Observations on Man (6th edition)/Part I/Chapter I/Section III
Section III
For first, Sensation, the perception of ideas, and a locomotive faculty, i.e. muscular motion, are the three most eminent marks of distinction between the animal and vegetable world: therefore, since it is already found that the two first are performed by the same means, i.e. vibrations, there is some presumption that the last will not require a different one.
Secondly, Of the two sorts of motion, viz. automatic and voluntary, the first depends upon sensation, the last upon ideas, as I shall shew particularly hereafter, and may appear, in general, to any one, upon a slight attention; whence it follows, that sensation, and automatic motion, must be performed in the same general manner, also the perception of ideas, and voluntary motion: and therefore, since sensation and perception, the two antecedents, agree in their causes, automatic and voluntary motion, the two consequents, i.e. all the four, must likewise.
Thirdly, It appears from the first and second propositions, that the white medullary substance is the common instrument of sensation, ideas, and motion; and by the fifth, that this substance is uniform and continuous every where. Hence it follows, that the subtle motions excited in the sensory nerves and medullary substance of the brain, during sensation and intellectual perception, must, of whatever kind they be, pass into the motory nerves; and when they are arrived there, it is probable that they must cause the contraction of the muscles, both because otherwise their arrival at the motory nerves would be superfluous, and because some such subtle motions are required for this purpose.
Cor. I. All arguments, therefore, which prove the performance of sensation and intellectual perception, by means of vibrations of the small medullary particles, must infer, that muscular motion is performed by vibrations also. And conversely, if vibrations can be shown to take place in muscular motion, they must also be instrumental in sensation and intellectual perception.
Cor. II. There are certain experiments and observations which favour the supposition of the performance of muscular motion by subtle agitations in the small particles of the muscular fibres, i.e. by vibratory motions. It follows therefore, that these experiments and observations are some additional evidence for the existence of sensory and ideal vibrations, as above explained. Such are, that the motion of the heart, and of other muscles, may be renewed in dying animals, and those that are newly dead, by heat, injection of a fluid, and punctures, it being easy to be conceived, that the two last causes should put the particles of the fibres into agitations for a short time, i.e. till they can recover their equilibrium, by altering their distances, and mutual actions: and the first cause, i.e. heat, is, by the common consent of all, judged to consist in, and to cause, subtle vibratory motions. It is also difficult to assign any other action which these causes can have. In like manner, the alternate contractions and relaxations of the hearts of frogs, vipers, and some other animals, which continue for long spaces of time after these have been entirely separated from their bodies, seem utterly inexplicable upon any of the common suppositions, but follow easily from the doctrine of vibrations, as it is applied to muscular motion, in the two next propositions.
Cor. III. Since the same motion which occasions sensation, and intellectual perception, passes through the seats of these into the motory nerves, in order to excite there the automatic and voluntary motions, thus pervading the whole medullary substance, in various ways, according to the variety of the circumstances, but in all with the greatest precision and exactness; it follows, that this must be a vibratory one, and that of the most subtle kind. For the same excess of softness, which renders the medullary substance totally inelastic as to sense, and consequently unfit for the grosser vibrations of the particles of the first or largest order (by the vibrations of which, in sonorous bodies, it seems that sound is excited in the air), may render it more susceptible of vibrations, in the particles of the second, third, &c. orders; and if we suppose a proper ultimate structure in the several parts of the medullary substance, these vibrations may be conveyed with all that precision and variety which the phænomena require. And unless we do suppose some such subtle vibrations as these, it will be extremely difficult to conceive, how so soft a pulp as the medullary substance is, should be the common instrument of sensation, thought, and motion; which yet all physicians and philosophers must allow, according to the first and second propositions. If we set aside subtle vibratory motions, the impulse of the objects of sense can communicate nothing, as it seems, to so soft a substance, but an uniform pressure, susceptible of few or no modifications, and consequently highly unsuitable to the great variety of the phænomena that are to be solved by it. This argument therefore tends to shew, that sensation, thought, and motion, must all be performed by vibrations.
In order to shew this, let us make the following suppositions:
First, That vibrations descend along the motory nerves, i.e. the nerves which go to the muscles, in some such manner as sound runs along the surfaces of rivers, or an electrical virtue along hempen strings.
Secondly, that these vibrations, when they arrive at the muscular fibres, are communicated to them, so that the small particles of these fibres shall be agitated with like vibrations.
Thirdly, That the vibrations thus excited in the fibres, put into action an attractive virtue, perhaps of the electrical kind, which lies concealed in the particles of the fibres, or in the blood-globules, or both. That the blood-globules of animals are electrical, may be conjectured from the electricity of those of the muscle-shell fish, observed by Dr. Hales; and that the red blood has a principal share in muscular contraction is highly probable, from the red colour of all the great muscles of the body, and from the weakness of all young animals, and of such as want a due share of red blood. At the same time, it appears from exanguious and transparent animals, that pale fibres, and colourless fluids, have all the necessary requisites for muscular contraction, in certain degrees.
