, “If a burning coal be nimbly moved round in a circle, with gyrations continually repeated, the whole circle will appear like fire; the reason of which is, that the sensation of the coal, in the several places of that circle, remains impressed on the sensorium until the coal return again to the same place. And so in a quick consecution of the colours,” (viz. red, yellow, green, blue, and purple, mentioned in the experiment, whence this passage is taken,) “the impression of every colour remains on the sensorium until a revolution of all the colours be completed, and that first colour return again. The impressions therefore of all the successive colours are at once in the sensorium—and beget a sensation of white.” Opt. b. I. p. 2. Experiment 10.
Thus also, when a person has had a candle, a window, or any other lucid and well-defined object, before his eyes for a ​considerable time, he may perceive a very clear and precise image thereof to be left in the sensorium, fancy, or mind (for these I consider as equivalent expressions in our entrance upon these disquisitions,) for some time after he has closed his eyes. At least this will happen frequently to persons who are attentive to these things in a gentle way; for, as this appearance escapes the notice of those who are entirely inattentive, so too earnest a desire and attention prevents it, by introducing another state of mind or fancy.
To these may be referred the appearance mentioned by Sir Isaac Newton, Opt. Qu. 16. viz. “When a man in the dark presses either corner of his eye with his finger, and turns his eye away from his finger, he will see a circle of colours like those in the feather of a peacock’s tail. And this appearance continues about a second of time after the eye and finger have remained quiet.” The sensation continues therefore in the mind about a second of time after its cause ceases to act.
The same continuance of the sensations is also evident in the ear. For the sounds which we hear are reflected by the neighbouring bodies, and therefore consist of a variety of sounds, succeeding each other at different distances of time, according to the distances of the several reflecting bodies; which yet causes no confusion or apparent complexity of sound, unless the distance of the reflecting bodies be very considerable, as in spacious buildings. Much less are we able to distinguish the successive pulses of the air, even in the gravest sounds.
As to the senses of taste and smell, there seems to be no clear direct evidence for the continuance of their sensations after the proper objects are removed. But analogy would incline one to believe, that they must resemble the senses of sight and hearing in this particular, though the continuance cannot be perceived distinctly, on account of the shortness of it, or other circumstances. For the sensations must be supposed to bear such an analogy to each other, and so to depend in common upon the brain, that all evidences for the continuance of sensations in any one sense, will extend themselves to the rest. Thus all the senses may be considered as so many kinds of feeling; the taste is nearly allied to the feeling, the smell to the taste, and the sight and hearing to each other. All which analogies will offer themselves to view when we come to examine each of these senses in particular.
In the sense of feeling, the continuance of heat, after the heating body is removed, and that of the smart of a wound, after the instant of infliction, seem to be of the same kind with the appearances taken notice of in the eye and ear.
But the greatest part of the sensations of this sense resemble those of taste and smell, and vanish to appearance as soon as the objects are removed.
These vibrations are motions backwards and forwards of the small particles; of the same kind with the oscillations of pendulums, and the tremblings of the particles of sounding bodies. They must be conceived to be exceedingly short and small, so as not to have the least efficacy to disturb or move the whole bodies of the nerves or brain. For that the nerves themselves should vibrate like musical strings, is highly absurd; nor was it ever asserted by Sir Isaac Newton, or any of those who have embraced his notion of the performance of sensation and motion, by means of vibrations.
In like manner we are to suppose the particles which vibrate, to be of the inferior orders, and not those biggest particles, on which the operations in chemistry, and the colours of natural bodies, depend, according to the opinion of Sir Isaac Newton. Hence, in the proposition, I term the medullary particles, which vibrate, infinitesimal.
Now that external objects impress vibratory motions upon the medullary substance of the nerves and brain (which is the immediate instrument of sensation, according to the first proposition) appears from the continuance of the sensations mentioned in the third; since no motion, besides a vibratory one, can reside in any part for the least moment of time. External objects, being corporeal, can act upon the nerves and brain, which are also corporeal, by nothing but impressing motion on them. A vibrating motion may continue for a short time in the small medullary particles of the nerves and brain, without disturbing them, and after a short time would cease; and so would correspond to the above-mentioned short continuance of the sensations; and there seems to be no other species of motion that can correspond thereto.
Cor. As this proposition is deduced from the foregoing, so if it could be established upon independent principles, (of which I shall treat under the next,) the foregoing might be deduced from it. And on this supposition there would be an argument for the continuance of the sensations, after the removal of their objects; which would extend to the senses of feeling, taste, and smell, in the same manner as to those of sight and hearing.
This proposition is chiefly an evidence and explanation of the foregoing; and accordingly might have been included in it. ​However, as it is of great importance in the present subject, I thought it best to give it a distinct place and consideration.
Before I enter upon the proof of it, it will be proper to premise something by way of explanation, concerning the æther, and the qualities of the medullary substance just mentioned.
Sir Isaac Newton supposes, that a very subtle and elastic fluid, which he calls æther, for the sake of treating upon it commodiously under an appropriated name, is diffused through the pores of gross bodies, as well as through the open spaces that are void of gross matter. He supposes likewise, that it is rarer in the pores of bodies than in open spaces, and even rarer in small pores and dense bodies, than in large pores and rare bodies; and also that its density increases in receding from gross matter; so, for instance, as to be greater at the hundredth of an inch from the surface of any body than at its surface; and so on. To the action of this æther he ascribes the attractions of gravitation and cohesion, the attractions and repulsions of electrical bodies, the mutual influences of bodies and light upon each other, the effects and communication of heat, and the performance of animal sensation and motion. My business in these observations is only with the last; but the reader will do well to consult what Sir Isaac Newton has himself advanced concerning the existence of this æther, and the properties and powers which he has ascribed to it in the last paragraph of his Principia, the Questions annexed to his Optics, and a Letter from him to Mr. Boyle, lately published in Mr. Boyle’s Life. As to myself, I am not satisfied that I understand him perfectly on this subject. I will hint a few things partly from him, partly from my own reflections, concerning the existence and properties of this æther.
Since a thermometer kept in vacuo varies with the heat and cold of the room in which it is placed, as much as another surrounded by air; and since the small parts of hot bodies probably vibrate to and fro, and by thus vibrating keep up the heat for a certain time; one may conjecture that a subtle medium remains after the air is exhausted, and that heat is communicated to the thermometer suspended in vacuo, by the vibrating motions of this medium. See Opt. Qu. 18.
The greater density of the æther at a distance from bodies than at their surface, may be conjectured from the various phænomena solved by this supposition; which phænomena may also be alleged as probable evidences of the existence of the æther. See Opt. Qu. and the Letter to Mr. Boyle.
The great subtlety and elasticity of the æther may be inferred from the motions of the planets and quick propagation of light, if we first suppose its existence, and concurrence in the propagation of light, and efficacy in causing gravity. And from its great elasticity we may infer, that it is extremely susceptible of vibrations and pulses, in the same manner as common air. See Opt. Qu.
Since the gross bodies that lie upon the surface of the earth ​emit air-particles, constituting a thin, elastic fluid, of great efficacy in performing the ordinary operations of nature, it seems not unnatural to expect, that the small particles of bodies should emit a proportionally attenuated air; i.e. an æther which may likewise have a great share in the subtle actions of the small particles of bodies over each other. The emission of odoriferous particles, light, magnetical and electrical effluvia, may also be some presumption in favour of the existence of the æther. Moreover, it is reasonable to expect that it should have a repulsive force in respect of the bodies which emit it; and for the same reasons its particles may repel each other. It may therefore be elastic, compressible, and apt to receive vibrations from the last cause; and from the first may be rarer within the pores of bodies than in large open spaces, and grow denser as the distance from gross matter increases. Our air is indeed denser near the earth than in the higher regions; but this is owing to its gravity prevailing against its expansive force. If we suppose the gravity of the æther to be very small, and its elasticity or expansive and repulsive force very great, both which must be supposed, if we admit it at all in the manner proposed by Sir Isaac Newton, its density may increase in receding from gross matter, and be much less in the pores of bodies than in open spaces void of gross matter. Thus we may suppose even the air, which remains in the large pores of such bodies as repel its particles, to be rarer than the common external air.