Fourthly, We must now suppose, in consequence of the three foregoing suppositions, that each muscular fibre, and consequently the whole muscle, is made shorter by this increase of attraction in its particles; whilst yet their approach to each other is so small, as that the whole bulk of the muscle is but little diminished; for though the length of the muscle is lessened, its other dimensions are increased.
Fifthly, If we suppose the small ultimate fibres of the muscles to bend alternately to the right and left as an eel does, at exceedingly short intervals, agreeably to Dr. Lower, this may somewhat assist us to conceive in what manner a muscle may be shortened, and yet so increased in breadth and thickness, as to remain of nearly the same dimensions. For if these flexures be increased by the increase of the attraction of the parts, the whole muscle will become shorter and thicker, as it is found to be in contraction; and conversely, when the flexures are drawn out, the muscle will be longer and thinner, i.e. in a state of relaxation. The small wrinkles which have been observed in the muscular fibres, by Leeuwenhoek, and others, the wavings and curls which frequently appear to the eye in muscles, after boiling or roasting, and the rhomboidal pinnulae taken notice of by Dr. Hales in the abdominal muscles of a living frog, when under contraction, all seem to favour this fifth supposition.
Dr. Pemberton conjectures, that the cause of the contraction of muscular fibres is no other than the common cause of the cohesion of the small particles of the muscular fibres increased. And this seems very probable; for the muscles are hard during contraction, soft during relaxation; and hardness and softness are evidently nothing but variations in the cohesion of the small particles of bodies. Neither is this conjecture at all repugnant to the supposition of an electrical attraction above made, or to the doctrine of vibrations; for electricity may reach to small distances, without being excited by friction, and flow from the same principle as the cohesion of bodies, as Sir Isaac Newton has observed. It may therefore be the general cause of cohesion, and may be excited in the muscular fibres in an extraordinary degree, whenever extraordinary vibrations are communicated to them. Or, if we suppose the cause of cohesion to be something distinct from electricity, it may, however, be increased by vibrations of the small cohering particles.
For, when the fibres are in a state of contraction, they are hard; and this hardness, if it be supposed to extend to the small particles (which is no unreasonable supposition), must render the particles of these particles, i.e. the particles supposed in these propositions to be agitated with vibrations, indisposed to receive these vibrations; but the free admission of these vibrations is by supposition the cause which excites the attractions of the particles, and the consequent contraction of the muscle. It follows therefore, that the hardness which impedes these vibrations, must also lessen the attraction and contraction; or, in other words, that the contraction of a muscle, when carried to a certain degree, must check itself, and bring on a relaxation, after a time sufficient for the proper causes to take effect.
In like manner, when a muscle is relaxed, the vibrations which descend along the motory nerves pass freely into the muscular fibres, increase the attractions of the particles, and bring on the opposite state, that of contraction; and so on alternately.
The fibres of the relaxed muscle may also be considered as under a state of distention to a certain degree, and, consequently, as liable to an increase of vibrations upon this account. To which we may farther add, that since vibrations are hindered from passing into the contracted muscle, in the manner just now explained, they will pass with greater force into the relaxed one, from the place of the common derivation of their nerves, wherever there are antagonist muscles that derive nerves from the same trunk, as in the limbs, and muscles of respiration.
Cor. It appears from this method of considering the contractions and relaxations of muscles, that there is a certain degree of hardness or contraction in muscular fibres, which may be supposed just to balance each degree of force with which vibrations descend into the muscular fibres; and that while this equilibrium subsists, the contraction can neither be increased nor abated.
In order to make this appear, it will be proper to distinguish the motory vibrations, or those which descend along the nerves of the muscles into their fibres, into the five following classes:
First, then, We are to conceive, that those sensory vibrations which are excited in the external organs, and ascend towards the brain, when they arrive, in their ascent, at the origins of motory nerves, as they arise from the same common trunk, plexus, or ganglion, with the sensory ones affected, detach a part of themselves at each of these origins down the motory nerves; which part, by agitating the small particles of the muscular fibres, in the manner explained in the sixteenth proposition, excites them to contraction.
Secondly, The remainder of the sensory vibrations, which arrives at the brain, not being detached down the motory nerves in its ascent thither, must be diffused over the whole medullary substance. It will therefore descend from the brain into the whole system of motory nerves, and excite some feeble vibrations, at least, in them. The same may be observed of ideal vibrations, generated in the brain by association; these must pervade the whole medullary substance, and, consequently, affect all the motory nerves in some degree.
Thirdly, The heat of the blood and pulsation of the arteries, which pass through the medullary substance, must always excite, or keep up, some vibrations in it; and these must always descend into the whole system of muscles. And I apprehend, that, from these two last sources taken together, we may account for that moderate degree of contraction, or tendency thereto, which is observable in all the muscles, at least in all those of healthy adults, during vigilance.
Fourthly, When vivid vibrations are excited in membranes of an uniform texture, by a stimulus of any kind, they seem to run over the whole extent of such membranes, and by this means to have a great influence in contracting all the muscles that lie near any part of this membrane, though they be remote from the place of the stimulus. The manner in which this is effected, I conceive to be as follows:—The repeated or continued action of the stimulus diffuses vibrations from the place of its action over the whole membrane, which, by their reciprocal influences, become equal, or nearly so, in every part of it, and are, at last, so exalted, as to contract every part. As soon as this contraction takes place, the vibrations in its small particles must cease, for reasons given above. They will therefore be propagated almost instantaneously over the neighbouring muscles, from the nervous communications between the membrane and the neighbouring muscles; by which all changes made in the nerves of the membrane must affect those of the neighbouring muscles. As therefore during the vivid vibrations of the particles of the membrane, we must suppose some to be propagated into the neighbouring muscles, agreeably to the first article of this proposition, so upon their sudden cessation, such a change may reasonably be supposed, in the communicating nervous fibrils, as shall agitate the æther contained in them with much more vivid vibrations than before; and these vibrations must now pass into the muscles alone, since the contraction of the membrane hinders them from returning into it. I shall hereafter produce several examples of this process, in detail. It may suffice, at present, just to mention the action of sneezing, and to desire the reader to compare this action, in a cursory way, with the foregoing account.
Fifthly, I have, in the last article, shewn how a cessation of vibrations in the particles of a membrane may increase those in the neighbouring muscles. But it seems also, that a cessation of vibrations in any other considerable part of the body, from whatever cause it proceeds, has a like tendency; and that this tendency is deducible from the change made in the nerves of the part affected, and thence propagated into the communicating branches, or even into the whole medullary substance. The yawnings and stretchings of persons disposed to sleep, the convulsive respiration of those that are just fallen asleep, and the convulsive motions which attend the extinction of the senses in epileptic fits, and the near approaches of death, may be derived, perhaps, in part, from this source, in part from some of the foregoing.
The particular detail of this obscure and intricate matter will be attempted in the proper places of the next chapter, which will contain the application of the general positions concerning sensation and motion, in this, to each of the most remarkable phænomena, considered separately. I will, however, present the reader here with a short sketch, to enable him to form some notion of the manner and plausibility of the attempt.
The ordinary motions of the heart appear to arise from the second and third classes of motory vibrations, mentioned in the last proposition; and it is remarkable, that its motions are found to be, in general, and cæteris paribus, stronger or weaker, as the sum total of these two classes is greater or less. The systole and diastole succeed each other, from the causes assigned in the seventeenth proposition. We are to conceive, however, that both the influx of the venal blood into the ventricles, and of the arterial into the coronary vessels, have a considerable share in bringing on the systole, in the way of distention and irritation.
May we not conjecture, from that experiment of Dr. Hook’s, in which he kept a dog alive, by a mere continued stream of fresh air passing through the lungs, without any such alternate motion of the chest as takes place in common respiration, that one principal use of the air, which is an electric per se, in respiration, is to restore to the blood, as it passes through the lungs, that electricity which it has lost in circulating through the body? For, upon this supposition, the blood which arrives at the left ventricle, will, in cessations of respiration, and also where foul air is respired, want its due electricity; whence, according to Prop. XVI. the muscles, and especially the heart, will want one of the principal requisites for contraction. However, convulsive motions may ensue after a syncope, from the fifth class of motory vibrations.
It is remarkable here, that the hearts of frogs, vipers, and several other such animals, as can live in great degrees of cold, and without respiration, continue to beat, as has been taken notice of above, for a long time after they are taken out of their bodies. We must therefore suppose, that the fibres of their hearts, and the blood globules which remain in them, are endued with an electric, or other attractive virtue, of a more durable kind than the fibres and blood globules of the more perfect animals; also, that this virtue may be put into action by a less degree of heat. All which is very agreeable to the other circumstances of their economy.
Respiration and crying are excited in the new-born child from the cold handling of the midwife, and other vivid sensations impressed immediately upon its coming into the world. These vivid sensations put the whole system of muscles, or at least those of the trunk and larynx, into action at once, as far as their mutual antagonism will permit, the stronger set of conspiring muscles overpowering the weaker for a certain short time, and then, after their force is exhausted, according to Prop. XVII. giving way for a shorter time to the weaker. But this alternate action of the muscles of the trunk and larynx will be an imperfect kind of respiration, with crying, as may be easily seen from the disposition of the muscles. Respiration is afterwards kept up, partly by the propensity of the muscles to alternate action, explained Prop. XVII.; partly, perhaps, by the power of habit, i.e. association; partly by the renewal of vivid impressions; and partly, as it seems, by vibrations excited in the pleura and peritonæum, and thence communicated to the diaphragm, and to the muscles of the breast and belly.