Lastly, let us suppose the existence of the æther, with these its properties, to be destitute of all direct evidence, still if it serve to explain and account for a great variety of phænomena, it will have an indirect evidence in its favour by these means. Thus we admit the key of a cypher to be a true one, when it explains the cypher completely; and the decypherer judges himself to approach to the true key, in proportion as he advances in the explanation of the cypher; and this without any direct evidence at all. And as the false and imperfect keys, which turn up to the decypherer in his researches, prepare the way for the discovery of the true and complete one, so any hypothesis that has so much plausibility as to explain a considerable number of facts, helps us to digest these facts in proper order, to bring new ones to light, and to make experimenta crucis for the sake of future inquirers. The rule of false affords an obvious and strong instance of the possibility of being led, with precision and certainty, to a true conclusion from a false position; and it is of the very essence of algebra to proceed in the way of supposition.
We come next to consider the uniformity and continuity of the white medullary substance of the brain, spinal marrow, and nerves. Now these are evident to the eye, as far as that can be a judge of them. The white medullary substance appears to be every where uniform and similar to itself throughout the whole brain, spinal marrow, and nerves; and though the cortical ​substance be mixed with the medullary in the brain, and spinal marrow, and perhaps in the ganglions and plexuses, yet it does not appear that the communication of any one part of the medullary substance with every other, is cut off any where by the intervention of the cortical. There is no part of the medullary substance separated from the rest, but all make one continuous white body; so that if we suppose vibrations apt to run freely along this body from its uniformity, they must pervade the whole, in whatever part they are first excited, from its continuity.
The excessive minuteness of the vessels of which the medullary substance consists, may also be conceived as inferring its uniformity and continuity. These vessels are, by all anatomists and physiologists, supposed to arise from those of the cortical substance, this being agreeable to the analogy of the other parts of the body. And it follows from the same analogy, that they must be smaller than those vessels from which they arise. But the finer orders of the vessels of the cortical substance are far too minute to admit of the most subtle injections, the best injectors having never penetrated farther than the grosser orders of vessels in the cortical substance. We may therefore well suppose, that the medullary substance consists of a texture of vessels so small and regular, as that it may have no vacuity or interval in it, sufficient to interrupt or disturb the vibrations of the æther, and concomitant ones of the small medullary particles, propagated through this substance in the manner to be described below.
The softness of the medullary substance is, in like manner, evident to the senses, and the natural consequence of the extreme smallness of the compounding vessels, and fluids circulating through them.
If we admit the foregoing account of the uniform continuous texture of the medullary substance, it will follow that the nerves are rather solid capillaments, according to Sir Isaac Newton, than small tubuli, according to Boerhaave. And the same conclusion arises from admitting the doctrine of vibrations. The vibrations hereafter to be described, may more easily be conceived to be propagated along solid capillaments, so uniform in their texture as to be pellucid when singly taken, than along hollow tubuli. For the same reasons, the doctrine of vibrations will scarce permit us to suppose the brain to be a gland properly so called; since the deformity of texture required in a gland appears inconsistent with the free propagation of vibrations. Neither can we conclude the brain to be a gland, from the great quantity of blood sent to it by the heart. It is probable, indeed, that this is required on account of the important functions of accretion, nutrition, sensation, and motion, which are plainly performed by the brain. But then these functions admit of as easy an explanation on the hypothesis here proposed, as on that of a glandular secretion, called nervous fluid, animal spirits, &c. ​In the mean time, I cannot but acknowledge many, or even most, things in the Boerhaavian doctrine concerning the structure and functions of the brain, to be beautiful, just, and useful. And it may even be that the doctrine of a glandular secretion, properly qualified, is not inconsistent with that of vibrations.
Sir Isaac Newton supposes the nerves, when singly taken, to be pellucid, because otherwise they could not be sufficiently uniform for the purpose of transmitting vibrations freely to and from the brain; the opacity of any body being, according to him, an argument that its pores are so large and irregular as to disturb and interrupt the vibrations of the æther. For the same reasons we must suppose the fibrils of the medullary substance of the brain to be pellucid, when singly taken. And this consideration may incline one to conjecture, that in palsies, the infinitesimal vessels of the fibrils of the brain, and capillaments of the nerves, are so obstructed, as to render these fibrils and capillaments white and opake, in the same manner as the hair in old age, or the cornea in an albugo.
Since the pia mater, with its blood-vessels, enters the interstices of the several folds of the brain, one may suspect, that it penetrates not only the cortical substance, but also the medullary, along with the several descending orders of vessels, and consequently that it divides and subdivides the medullary substance into various greater and lesser regions. One may affirm, at least, that such a distribution of the pia mater would be greatly analogous to that of the cellular membrane, through the system of muscles, their separate portions, fibres, and fibrils. But then we may reasonably suppose the pia mater to be so attenuated in these its processes, as that the medullary substance may still remain sufficiently uniform for the free propagation of vibrations. Or, if there be some little impediment and confinement in certain regions, on account of some exceedingly small discontinuity, arising from this intervention of the pia mater between certain regions, it may, as it seems to me, suit this theory rather better than an absolute and perfect continuity, as before supposed. It is reasonable also to think, that the nerves of different parts have innumerable communications with each other in the brain, in the ganglions, (which are, as it were, little brains, according to the opinion of Winslow,) and even in the plexuses; and that many phænomena, particularly those of the sympathetic kind, are deducible from these communications. But as it seems impossible to trace out these communications anatomically, on account of the great softness of the brain, we must content ourselves with such conjectures as the phænomena shall suggest, trying them by one another, and admitting for the present those which appear most consistent upon the whole, till farther light appears. The same, or even a greater, obscurity attends all inquiries into the uses of the particular shape and protuberances of the medullary substance of the brain.
​We come, in the last place, to consider what active properties may belong to the small particles of the medullary substance, i.e. to the small particles which compose either the ultimate vessels of this substance, or the fluid which circulates in these ultimate vessels. The common doctrine concerning the powers of the nervous system supposes the fluid secreted by, and circulating through, the medullary substance to be of a very active nature; and this may be, though the taste of the medullary substance in brute animals discovers no such activity. For the power of impressing tastes seems to reside in particles much larger than those which we are here considering. And it is sufficiently obvious, that many poisons, mineral, vegetable, and animal, have the most active properties concealed under insipid or at least moderate savours. Now that some powers of attraction or repulsion, or rather of both at different distances, reside in the small particles of the medullary substance, can scarce be doubted, after so many instances and evidences, as Sir Isaac Newton has produced, of attractive and repulsive powers in the small particles of various bodies, Opt. Qu. 31; meaning, as he does, by attraction and repulsion, a mere mathematical tendency to approach and recede, be the cause what it will, impulse, pressure, an unknown one, or no physical cause at all, but the immediate agency of the Deity. The smallness also of the particles of the medullary substance may not improbably increase their activity, in respect of their bulk, agreeably to Sir Isaac Newton’s conjecture concerning the particles of the æther. Which may be farther inferred from the nature of these attractions and repulsions; for since they seem to be as some reciprocal power of the distance, we may judge that only the nearest parts of large particles will be eminently active, and that the more remote ones will be an impediment to their actions; whence small particles, having nearly as great active powers, and much less matter to be moved, will, upon the whole, be more active in proportion to their bulk, than large ones. If we farther suppose the particles of the fluids, which circulate through the ultimate vessels of the medullary substance, to be smaller than the particles which compose these vessels, then will they also be more active. And thus we seem to approach to all that is probable in the received doctrines concerning the nervous fluid, and the animal spirits, supposed to be either the same or different things; and all the arguments which Boerhaave has brought for his hypothesis, of a glandular secretion of a very subtle active fluid in the brain, may be accommodated to the Newtonian hypothesis of vibrations.