That the last cause has a real efficacy, may appear from the following instance:—Let respiration be supposed to be at a stand for a small time, on account of the person’s running, or exerting an act of great strength. It is evident, that the blood will both be accumulated in the lungs, and heated there, during this interruption of respiration, since respiration both ventilates the blood, and promotes its motion through the lungs. The external membrane of the lungs will therefore be both distended and heated, i.e. will have an increase of vibrations communicated to it. But this membrane is continuous to the pleura, and, indeed, is the same membrane with it. An increase of vibrations will therefore be communicated to the pleura, and consequently to the diaphragm, and muscles of the breast, which it invests.
The peristaltic motion of the intestines is, in part, to be deduced from the second and third classes of motory vibrations, in the same manner as the motion of the heart, since that motion, like this, returns at intervals incessantly. And there is reason to believe, that vigorous vibrations, either of the sensory or ideal kind, impart an extraordinary degree of activity to the stomach and bowels. However, they derive also a great part of their motions, probably the major part, from the impressions which the aliment, bile, and fæces, make upon the villous coat; the vibrations excited by these impressions both running directly into the muscular coat, for the purpose of contracting that part which adjoins to the seat of impression, and also running upwards and downwards along the villous coat, so as to exert some efficacy at a distance from this seat.
It is very remarkable, that the pale fibres of the intestines, in men, and many other animals, preserve their power of alternate contraction and relaxation for a considerable time after death, whereas the red fleshy muscles of the same animals lose theirs soon after the effusion of their blood. It is a phænomenon of a like kind with this, that the whole muscular system of some animals that are exanguious, or nearly so, retain their activity for a considerable time after these animals are cut into pieces. And both may serve to intimate that the electricity, or other attractive virtue, of pale fibres and fluids, at the same time that it is feebler than that of red ones, is however, of a more durable kind, and, as was observed above of the hearts of frogs and vipers, capable of being put into action by a less degree of heat.
The actions of sneezing, swallowing, coughing, hiccoughing, vomiting, and expelling the fæces and urine, with others of a like nature, are to be deduced from the first and fourth classes of motory vibrations; i.e. either from those vibrations which first ascend up the sensory nerves, and then are detached down the motory nerves, which communicate with these by some common trunk, plexus, or ganglion, or else from those vibrations that run along the surfaces of uniform membranes, and so affect all the muscles which lie contiguous to any part of these membranes. It is a strong argument in favour of the hypothesis here delivered, that all the above-mentioned motions arise in the neighbourhood of vivid sensations, increase when they increase, and languish when they languish.
In examining this hypothesis by the actions of sneezing, swallowing, and coughing, regard must be had to the nose, uvula, and epiglottis, respectively, as being extreme and pointed parts, and consequently liable to be affected with extraordinary vibrations, agreeably to the ninth phænomenon of the sixth proposition.
In like manner, the numerous plexuses and ganglions of the eight pair of nerves and intercostal nerve must have great influence in the motions and functions of the parts contained in the thorax and belly.
As the motory vibrations of the second and third classes are of a gentle kind, for the most part, and descend constantly into the whole system of the muscles, it may be expected, that young children should move all their limbs at times, with some irregular kind of succession, from this cause. And this seems to be the fact. Strong contractions of the limbs are often excited by frictions, gripes, and other vivid sensations; but then the motory vibrations here are those of the first and fourth classes. General convulsions, from acidities, and other irritations in the bowels, seem to be excited in the same way, the intercostal nerve serving to communicate the vibrations with more readiness to the muscles of the trunk and limbs.
It appears to me also, that the intercostal nerve, which makes those of each side a separate system, as it were, has some share in determining hemiplegias to one side. In like manner, the great brachial and crural ganglions make all the nerves of the same limb sympathize with one another.
Whether the nerves of the same names throughout the body have not some sympathetic influences over each other, may be doubted. If those of the right side arise from the left part of the brain, and vice versâ, which seems to be the opinion of the best anatomists, then one would imagine, that the homonymous nerves of the right and left sides must, in crossing over, lie somewhere contiguous to each other, and so impart vibrations to each other. And there seem to be some facts from whence this may be inferred; but we cannot expect to be able to distinguish, with certainty, so feeble an influence, amidst so many others that are far stronger.
Yawning and stretching may, perhaps, when considered in all their circumstances, take in all the five classes of motory vibrations. When they happen in the attacks of fever-fits, and other morbid cases, the first seems to be owing to pretty sudden and strong contractions in the membrane of the mouth, fauces, aspera arteria, and œsophagus; the last to contractions in the whole skin.
As the bowels derive their peristaltic motion, in part, from the second and third classes; so it seems that the secretory and excretory vessels of the glands must be constantly agitated with a like motion, from the same causes, performing their ordinary secretions and excretions thereby. Their extraordinary ones are generally owing to irritations in the membranes, in which the mouths of their excretory vessels lie. And this agrees remarkably with the doctrine of vibrations. For the vivid vibrations excited in the membranes by the irritating cause must diffuse themselves every way; and when they come to the mouths of the excretory vessels, penetrate them, and, by passing up into the vessels, both excretory and secretory, greatly increase their peristaltic motion, and, by consequence, their secretions and excretions. All this seems equally to hold, in respect of the exhaling and absorbing vessels dispersed throughout the body.
The external motions of the eyes in young children are probably owing, in part at least, to the immediate action of light upon the tendinous expansions of the four strait muscles, and particularly upon those of the adducens and abducens. But the light which passes through the pupil seems also to have some share, as will be shewn hereafter. As to the internal motions, it appears, that the light which falls upon the cornea and uvea must excite the greater and lesser rings to contraction, in proportion to its strength; and, consequently, prepare the eye to see distinctly, at different distances, in the manner explained by Dr. Jurin. The hypothesis of this proposition does therefore give and receive light from his ingenious theory of this matter.
The two muscles which relax the membrana tympani, are much more exposed to the air than the musculus internus, or the musculus stapedis. When therefore the air is agitated with strong vibrations, as in loud sounds, it will excite the first-named muscles to action, and consequently relax the membrana tympani, as it ought to do. For what reasons the last-named muscles are contracted in feeble sounds, is a question of a more difficult nature, as is the parallel one in the eye, viz. why the radiated fibres of the uvea are contracted in small degrees of light, so as then to dilate the pupil.
The reader is desired to take notice, that, in all the instances of this proposition, I consider the motions as merely automatic. Their voluntary and semi-voluntary state will be accounted for in the two next propositions.
This proposition is the immediate consequence of admitting the doctrines of vibrations and association, in the manner in which they have been asserted in the foregoing propositions. It contains the theory of the voluntary and semi-voluntary motions; to facilitate the application of which theory in the next proposition, I shall deliver the principal cases of this, in the following corollaries.
Cor. I. The motory vibrations of the five classes mentioned Prop. XVIII. will generate a propensity to corresponding motory vibratiuncles.
Cor. II. These motory vibratiuncles will affect the brain, as well as the motory nerves along which they descend; and, indeed, their descent along the motory nerves will be principally owing to their being first excited in the brain. This is sufficiently evident in the motory vibratiuncles which are derived from the motory vibrations of the second and third classes. As to the motory vibrations of the other classes, it is evident, that the brain is strongly affected by the sensory vibrations which give birth to them, and consequently, that a proportional affection of the brain must take place in the motory vibratiuncles derived from them.
Cor. III. The motory vibratiuncles will cohere to one another, by associations both synchronous and successive. Hence the simple parts, of which complex and decomplex motions are compounded, may cohere closely, and succeed readily to each other.
Cor. IV. The motory vibratiuncles will also cohere to ideal ones by association. Common ideas may therefore excite motory vibratiuncles, and consequently be able to contract the muscles, provided the active powers lodged in their fibres and blood globules be sufficiently exalted for this purpose.
Cor. V. If we suppose the ideal vibratiuncles to be so much increased from the causes mentioned, Prop. XIV. as to be equal in strength to the usual sensory vibrations, the motory vibratiuncles connected with them by aossciation must be supposed to be increased proportionably. Hence ideas may occasion muscular motions of the same strength with the automatic motions.
Cor. VI. The third and last connexion of the motory vibratiuncles is that with sensory vibrations, foreign to them, i.e. such as had no share in generating the motory vibratiuncles under consideration. Particular motions of the body may therefore by association be made to depend upon sensations, with which they have no natural and original connexion.
Cor. VII. As muscular motion has three connexions deducible from association, viz. those mentioned in the third, fourth, and sixth corollaries, so the sensations and ideas have the same three connexions. Hence the whole doctrine of association may be comprised in the following theorem, viz.
If any sensation A, idea B, or muscular motion C, be associated for a sufficient number of times with any other sensation D, idea E, or muscular motion F, it will, at last, excite d, the simple idea belonging to the sensation D, the very idea E, or the very muscular motion F.
The reader will observe, that association cannot excite the real sensation D, because the impression of the sensible object is necessary for this purpose. However, in certain morbid cases, the idea is magnified so as to equal, or even overpower, sensible impressions.
In order to verify this proposition, it is necessary to inquire, what connexions each automatic motion has gained by association with other motions, with ideas, or with foreign sensations, according to the third, fourth, and sixth corollaries of the last proposition, so as to depend upon them, i.e. so as to be excited no longer, in the automatic manner described in the nineteenth proposition, but merely by the previous introduction of the associated motion, idea, or sensation. If it follow that idea, or state of mind (i.e. set of compound vibratiuncles), which we term the will, directly, and without our perceiving the intervention of any other idea, or of any sensation or motion, it may be called voluntary, in the highest sense of this word. If the intervention of other ideas, or of sensations and motions (all which we are to suppose to follow the will directly), be necessary, it is imperfectly voluntary; yet still it will be called voluntary, in the language of mankind, if it follow certainly and readily upon the intervention of a single sensation, idea, or motion, excited by the power of the will: but if more than one of these be required, or if the motion do not follow with certainty and facility, it is to be esteemed less and less voluntary, semi-voluntary, or scarce voluntary at all, agreeably to the circumstances. Now, if it be found, upon a careful and impartial inquiry, that the motions which occur every day in common life, and which follow the idea called the will, immediately or mediately, perfectly or imperfectly, do this, in proportion to the number and degree of strength in the associations, this will be sufficient authority for ascribing all which we call voluntary in actions to association, agreeably to the purport of this proposition. And this, I think, may be verified from facts, as far as it is reasonable to expect, in a subject of inquiry so novel and intricate.