Having thus endeavoured to settle our notions concerning the æther, and establish our evidences for its existence and properties, and for the uniformity, continuity, softness, and active powers of the medullary substance; we come, in the next place, to inquire in what manner these may serve to explain or evince ​the vibrations of the medullary particles, asserted in the foregoing proposition.
First, then, We are to conceive, that when external objects are impressed on the sensory nerves, they excite vibrations in the æther residing in the pores of these nerves, by means of the mutual actions interceding between the objects, nerves, and æther. For there seem to be mutual actions of all the varieties between these three, in all the senses, though of a different nature in different senses. Thus it seems that light affects both the optic nerve and the æther; and also, that the affections of the æther are communicated to the optic nerve, and vice versâ. And the same may be observed of frictions of the skin, tastes, smells, and sounds. The impulse, attraction, or whatever else be the action of the object, affects both the nerves and the æther; these affect each other, and even the object or impression itself, in most or all cases, so as to alter or modify it. And the result of these actions, upon the whole, may be supposed such a compression or increase of density in the æther, as must agitate its particles with vibrations analogous to those which are excited in the air by the discharge of guns, by thunderclaps, or by any other method of causing a sudden and violent compression in it.
Secondly, We are to conceive, that the vibrations thus excited in the æther will agitate the small particles of the medullary substance of the sensory nerves with synchronous vibrations, in the same manner as the vibrations of the air in sounds agitate many regular bodies with corresponding vibrations or tremblings. And here the uniformity, softness, and active powers of the medullary substance, must be considered as previous requisites and assistances. A want of uniformity in the medullary substance would argue a like want of uniformity in the æther contained within it. The hardness of it, if it extended to the particles, would cause an ineptitude to vibratory motions in the particles of these particles, i.e. in the infinitesimal particles considered in this and the foregoing proposition. And a want of active powers in these particles would suffer the excited motions to die away prematurely.
One may conjecture, indeed, that the rays of light excite vibrations in the small particles of the optic nerve, by a direct and immediate action. For it seems probable, from the alternate fits of easy transmission and reflexion, that the rays of light are themselves agitated by very subtle vibrations, and consequently that they must communicate these directly and immediately to the particles of the optic nerves. And it may be also, that sapid and odoriferous particles are agitated with specific vibrations, and that they communicate these directly and immediately to the small particles of the gustatory and olfactory nerves respectively, as well as to the interjacent æther. Upon this supposition, the vibrations of the æther must be conceived as regulating ​and supporting the vibrations of the particles, not as exciting them originally.
Thirdly, The vibrations thus excited in the æther, and particles of the sensory nerves, will be propagated along the course of these nerves up to the brain. For the æther residing in the medullary substance, being of an uniform density on account of the smallness of the pores of the medullary substance, and uniformity of its texture, before taken notice of, will suffer the excited vibrations to run freely through it. And the same uniformity, together with the continuity, softness, and active powers of the medullary substance, will farther contribute to the free propagation of the vibrations; since, on these accounts, it follows, that the particles, which were last agitated, may easily communicate their agitations or vibrations to the similarly posited and equal contiguous ones, without interruption, and almost without any diminution of force. This free propagation of vibrations along the course of the nerves may be illustrated and confirmed by the like free propagation of sounds along the surface of water, which has sometimes been observed in still, calm nights.
Fourthly, The vibrations here described are confined to the medullary substance, or at least are only propagated feebly and imperfectly into the neighbouring parts, on account of the heterogeneity and geater hardness of the neighbouring parts. The first will make the æther of different densities, and in some cases there may be almost an interruption or discontinuity of it; and the last will indispose the particles to receive and communicate vibrations; and we may suppose from both together, agreeably to what has been already remarked, that only small vibrations, and such irregular ones as oppose each other, will just begin to take place in the immediately contiguous parts, and there cease without proceeding farther. It is somewhat analogous to this in sounds, that they are much sooner lost in passing over rough surfaces than smooth ones; and particularly, that they receive a much greater diminution from the irregular surface of the earth, than from that of still water. However, a particular exception is here to be made in respect of the fibres of the muscles and membranes, into which the vibrations of the æther and medullary particles seem to be propagated with great freedom and strength, as will be seen hereafter. Which may perhaps be some argument that muscular fibres are, according to Boerhaave’s opinion, mere productions of the ultimate nerves.
Fifthly, As soon as the vibrations enter the brain, they begin to be propagated freely every way over the whole medullary substance; being diminished in strength, in proportion to the quantity of matter agitated, just as in sounds, i.e. as it were in a reciprocal duplicate ratio of the distance from the place where the sensory nerve affected by the vibrations enters the brain. Or, if we suppose the pia mater to make some small discontinuity ​in the medullary substance by its processes, as has been hinted above, then we must also suppose that the vibrations, which ascend along any sensory nerve, affect the region of the brain which corresponds to this sensory nerve more, and the other regions less, than according to this proportion.
Sixthly, Since the vibrations, or reciprocal motions, of the small particles of each nerve are made in the same line of direction with the nerve, they must enter the brain in that direction, and may preserve some small regard to this direction at considerable distances within the brain; especially if this be favoured by the structure of the nervous fibrils in the brain. Hence the same internal parts of the brain may be made to vibrate in different directions, according to the different directions of the nerves by which the vibrations enter.
And thus it appears, that, admitting the existence and subtlety of the æther, and the qualities of the medullary substance here alleged, a probable account may be given, how the vibrations, asserted in the last proposition, may be excited in the sensory nerves, and propagated thence over the whole medullary substance, and over that alone. And the suitableness of this proposition to the last, and of both to a variety of phænomena, which will be seen in the course of these observations, may be considered as some evidence for both.
Let it be remarked also, that, if the performance of sensation by vibratory motions of the medullary particles be admitted, the existence of a subtle elastic fluid must be admitted in consequence thereof, as the only means that can be conceived for their rise and free propagation, so as to answer to the phænomena of sense, motion, and ideas; and reciprocally, if the existence of so subtle and elastic a fluid, as the æther described by Sir Isaac Newton, can be established upon independent principles, it may reasonably be supposed to penetrate the pores of the medullary substance, how small soever they be, in the same manner as air penetrates grosser cavities and pores; and, like air, both be itself agitated by vibrations from a variety of causes, and also communicate these to the medullary particles. We may, therefore, either deduce the doctrine of vibrations here proposed from the consideration of the æther, or the existence of the æther from the doctrine of vibrations, according as either of these can be first established.
There is also some light and evidence to be cast upon one or both of these propositions, from several natural phænomena; as I will endeavour to shew in the following remarks:—
1. Heat in natural bodies is probably attended by vibrations of the small parts. This may be inferred from the duration and gradual declension of heat, and from the gross general proportion which is observed between this duration and the density of the heated body. For a vibratory motion would subsist for some time, decline gradually, and be kept up longer, cæteris paribus, ​where the number of vibrating particles were many, than where few. The same hypothesis is well suited to the rarefaction, fluidity, dissolution, and other changes of texture which heat produces in bodies, according to their various natures. And if we farther consider that all bodies, contiguous to each other, come, after a short time, to the same degree of heat, viz. that of the circumambient air; those which are hotter losing something, and those which are colder gaining; and yet that the air is not necessary for the conveyance of heat, as appears from Sir Isaac Newton’s experiment of the two thermometers above-mentioned; it will appear highly probable, both that heat in bodies is attended by vibratory motions of the small parts, and also that these are communicated to contiguous bodies by vibrations of a subtle fluid, by an argument something different from that urged above, in speaking of the two thermometers; at least the consideration of the equality of temperature, to which all contiguous bodies are known, by common observations, to arrive, will cast some light upon that argument. And upon the whole it will follow, that heat in us is caused or attended by subtle vibrations of the medullary substance, which is the immediate instrument of all the sensations; and that a subtle fluid is concerned in the production of this effect. And what is thus proved of heat, may be inferred to hold in respect of all the other sensations, from the argument of analogy.
2. Light is so nearly related to heat, that we must suppose the argument of analogy to be particularly strong in respect of it: but besides this, we have an independent argument for the existence of vibrations here, also for their communication by a subtle fluid, if we admit Sir Isaac Newton’s hypothesis concerning the cause of the alternate fits of easy reflection and transmission, as I have above remarked.
3. As sounds are caused by pulses or vibrations excited in the air by the tremors of the parts of sounding bodies, they must raise vibrations in the membrana tympani; and the small bones of the ear seem peculiarly adapted, by their situation and muscles, to communicate these vibrations to the cavities of the vestibulum, semicircular canals, and cochlea, in which the auditory nerve is expanded; i.e. to the nerve itself. Now though these are gross vibrations, in respect of those which we must suppose to take place in the æther itself, yet they prepare the way for the supposition of the more subtle vibrations of the æther, and may be the instrument of these, in the same manner as very gross reciprocal motions of bodies in the air are observed to produce, by percussion, those quicker vibrations in which sound consists.
4. We are, in some measure, prepared also for admitting the doctrine of vibrations in the animal functions, from that disposition to yield a sound upon percussion, which appears in some degree in almost all bodies; since this shews, that the disposition ​to vibrate is general, or even universal, in the bigger orders of particles; and therefore makes it more easy to conceive, that there may be a like disposition in the lesser orders, i.e. in the infinitesimal medullary particles, considered in this and the foregoing proposition.
5. The mutual attractions and repulsions which seem to intercede between all small particles, concur to the same purpose. For when the attractions and repulsions are changed, by changing the distances of the particles, these must oscillate to and fro for some time, before they can gain their former equilibrium.
6. Elasticity seems to result from mutual attractions and repulsions of some kind, and is evidently the cause of vibrations in musical strings, and many other bodies. It seems also, that there is scarce any body entirely devoid of elasticity. And thus elasticity is connected with the doctrine of vibrations in different ways.
7. The effluvia of electric bodies seem to have vibrating motions. For they are excited by friction, patting, and heat; and excite light, sound, and a pricking sensation. They have also a repulsive power in respect of each other, as the particles of air have; and therefore must, like them, be easily susceptible of vibrations. Their motions along hempen strings resemble the motions along the nerves in sensation and muscular contraction; and their attractive powers, at the end of such strings, resemble the powers of the sensations over the muscles for contracting them. So that electricity is also connected in various ways with the doctrine of vibrations.
Lastly, To sum up in one what has been remarked in the last five paragraphs: as the attractions of gravitation, electricity, magnetism, and cohesion, with the repulsions which attend upon the three last, intimate to us the general tenor of nature in this respect; viz. that many of its phænomena are carried on by attractions and repulsions; and that these may be expected to take place in the small descending orders of particles, as well as in gross bodies, and in the biggest component particles; so the pulses of the air, the tremors of sounding bodies, the propagation of sounds both through the air, and along contiguous solid bodies, the oscillations of elastic bodies, and the phænomena of electricity, may, in like manner, serve as a clew and guide to the invention, and afford a presumption, that other reciprocal motions or vibrations have a great share in the production of natural phænomena.
Nor is it an objection to this, but rather a confirmation of it, that these principles of attraction and repulsion of the several kinds, and of vibrations, are dependent upon and involved within each other, since this also is agreeable to the tenor of nature, as it is observed in the body, in the mind, in science in general, and in the several branches of each science in particular. Each part, faculty, principle, &c. when considered and pursued ​sufficiently, seems to extend itself into the boundaries of the others, and, as it were, to enclose and comprehend them all. Thus magnetism mixes itself with the gravitation both of bodies upon the surface of the earth, and with that of the moon to the earth: a polar virtue of the same kind seems to have a principal share in the formation of natural bodies, especially those whose parts cohere in regular figures: electricity may also extend, without being excited by friction or otherwise, to small distances, and join with the just-mentioned polar virtue in making the parts of bodies cohere, and in some cases in regular figures. The effervescence which attends the mixture of acids and alcalis, and the solution of certain bodies in menstruums, fermentation, and putrefaction, are all general principles of very extensive influence, nearly related to each other, and to the fore-mentioned mutual attractions and repulsions, and are possessed of the same unlimited power of propagating themselves, which belongs to the several species of plants and animals. A repulsion which should throw off indefinitely small corpuscles with indefinitely great velocity from all the bodies of the universe, (a thing that would be very analogous to the emission of light, odoriferous particles, and magnetical and electrical effluvia, and to the generation of air and vapour,) might cause the gravitation of all the great bodies of the universe to each other, and perhaps other kinds of attraction. Some of these corpuscles, by stopping each other in the intermundane spaces, or other mutually repulsive corpuscles lodged there from causes not yet discovered, may compose a subtle vibrating medium. The vibrations of this medium, being continued to the great bodies of the universe, may so far agitate their small parts, as to give their attractive and repulsive powers an opportunity to exert themselves with great vigour; and the emission of the above-mentioned corpuscles may be, in part, occasioned by the attractions and consequent collisions of small parts thus agitated; so that elastic corpuscles may be thrown off from these small parts with indefinitely great velocity. And it would be no objection to these or such like suppositions, that we could not explain, in any definite manner, how these things are affected, nor put any limits to the sizes of decreasing corpuscles, or their active powers in respect of each other. Nor would this be to reason in a circle, more than when we argue, that the heart and brain, or the body and the mind, depend upon each other for their functions; which are undeniable truths, however unable we may be to give a full and ultimate explanation of them. However it is not impossible, on the other hand, but future ages may analyse all the actions of bodies upon each other, up to a few simple principles, by making such suppositions as the phænomena shall suggest, and then trying and modelling them by the phænomena. At least this is what one is led to hope, from the many simple and easy solutions of very complex problems, which have been produced within the two last centuries.
​We may draw the following corollaries from the hypothesis of vibrations, as laid down in the two foregoing propositions.
Cor. I. The vibrations of the medullary particles may be affected with four sorts of differences; viz. those of degree, kind, place, and line of direction. Vibrations differ in degree, according as they are more or less vigorous; i.e. as the particles oscillate to and fro, through a longer or shorter very short space; i.e. as the impression of the object is stronger or weaker, and thus affects the medullary particles more or less vigorously, either directly and immediately, or mediately, by generating a greater or less degree of condensation in the pulses of the æther. Vibrations differ in kind, according as they are more or less frequent; i.e. more or less numerous, in the same space of time. They differ in place according as they affect this or that region of the medullary substance of the brain primarily. And they differ in the line of direction, according as they enter by different external nerves.
Cor. II. The magnitude of each sensation is chiefly to be estimated from the vibrations which take place in the medullary substance of the brain; those which are excited in the spinal marrow and nerves being, for the most part, so inconsiderable, in respect of the just-mentioned ones, that they may be neglected.