In the same manner as any action may be rendered voluntary, the cessation from any, or a forcible restraint upon any, may be also, viz. by proper associations with the feeble vibrations in which inactivity consists, or with the strong action of the antagonist muscles.
After the actions, which are most perfectly voluntary, have been rendered so by one set of associations, they may, by another, be made to depend upon the most diminutive sensations, ideas, and motions, such as the mind scarce regards, or is conscious of; and which therefore it can scarce recollect the moment after the action is over. Hence it follows, that association not only converts automatic actions into voluntary, but voluntary ones into automatic. For these actions, of which the mind is scarce conscious, and which follow mechanically, as it were, some precedent diminutive sensation, idea or motion, and without any effort of the mind, are rather to be ascribed to the body than the mind, i.e. are to be referred to the head of automatic motions. I shall call them automatic motions of the secondary kind, to distinguish them both from those which are originally automatic, and from the voluntary ones; and shall now give a few instances of this double transmutation of motions, viz. of automatic into voluntary, and of voluntary into automatic.
The fingers of young children bend upon almost every impression which is made upon the palm of the hand, thus performing the action of grasping in the original automatic manner. After a sufficient repetition of the motory vibrations which concur in this action, their vibratiuncles are generated, and associated strongly with other vibrations or vibratiuncles, the most common of which, I suppose, are those excited by the sight of a favourite plaything which the child uses to grasp, and hold in his hand. He ought, therefore, according to the doctrine of association, to perform and repeat the action of grasping, upon having such a plaything presented to his sight. But it is a known fact, that children do this. By pursuing the same method of reasoning, we may see how, after a sufficient repetition of the proper associations, the sound of the words grasp, take hold, &c. the sight of the nurse’s hand in a state of contraction, the idea of a hand, and particularly of the child’s own hand, in that state, and innumerable other associated circumstances, i.e. sensations, ideas, and motions, will put the child upon grasping, till, at last, that idea, or state of mind which we may call the will to grasp, is generated and sufficiently associated with the action to produce it instantaneously. It is therefore perfectly voluntary in this case; and by the innumerable repetitions of it in this perfectly voluntary state, it comes, at last, to obtain a sufficient connexion with so many diminutive sensations, ideas, and motions, as to follow them in the same manner, as originally automatic actions do the corresponding sensations, and consequently to be automatic secondarily. And in the same manner, may all the actions performed with the hands be explained, all those that are very familiar in life passing from the original automatic state through the several degrees of voluntariness till they become perfectly voluntary, and then repassing through the same degrees in an inverted order, till they become secondarily automatic on many occasions, though still perfectly voluntary on some, viz. whensoever an express act of the will is exerted.
I will, in the next place, give a short account of the manner in which we learn to speak, as it may be deduced from the foregoing proposition. The new-born child is not able to produce a sound at all, unless the muscles of the trunk and larynx be stimulated thereto by the impression of pain on some part of the body. As the child advances in age, the frequent returns of this action facilitate it; so that it recurs from less and less pains, from pleasures, from mere sensations, and lastly from slight associated circumstances, in the manner already explained. About the same time that this process is thus far advanced, the muscles of speech act occasionally, in various combinations, according to the associations of the motory vibratiuncles with each other. Suppose now the muscles of speech to act in these combinations at the same time that sound is produced from some agreeable impression, a mere sensation, or a slight associated cause, which must be supposed to be often the case, since it is so observable, that young children, when in a state of health and pleasure, exert a variety of actions at the same time. It is evident, that an articulate sound, or one approaching thereto, will sometimes be produced by this conjoint action of the muscles of the trunk, larynx, tongue, and lips; and that both these articulate sounds, and inarticulate ones, will often recur, from the recurrence of the same accidental causes. After they have recurred a sufficient number of times, the impression, which these sounds, articulate and inarticulate, make upon the ear, will become an associated circumstance (for the child always hears himself speak, at the same time that he exerts the action) sufficient to produce a repetition of them. And thus it is, that children repeat the same sounds over and over again, for many successions, the impression of the last sound upon the ear exciting a fresh one, and so on, till the organs be tired. It follows, therefore, that if any of the attendants make any of the sounds familiar to the child, he will be excited from this impression, considered as an associated circumstance, to return it. But the attendants make articulate sounds chiefly; there will therefore be a considerable balance in favour of such, and that of a growing nature: so that the child’s articulate sounds will be more and more frequent every day—his inarticulate ones grow into disuse. Suppose now, that he compounds these simple articulate sounds, making complex ones, which approach to familiar words at some times, at others such as are quite foreign to the words of his native language, and that the first get an ever-growing balance in their favour, from the cause just now taken notice of; also, that they are associated with visible objects, actions, &c. and it will be easily seen, that the young child ought, from the nature of association, to learn to speak much in the same manner as he is found in fact to do. Speech will also become a perfectly voluntary action, i.e. the child will be able to utter any word or sentence proposed to him by others, or by himself, from a mere exertion of the will, as much as to grasp: only here the introductory circumstance, viz. the impression of the sound on the ear, the idea of this sound, or the preceding motion in pronouncing the preceding word is evident; and therefore makes it probable, that the same thing takes place in other cases. In like manner, speech, after it has been voluntary for a due time, will become secondarily automatic, i.e. will follow associated circumstances, without any express exertion of the will.