Cor. III. The brain may therefore, in a common way of speaking, be reckoned the seat of the sensitive soul, or the sensorium, in men, and all those animals where the medullary substance of the nerves and spinal marrow is much less than that of the brain; and this even upon the supposition laid down in the first proposition; viz. that the whole medullary substance of the brain, spinal marrow, and nerves, is the immediate instrument of sensation, and equally related to the sensitive soul, or principle. But if there be any reason to suppose that the first proposition is not strictly true, but that the spinal marrow and nerves are only instruments subservient to the brain, just as the organs of the hand, eye, ear, &c. are to them, and the brain itself to the soul, we may conclude absolutely, that the sensorium of such animals is to be placed in the brain, or even in the innermost regions of it. Now there are some phænomena which favour this, by shewing, that whatever motions be excited in the nerves, no sensation can arise unless this motion penetrate to, and prevail in, the brain. Thus, when a nerve is compressed, we lose the sense of feeling in the parts to which it leads: a person much intent upon his own thoughts does not hear the sound of a clock; i.e. the vibrations excited by this sound in the auditory nerve cannot penetrate to, and prevail in, the brain, on account of those which already occupy it: and a person who has lost a limb often feels a pain which seems to proceed from the amputated limb; probably because the region of the brain corresponding to that limb is still affected.
If it be certain, that some of the medullary parts have been ​discharged in abscesses of the brain, one would incline to think, that the external parts of the medulla are instrumental, in respect of the internal. And, on the other hand, one may question, whether in animals of the serpentine form, and those whose brains are comparatively small, and in all those of the polypous kind, the sensorium be not equally diffused over the whole medullary substance, or even over all the living parts. I only hint these things, not presuming even to conjecture, but only to excite those who have proper opportunities, to inquire carefully into these matters.
Cor. IV. If we allow the existence of the æther, and its use in performing sensation, thought, and motion, as it may be inferred from the two foregoing propositions, compared with such other things as follow in these observations, in favour of the doctrine of vibrations; we may conclude that the æther must have a considerable share in the production of many other natural phænomena; and therefore shall have a sufficient foundation for trying how far it will carry us agreeably to the facts. I would recommend this, in a particular manner, to those persons who are much conversant with electrical phænomena; especially as Sir Isaac Newton himself, whose great caution and reservedness, in difficult and doubtful matters, are sufficiently known, has made no scruple to affirm, that the powers of electrical bodies are owing to the action of the æther. See the last paragraph of the Principia.
Scholium. It may be proper to remark here, that I do not, by thus ascribing the performance of sensation to vibrations excited in the medullary substance, in the least presume to assert, or intimate, that matter can be endued with the power of sensation. It is common to all systems, to suppose some motions attendant upon sensation, since corporeal objects must, by their actions, impress some motion upon our bodies, and particularly upon that part which is most nearly related to the sentient principle; i.e. upon the medullary substance, according to the first and second propositions. I lay down these propositions, therefore, as established by the common consent of physicians and philosophers; and upon that foundation proceed to inquire into, and determine, some matters of a more difficult nature; such as, the complex problems concerning sensations, ideas, and motions, and their mutual influences and relations.
The following instance may illustrate this:—The quantity of matter in bodies is always found to be proportional to their gravity: we may therefore either make the quantity of matter the exponent of the gravity, or the gravity the exponent of it, according as either can be best ascertained; notwithstanding that we are entirely at a loss to determine in what mechanical way each atom contributes to the gravity of the whole mass; and even though we should, with some, suppose this effect to be immechanical, and to arise from the immediate agency of God. ​And by parity of reason, if that species of motion which we term vibrations, can be shown, by probable arguments, to attend upon all sensations, ideas, and motions, and to be proportional to them; then we are at liberty either to make vibrations the exponent of sensations, ideas, and motions, or these the exponents of vibrations, as best suits the inquiry; however impossible it may be to discover in what way vibrations cause, or are connected with, sensations, or ideas; i.e. though vibrations be of a corporeal, sensations and ideas of a mental, nature.
If we suppose an infinitesimal elementary body to be intermediate between the soul and gross body, which appears to be no improbable supposition, then the changes in our sensations, ideas, and motions, may correspond to the changes made in the medullary substance, only as far as these correspond to the changes made in the elementary body. And if these last changes have some other source besides the vibrations in the medullary substance, some peculiar original properties, for instance, of the elementary body, then vibrations will not be adequate exponents of sensations, ideas, and motions. Other suppositions to the same purpose might be made; and upon the whole, I conjecture, that though the first and second propositions are true, in a very useful practical sense, yet they are not so in an ultimate and precise one.
The most vigorous of our sensations are termed sensible pleasures and pains, as noted above, in the Introduction. And the vivid nature of these engages us to be very attentive to their several properties, relations, and oppositions. It is requisite therefore, in our inquiry into the doctrine of vibrations, to examine, how far the phænomena of sensible pleasure and pain can be deduced from, or explained by, it.
First, then, the doctrine of vibrations seems to require, that each pain should differ from the corresponding and opposite pleasure, not in kind, but in degree only; i.e. that pain should be nothing more than pleasure itself, carried beyond a due limit. For of the four differences of vibrations mentioned in the first corollary of the foregoing proposition, three are given, viz. those of kind, place, and line of direction, in the pleasures and pains which correspond, as opposites to each other: there is therefore nothing left, from whence the difference of such pleasures and pains can arise, except the difference of degree. But the phænomena appear to be sufficiently suitable to this reasoning, inasmuch as all pleasure appears to pass into pain, by increasing its cause, impression, duration, sensibility of the organ upon which it is impressed, &c. Thus an agreeable warmth may be made to pass into a troublesome or burning heat, by ​increase, or continuance; and the same thing holds in respect of friction, light, and sounds. And as medicinal bodies appear, from observations both philosophical and vulgar, to be endued with more active properties than common aliments; i.e. to be fitted for exciting stronger vibrations; so their tastes and smells are, for the most part, ungrateful; whereas those of common aliments are pleasant. It may be observed also, that some painful sensations, as they decrease by time, or the removal of the cause, pass into positive local pleasures, of the same species as the preceding pain; thus shewing the near alliance between pleasure and pain; and that a mere difference in degree puts on the appearance of one in kind, at a certain point. I suppose it may be referred to this head, that some bitter and acrimonious tastes leave an agreeable relish of the sweet kind upon the tongue after some time.
Secondly, It agrees well with the doctrine of vibrations, that all evident solutions of continuity in the living parts occasion pain, inasmuch as a solution of continuity cannot happen without a violent impression of some sensible object, nor, by consequence, without violent mutual actions between the object, nerves, and æther. The solution of continuity does therefore pre-suppose that degree of violence in the vibrations, which exceeds the limit of pleasure, and is proper to pain, according to the foregoing paragraph.
Thirdly, We may, in like manner, give a reason, from the doctrine of vibrations, both why a moderate degree of distention in the parts is necessary to their growth and pleasurable state; and also, why all great distentions are attended with pain for a considerable time before they are raised to such a pitch as to cause a visible solution of continuity. For a great distention is equivalent to a vigorous impression of a sensible object, being often caused by such; and as the situation of the small particles is changed in great distentions, their mutual actions will be changed also, and so may give rise to more vigorous vibrations; and these increased vibrations may either fall within the limits of pleasure, or go beyond them, according to their degree. We are also to consider, that in all considerable distentions there is an increase of friction between the vessels and circulating fluids, and consequently of heat, i.e. of vibrations.
But besides this, it seems not improbable, that in preternatural and painful distentions, the small particles are perpetually separating themselves from their former cohesions, and running into new ones; so that a minute and invisible solution of continuity is carried on during the whole distention, till such time as this degree of distention becomes familiar to the parts, and the situation and mutual actions of the small particles be accommodated to it. Thus, the cause of the pain in distentions will arise from the solution of continuity, and may be referred to the foregoing head. And conversely it appears, that in manifest ​solutions of continuity, occasioned by wounds, burns, &c. there always arises in the neighbouring parts, which are inflamed, a preternatural distention of the small fibres and vessels; by which means the pain is renewed and continued. Every manifest solution of continuity does therefore, according to the explanation of distention just laid down, include within itself an infinite number of minute invisible solutions.
Hence we may ask, whether this minute invisible solution of continuity in the infinitesimal medullary particles of the brain be not that common limit, and middle point, which separates pleasure from pain, and of which the visible solutions of continuity, which are caused by external injuries, are a type, and also a means, viz. by propagating violent vibrations up to the brain? It is some presumption in favour of this position, that all conjectures concerning invisible things ought to be taken from visible ones of the same kind; also, that it is particularly suitable to the doctrine of vibrations; inasmuch as, laying down this doctrine, one may easily conceive how moderate and pleasant impressions may agitate the medullary particles in so moderate a degree, as that they shall again return to their former situations and connexions, when the agitation is over; whereas violent and painful ones may force the particles from thence, and give rise to new ones, i.e. to the solution of continuity. And as the body is so formed, that great and visible solutions of continuity may be healed again, and the parts restored, in great measure, to their primitive integrity and perfection, by the power of nature, unless where there is a loss of substance, (and yet even here the same end is obtained in part;) so we may suppose, that the power of nature restores all minute solutions of continuity in the constituent infinitesimal particles almost instantaneously, and so that the body receives no perceptible detriment from single instances, though it probably does from frequent repetitions; agreeably to which, it is generally supposed, that pain, by often returning, impairs the faculties, both bodily and mental.
Fourthly, The bones, nails, hair, and cuticle, may, consistently with the doctrine of vibrations, have a solution of continuity produced in their parts, without pain ensuing; for they are hard, and therefore incapable, as it seems, of receiving and communicating to the contiguous nerves, and thence to the brain, vibrations of any considerable degree of strength. We are also to suppose, that in palsies, mortifications, &c. changes of texture of somewhat a like kind are produced, so as to render the parts affected thereby incapable of conveying sensation to the medullary substance of the brain. Old age, inactivity, inflammation, pain, &c. are in like manner to be considered, as inducing such a degree of condensation, fixation, and callosity, in the medullary substance itself, as must end at last in insensibility and death.
Fifthly, It is not unsuitable to the doctrine of vibrations, that the frequent repetition of the same external impressions should ​have the power of converting original pains into pleasures, and pleasures into mere sensations, i.e. into evanescent pleasures; as we find it has in fact. For this may be effected by such a change in the organ and brain, as that the organ shall send weaker and weaker vibrations perpetually to the brain, upon every successive renewal of the same impression, and the brain become perpetually less and less disposed to receive strong vibrations, though the power of communication from the impressions should continue the same. It remains therefore to be inquired, what general tendencies in the small medullary particles might dispose them to undergo such changes. And it appears to me, that a change of the spheres of attraction and repulsion in these particles, upon every change in their situations, so as always to lean towards the situation last superinduced, might be sufficient for this purpose. However, this is a mere supposition, and that of a very recluse nature. Only let it be observed, that the fact to be here accounted for, viz. the decrease in the efficacy of impressions frequently repeated, is both an evident one, and also must have its rise from some powers in the small parts of matter over each other. It must therefore admit of an explanation, either from the doctrine of vibrations, or from some other law of matter and motion. And if the doctrine of vibrations be found suitable to other phænomena, it may be presumed not to be unsuitable to this, till such time as some manifest inconsistency between them shall appear.
It ought to be remarked here, that this transit of original pains into pleasures, and of vivid pleasures into faint ones, by frequent repetition, bears some relation to the above-mentioned transition of pains into positive local pleasures, of the same kind with themselves.
To this head of consideration may be referred Dr. Jurin’s observation; viz. “That when we have been for some time affected with one sensation, as soon as we cease to be so affected, a contrary sensation is apt to arise in us, sometimes of itself, and sometimes from such causes as at another time would not produce that sensation at all, or at least not to the same degree.” For the continued impression of the same object will so fix upon the sensory nerve, and region of the brain corresponding thereto, a tendency to one peculiar sort of vibrations, that an impression of an opposite or very different sort, must do more than usual violence to the brain; i.e. will excite a glaring sensation of an opposite nature. See the instances mentioned by Dr. Jurin, in his Essay on distinct and indistinct Vision.
Sixthly, We may account for the different kinds and degrees of pleasure and pain, from the four differences of vibrations mentioned above, viz. those of degree, kind, place, and line of direction, and their various combinations with each other. For it is obvious to conceive, that these combinations may be sufficiently numerous and distinct from each other, to answer to the ​facts. If the vibrations go beyond the common limit of pleasure and pain in one part of the brain, at the same time that they fall short of it in others, the result will be a pleasure or pain, according as this or that sort of vibrations prevails; and if they be nearly equal, it will be difficult to determine of which kind it is. If the vibrations fall a little short in all the parts, they will generate a high degree of pleasure; which, however, must be less than the least general pain, i.e. such a one wherein the vibrations go beyond the limits in all the parts: but it may be far greater than partial pains, or than those which affect only one particular region of the brain. Hence we may see, that the pains are in general greater than the pleasures; but then they are more rare for the same reason, being such violent states as cannot arise from common impressions. Or, if we suppose the pains to be frequent, they will then so far alter the disposition of the medullary substance, according to what was said above, as that many original pains will be converted into pleasures. Which indeed seems to be the case not unfrequently; for the organs of the new-born infant are so delicate, as to receive pain from many of those impressions which afterwards yield pleasure. But then, his sources of pleasure seem to be multiplied more than in proportion to what he suffers by this previous passage through pain.
In certain cases of excessive pains, the violent vibrations appear at last to excite a latent attractive power in the medullary particles, in the manner hereafter to be described, in respect of the fibres of the muscles and membranes, which puts a stop to those very vibrations that excited it. Hence faintings and stupors; i.e. the cessation of pain from violent pains. However, a greater degree of vibrations is probably required for exciting this attractive power in the medullary particles than in white fibres, and in white fibres than in red ones, as will appear hereafter.
It follows also, from the principles here laid down, that all the pleasures, though particularly different from each other, ought to have a general resemblance, in their circumstances and consequences; and the pains likewise.
Seventhly, All the mere sensations, which enter the mind by the five external senses, admit of a general analysis, upon the same principles as the pleasures and pains do. For all the mere sensations were, in their original state, either pleasures or pains; and vary now from their original state only by the diminution of the degree. Let therefore all the differences of kind, place, and line of direction, be combined in all their varieties, the degree being supposed every where evanescent; and we shall have all the particular vibrations from whence each mere sensation arises. This is the general account. But it is a most difficult problem to explain, by what differences of kind the particular sensations, either of the same or of different senses, are distinguished from each other.
​It seems probable to me, that the limits of the seven primary colours, viz. the extreme red, the limit of the red and orange, of the orange and yellow, yellow and green, green and blue, blue and indigo, indigo and violet, and the extreme violet, excite vibrations in the optic nerve, whose times are proportional to the times of vibration of a string which sounds the notes in order, according to the key mentioned by Sir Isaac Newton in his Optics, i.e. the notes D, E, F, G, A, B, C, D. This hypothesis affords at least a probable reason for the several very particular breadths of the primary colours, in the prismatic oblong image of the sun, as I shall endeavour to show in its place, Prop. 56.
If the frequency of the vibrations excited by the several sapid and odorous bodies in the nerves of the tongue, and membrana Schneideriana, could be discovered, it is not improbable but this would be a clew to lead us into the inner constitution of natural bodies, since one may reasonably suppose, that each sapid and odorous body excites vibrations of the same frequency as those which take place in it before it is tasted or smelt to.