From the account here given of the actions of handling and speaking, we may understand in what manner the first rudiments are laid of that faculty of imitation, which is so observable in young children. They see the actions of their own hands, and hear themselves pronounce. Hence the impressions made by themselves on their own eyes and ears become associated circumstances, and consequently must, in due time, excite to the repetition of the actions. Hence like impressions made on their eyes and ears by others will have the same effect; or, in other words, they will learn to imitate the actions which they see, and the sounds which they hear.
In the same manner may be explained the evident powers which the will has over the actions of swallowing, breathing, coughing, and expelling the urine and fæces, as well as the feeble and imperfect ones over sneezing, hiccoughing, and vomiting. As to the motion of the heart, and peristaltic motion of the bowels, since they are constant, they must be equally associated with every thing, i.e. peculiarly so with nothing, a few extraordinary cases excepted. They will therefore continue to move solely in the original automatic manner, during the whole course of our lives. However, association may, perhaps, have some share in keeping these motions, and that of respiration, up for a time, when the usual automatic causes are deficient in any measure; and may thus contribute to their equability and constancy. It seems certain, at least, that where unequable and irregular motions of the heart and bowels are generated, and made to recur for a sufficient number of times, from their peculiar causes, in full quantity, a less degree of the same causes, or even an associated circumstance, will suffice to introduce them afterwards. And the same thing may be observed of hysteric and epileptic fits. These recur from less and less causes perpetually, in the same manner, and for the same reasons, as original automatic motions are converted into voluntary ones.
I will add one instance more of the transition of voluntary actions into automatic ones of the secondary kind, in order to make that process clearer, by having it singly in view. Suppose a person who has a perfectly voluntary command over his fingers, to begin to learn to play upon the harpsichord; the first step is to move his fingers from key to key, with a slow motion, looking at the notes, and exerting an express act of volition in every motion. By degrees the motions cling to one another, and to the impressions of the notes, in the way of association so often mentioned, the acts of volition growing less and less express all the time, till at last they become evanescent and imperceptible. For an expert performer will play from notes, or ideas laid up in the memory, or from the connexion of the several complex parts of the decomplex motions, some or all; and, at the same time, carry on a quite different train of thoughts in his mind, or even hold a conversation with another. Whence we may conclude, that the passage from the sensory, ideal, or motory vibrations which precede, to those motory ones which follow, is as ready and direct, as from the sensory vibrations to the original automatic motions corresponding to them; and consequently, that there is no intervention of the idea, or state of mind, called will. At least, the doctrine of association favours this, and the fact shews, that there is no perceptible intervention, none of which we are conscious.
And thus, from the present proposition, and the nineteenth taken together, we are enabled to account for all the motions of the human body, upon principles which, though they may be fictitious, are, at least, clear and intelligible. The doctrine of vibrations explains all the original automatic motions, that of association the voluntary and secondarily automatic ones. And if the doctrine of association be founded in, and deducible from, that of vibrations, in the manner delivered above, then all the sensations, ideas, and motions, of all animals, will be conducted according to the vibrations of the small medullary particles. Let the reader examine this hypothesis by the facts, and judge for himself. There are innumerable things, which, when properly discussed, will be sufficient tests of it. It will be necessary, in examining the motions, carefully to distinguish the automatic state from the voluntary one, and to remember, that the first is not to be found pure, except in the motions of the new-born infant, or such as are excited by some violent irritation or pain.
Cor. I. The brain, not the spinal marrow, or nerves, is the seat of the soul, as far as it presides over the voluntary motions. For, by Cor. II. of the last proposition, the efficacy of the motory vibratiuncles depends chiefly on that part of them which is excited within the brain.
Cor. II. The hypothesis here proposed is diametrically opposite to that of Stahl, and his followers. They suppose all animal motions to be voluntary in their original state, whereas this hypothesis supposes them all to be automatic at first, i.e. involuntary, and to become voluntary afterwards by degrees. However, the Stahlians agree with me concerning the near relation of these two sorts of motion to each other, as also concerning the transition (or rather return, according to my hypothesis) of voluntary motions into involuntary ones, or into those which I call secondarily automatic. As to final causes, which are the chief subject of inquiry amongst the Stahlians, they are, without doubt, every where consulted, in the structure and functions of the parts; they are also of great use for discovering the efficient ones. But then they ought not to be put in the place of the efficient ones; nor should the search after the efficient be banished from the study of physic, since the power of the physician, such as it is, extends to these alone. Not to mention, that the knowledge of the efficient causes is equally useful for discovering the final, as may appear from many parts of these observations.