The solution of the same problem in the several senses might also a little unfold to us the different internal structure of the several nerves, and of the parts of the brain that correspond thereto. For it seems probable to me, that each nerve and region is originally fitted to receive, and, as one may say, sympathize with, such vibrations as are likely to be impressed upon them in the various incidents of life; and not that the auditory nerve could perform the office of the optic, if put into its place, or vice versâ, &c. according to Raw.
Eighthly, It is observed by medical writers, that pain is apt to excite a contraction in the fibres of the neighbouring membranes. Now this is very agreeable to that power which sensory vibrations have, in general, over the red fleshy muscles, for contracting them, in the manner to be described hereafter. For since vibrations of a middle strength, by descending into the red fibres of the muscles, are sufficient to contract them in the ordinary functions and actions of life; it is not unreasonable to expect, that the stronger vibrations, which attend pain, should be sufficient to contract the pale fibres of membranes, though these be in themselves of a less contractile disposition.
It is agreeable to this, that titillation and itching, which lie, as it were, upon the confines of pleasure and pain, are more apt to diffuse themselves over the neighbouring parts than pain. For titillation and itching only agitate the small particles of the membranes, and therefore run along their surfaces, by the successive communication of these agitations; whereas, pain, by contracting the fibres, puts a stop to these agitations, and consequently to its own diffusion over the neighbouring parts.
Ninthly, Extreme and pointed parts, such as the extremity of the nose, the uvula, the epiglottis, the nipples, and the ends of ​the fingers, are in general more subject to irritation, itching, and inflammation, and endued with a greater degree of sensibility, than the other parts. Now this phænomenon agrees with the doctrine of vibrations, inasmuch as such parts must, according to the Newtonian hypothesis, be surrounded with an æther of a greater density than that within their pores, and which also grows denser and denser, in a regular manner. For one may conceive, that the vibrations communicated to this denser æther will be stronger in proportion to its density; and consequently, that they will agitate the small particles of the extreme parts also with vibrations stronger than ordinary.
It is true, indeed, that the sensibility of each part does depend, in great measure, on the number, structure, and disposition, of the nervous papillae, which are the immediate organ in the senses of feeling, taste, and smell; but then we may remark, that the same observation holds in respect of these nervous papillae. For they are also extreme and pointed parts; and that especially, if we suppose, which seems probable, that when any part is in a state of exquisite sensibility, the nervous papillae are erected, (in some such manner as the hairs of the neck and back in certain animals, when enraged,) so as to recede from each other, and, consequently, to admit the denser æther between them. They may also, upon the same occasions, be made turgid, by the constriction of their bases, and thus have their sensibility or power of receiving vibrations, increased by distention.
We may remark likewise, in pursuing this method of reasoning, that the æther which lies contiguous to the medullary substance in the ventricles of the brain, is denser than that which lies in the medullary substance itself. May we not therefore conjecture, that one use of the cavities of the ventricles is to increase and keep up all the vibrations propagated from the external nerves into the medullary substance of the brain, by means of the denser æther lodged in those cavities; that blood and serum extravasated, and lying in the ventricles, suffocate sensations, by excluding this denser æther, as well as by pressing on the medullary substance; and lastly, that those brutes whose olfactory nerves have cavities within them continued from the ventricles, are more acute than men, in perceiving odours, and distinguishing them from each other, in part, upon this account?
Boerhaave is, indeed, of opinion, that the opposite sides of the ventricle always touch each other, so as to leave no cavity. But it seems more reasonable to suppose, that a subtle vapour, which is exhaled from the vessels of the investing membrane, and whose particles, like the vapour of water, have a repulsive power, in respect of each other, prevents the absolute mutual contact of the opposite sides, in common cases. And the same thing is favoured by the experiments tried upon the Parisian beggar. Since the brain in him could be somewhat pressed in, it seems that the skull was not entirely full before.
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 ​the cortical; so that, if we suppose its functions to consist in receiving, retaining, and communicating vibrations, it will be rendered peculiarly unfit for these functions, from the compression here mentioned; i.e. the animal will be indisposed to sensation and motion, agreeably to observation.
There are many other arguments which might be brought to shew, that during sleep, and sleepy distempers, the brain is particularly compressed, if it were necessary. But the instance of the Parisian beggar above noted, is most to this purpose. This person had a perforation in his skull, which did not ossify; whence, by external pressure upon that part, the internal regions of the brain might be affected; and it was constantly observed, that, as the pressure increased, he grew more and more sleepy, and at last fell into a temporary apoplexy.
In young children, there seems to be a constant moderate pressure of the skull upon the brain. For the brain is of a great relative magnitude in them; and by its endeavour to expand itself, it keeps the sutures from uniting too firmly, till such time as it is arrived at its full growth. It must therefore be compressed in return, by the re-action of the skull. And this may be considered as a circumstance, which concurs to render young children more apt to sleep than adults. When old persons are sleepy, it is a morbid affection, and may arise either from an hydropical disposition, whereby the turgescence of the neighbouring parts compresses the medullary substance; or from a defect of nutrition in this substance, which renders it soft and compressible in a preternatural degree. If the venal sinuses, and other blood-vessels, of the brain, be by any accident preternaturally distended, and continue so for a considerable time, they will scarce ever recover their pristine tone and dimensions; and this so much the more, as the person approaches to old age.
For the same reason, as the medullary substance within the skull and vertebrae is compressed during sleep, that of the ganglions, plexuses, and trunks of the nerves, in other parts of the body, will be compressed also, though in a less degree. For this substance has no blood or gross fluids within it, and is far the softest of all the parts of the body; and the membranes, which invest all the parts of the body, perform the same office to them, in a less degree, as the skull does to the brain, i.e. check their distention. The surrounding membranes must therefore compress the soft medullary substance in the ganglions, plexuses, and trunks of the nerves, during sleep, on account of the rarefaction of the humours at that time; whence, according to the doctrine of vibrations, sensory ones can neither ascend freely from the external organs to the brain, nor motory ones descend into the limbs; i.e. the animal will be insensible and inactive, as it is found to be in fact.
Is it not probable, that, as sleep comes on, the opposite sides of the ventricles of the brain approach towards each other, on ​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 ​effect; and even from immediate and direct sensations. A person may even feel, that the stomach is the seat of the pleasurable impressions made by opiates. We are to suppose therefore, that vivid vibrations, which, however, lie within the limits of pleasure, ascend perpetually from the stomach and bowels, along the par vagum, and intercostal nerves, up to the brain and spinal marrow, diffuse themselves over these, and from thence descend along the nerves into all the parts of the body. Hence it follows, that they will obscure and overpower all moderate sensations, or vibrations, which subsisted before, or which external objects may from time to time endeavour to excite, and introduce a general pleasurable state over the whole nervous system; with trains of pleasurable ideas, in the manner to be explained hereafter, when we come to treat of ideas, their generation, associations, and dependencies on bodily states. During this pleasurable state, the body will of course be composed to rest; restlessness, tossings, and changes of posture, being caused, for the most part, by uneasy sensations. Hence the blood will be accumulated in the veins and venal sinuses, and grow warm both from the vigorous vibrations excited by the opiate, and from the absolute rest of the body. For absolute rest conduces, in a peculiar manner, to make the body grow warm by the heat reflected from the contiguous coverings; as, on the contrary, the slightest motions frequently returning ventilate and cool the parts. And thus the compression of the medullary substance requisite for sleep will be induced by the action of the opiate upon the stomach and bowels.
But, besides this, we may conceive, that the opiate particles excite vibrations of the same kind in all the parts of the body, after they are taken into the blood, and circulate with it, till such time as, by a perfect assimilation, they lose all their peculiar qualities.