Cor. III. It may afford the reader some entertainment, to compare my hypothesis with what Des Cartes and Leibnitz have advanced, concerning animal motion, and the connexion between the soul and body. My general plan bears a near relation to theirs. And it seems not improbable to me, that Des Cartes might have had success in the execution of his, as proposed in the beginning of his Treatise on Man, had he been furnished with a proper assemblage of facts from anatomy, physiology, pathology, and philosophy, in general. Both Leibnitz’s preestablished harmony, and Malebranche’s system of occasional causes, are free from that great difficulty of supposing, according to the scholastic system, that the soul, an immaterial substance, exerts and receives a real physical influence upon and from the body, a material substance. And the reader may observe, that the hypothesis here proposed stands clear also of this difficulty. If he admit the simple case of the connexion between the soul and body, in respect of sensation, as it is laid down in the first proposition, and only suppose, that there is a change made in the medullary substance, proportional and correspondent to every change in the sensations, the doctrine of vibrations, as here delivered, undertakes to account for all the rest, the origin of our ideas and motions, and the manner in which both the sensations and these are performed.
Cor. IV. I will here add Sir Isaac Newton’s words, concerning sensation and voluntary motion, as they occur at the end of his Principia, both because they first led me into this hypothesis, and because they flow from it as a corollary. He affirms then, “both that all sensation is performed, and also the limbs of animals moved in a voluntary manner, by the power and actions of a certain very subtle spirit, i.e. by the vibrations of this spirit, propagated through the solid capillaments of the nerves from the external organs of the senses to the brain, and from the brain into the muscles.”
Cor. V. It follows, from the account here given of the voluntary and semi-voluntary motions, that we must get every day voluntary and semi-voluntary powers, in respect of our ideas and affections. Now this consequence of the doctrine of association is also agreeable to the fact. Thus we have a voluntary power of attending to an idea for a short time, of recalling one, of recollecting a name, a fact, &c. a semi-voluntary one of quickening or restraining affections already in motion, and a most perfectly voluntary one of exciting moral motives, by reading, reflection, &c.
For the motions which are previous and subservient to the obtaining of pleasure, and the removal of pain, will be much more frequent, from the very instant of birth, than those which occasion pain. The number also of the first will be perpetually increasing, of the last decreasing. Both which positions may be evinced by the following arguments:
First, The pleasures are much more numerous than the pains. Hence the motions which are subservient to them are much more numerous also.
Secondly, The associated circumstances of the pleasures are many more in number than the pleasures themselves. But these circumstances, after a sufficient association, will be able to excite the motions subservient to the pleasures, as well as these themselves. And this will greatly augment the methods of obtaining pleasure.
Thirdly, It favours the position here advanced, that the motions subservient to pleasure are of a moderate nature; and therefore, that they can be excited with the more ease, both in an automatic and voluntary manner.
Fourthly, The pains, and consequently the motions subservient to them, are few, and of a violent nature. These motions are also various, and therefore cannot be united to objects and ideas with constancy and steadiness; and, which is most to be regarded, they end, at last, from the very make of the body, in that species of motion which contributes most to remove or assuage the pain. This species therefore, since it recurs the most frequently, and continues longest, must be confirmed by association, to the exclusion of the rest.
Cor. I. Many changes in the actions of young children, very difficult to be explained, according to the usual methods of considering human actions, appear to admit of a solution from this proposition. These changes are such as tend to the ease, convenience, pleasure, of the young child; and they are sufficiently observable in the transition of the originally automatic actions into voluntary ones, as matters of fact, whatever be determined concerning their cause. I shall therefore refer to them occasionally, in the course of these papers, as allowed matters of fact.
Cor. II. It seems also, that many very complex propensities and pursuits in adults, by which they seek their own pleasure and happiness, both explicitly and implicitly, may be accounted for, upon the same, or such-like principles.
Cor. III. To similar causes we must also refer that propensity to excite and cherish grateful ideas and affections, and trains of these, which is so observable in all mankind. However, this does not hold in so strict a manner, but that ungrateful trains will present themselves, and recur on many occasions, and particularly whenever there is a morbid, and somewhat painful, state of the medullary substance.
Cor. IV. Since God is the source of all good, and consequently must at last appear to be so, i.e. be associated with all our pleasures, it seems to follow, even from this proposition, that the idea of God, and of the ways by which his goodness and happiness are made manifest, must, at last, take place of, and absorb all other ideas, and he himself become, according to the language of the Scriptures, All in all.
Cor. V. This proposition, and its corollaries, afford some very general, and perhaps new, instances of the coincidence of efficient and final causes.
Cor. VI. The agreement of the doctrines of vibrations and association, both with each other, and with so great a variety of the phænomena of the body and mind, may be reckoned a strong argument for their truth.