It seems, also, that the continued descent of vibrations, from the brain, and spinal marrow, into the limbs, and external parts, agitates them so much, as to render them unfit for receiving sensation and motion, in the same manner as continued friction of the head, when newly shaved, or shaking the hand, occasion a kind of numbness in the head and hand respectively. For a disorder raised in the motory nerves and muscular fibres, analogous to numbness in the sensory nerves, and sentient papillæ, must produce ineptitude to motion. It seems, therefore, that the insensibility and immobility which proceed from opiates, and which concur in hastening the sleep, and increasing its degree, arise in great part from this cause. The numbness, and paralytic weaknesses, which frequently succeed after opiates, are evidences for what is here alleged.
Opium seems to have an intermediate degree of activity between narcotics, or stupifying poisons, on one hand, and grateful aliments, particularly vinous liquors, on the other. Narcotics operate so violently on the stomach and bowels, the brain, and ​the external parts, as to bring confusion on the sensations, and trains of ideas, and convulsions on the muscular system. And that these effects are produced by a local influence on the stomach, in the manner proposed concerning opiates, appears, because they cease, or abate much, soon after the narcotic is ejected by vomiting; also because whipping a dog, after he has taken the nux vomica, contributes to obviate its ill effects. Wines, and grateful aliments, dispose to sleep, partly by their immediate effects on the stomach, partly by their effects after they are absorbed. But the degree not being so great as in opiates, it may more easily be overcome by a variety of common or vigorous impressions; in which case the vivid vibrations, excited by the wine or aliment, will illuminate all the impressions, and add strength to all the motions. The same thing is observed of opiates, in those who take them frequently.
Sixthly, Chylification, sanguification, nutrition, and growth, seem to proceed best during sleep. This may be conjectured from the sleepiness of all animals after eating, since sleep and chylification, &c. must here concur; and from the almost constant sleep of new-born children, above taken notice of, since nature seems chiefly intent on the due performance of these functions, for some time after birth. Now the doctrine of vibrations may be made to illustrate these points, in some measure. For since respiration becomes strong and convulsive at the instant of going to sleep, it will renew and increase the vibrations excited in the nerves of the stomach and bowels by their contents, which we must suppose to have languished before, in the same manner as those which subsisted in the external senses. The organs of digestion therefore, as well as those of respiration, are in a state of vigilance, and are intent upon the performance of their proper functions, while the other parts are in a state of sleep and inaction, and recruiting, in order to perform their functions in a due manner, upon a return of vigilance. And this holds most particularly in respect of the medullary substance of the brain, spinal marrow, and nerves, which, by the consent of all, is the chief instrument of nutrition and growth. Since the vibrations which take place in it during sleep are languid, it will then be filled and recruited, and consequently fitted for nutrition and growth; which will be farther favoured by the concurrence of a complete chylification and sanguification, at the same time.
The increased convulsive respiration, and increased force of the heart, which take place at the instant of going to sleep, and continue frequently during sleep, may perhaps be thus accounted for, agreeably to the doctrine of vibrations. When vivid vibrations cease in the external senses, and regions of the brain belonging to them, also in the muscles of the limbs, and parts of the spinal marrow corresponding thereto, this abatement of vibrations must either extend to the whole medullary substance, which seems to be the case in the night-mare; or if the ​nerves of the heart, and organs of respiration, and the regions of the brain and spinal marrow corresponding thereto, be exempted, they may be agitated even with more vigorous vibrations, on account of the abatement in the other parts, because the vibrations raised in these regions during sleep, by such of their causes as take place then, will be hindered from diffusing themselves freely, and abating their own force thereby, as soon as the other regions are collapsed and compressed. These causes are, first, the heat of the blood, and pulsation of the arteries of the medullary substance; both which, when increased on any account, must farther increase themselves by a reflected influence, since both increase the force of the heart. Secondly, the fulness and distention of the lungs. These arise from the rarefaction of the blood, and accumulation of it in the veins just before sleep (at which time respiration is languid), and must at last stimulate the organs of respiration to a vigorous exertion of themselves; i.e. raise vigorous vibrations in the region of the brain corresponding thereto, just as in the cases of sighing, and recovering from the night-mare. Thirdly, we may suppose, that the heart, and muscles of respiration, do not exert themselves during vigilance, with a degree of force at all approaching to their utmost powers, as the limbs do; and therefore, that they, and the corresponding regions of the brain, may be qualified for a vigorous exertion during sleep. Fourthly, an increase in the force of respiration must also increase the force with which the heart moves, because it propels the blood in greater quantities upon the heart. Fifthly, an increase of force in the heart must increase both itself and the action of respiration, because the blood-vessels of the heart and organs of respiration are particularly near to the heart, and therefore must be particularly under its influence.
Cor. I. By laying together what has been delivered concerning sleep, in this proposition, the difference between the states of sleep and vigilance may be thus set before the reader, in one view. In sleep, the nerves of the five external senses are indisposed to receive vibrations, and the objects themselves are either absent, or impressed feebly. The nerves of the stomach and bowels sympathize with these at first, but recover themselves at the instant of sleep, the impressions of the aliment, &c. being then made with unusual vigour; and this continues during the time of sleep. In like manner the muscular system becomes inactive in general; the heart, however, and muscles and respiration, are excepted, and even exert themselves with an extraordinary degree of force. The blood is rarefied so as to take up more space upon the whole; and as there is more in the veins, and particularly in those of the brain, and spinal marrow, than in a state of vigilance, the medullary substance is hereby exposed to a constant uniform compression; whereas, in vigilance, the action of the muscles squeezes the blood out of the veins, and cools it, unless this action be violent, or long continued. ​The glands are filled during sleep, and consequently, by drawing off from the fulness of the blood-vessels, prepare the body for vigilance, and are themselves fitted for the functions to be then performed; i.e. to excrete their proper fluids from muscular compression, or vibrations running up their excretory ducts, in the manner to be hereafter explained. The medullary substance is, in like manner, fitted and prepared for vigilance, whether it be of a glandular nature, or not. However, some vibrations must take place in it throughout, and they are particularly vivid in the regions corresponding to the heart, organs of respiration, and organs of digestion; also in the regions corresponding to the eye and ear, where they excite the trains of images which are presented to us in our dreams. But the nature of these cannot be unfolded till we have treated of ideas, their generation, and associations, and the nature of true and erroneous judgments, assent, dissent, imagination, and memory.
Cor. II. It appears also to follow, from the foregoing account of sleep, and the effect of heat, labour, pain, and opiates, in disposing to it, that, in many cases of sleep, the medullary substance tends to a subtle kind of inflammation, and is preserved from it, and restored to its natural state and degree of heat, by means of sleep sufficiently continued. Thus, in the access of most fevers, the patient is listless and sleepy, the external senses, muscles, and brain, being affected, in some respects, as by opiates. If the patient sleep, the distemper is cut short; but if the subtle inflammation be so great as to prevent that, the distemper increases, and comes to its period in some other way, according to the nature of the fever, and circumstances of the patient. In a coma vigil it seems to me, that the approach of the opposite sides of the ventricles excites such violent vibrations, on account of the inflammation of the medullary substance, perhaps of these sides particularly, as to awake the patient, and throw him into great confusion and consternation. In a phrenzy, the medullary substance itself seems to labour under an acute temporary inflammation, the other parts having often no more than a due heat, whereas, in the delirium of a fever, the medullary substance only sympathizes with the other parts. If the inflammation of the medullary substance be very subtle, moderate, and permanent, madness of some species ensues. And it seems to agree very well with the theory here proposed, that in deliriums, phrenzies, and some kinds of madness, the patient does not sleep at all, or if he do, in a quiet manner, is freed from his distemper; and that, in other kinds of madness, and in cases of melancholy, the sleep is very deep, and the patient extremely sluggish.