Observations on Man (6th edition)/Part I/Chapter II/Section VII
Section VII
[edit]OF OTHER MOTIONS, AUTOMATIC AND VOLUNTARY, NOT CONSIDERED IN THE FOREGOING SECTIONS OF THIS CHAPTER.
Here I observe,
First, that the motion of the heart is constant, more equable than any other in the body, and cannot be supposed to proceed from impressions made on the neighbouring parts; which things agree well together upon the supposition of the doctrine of vibrations: for thus it cannot proceed from the first or fourth source of motory vibrations; and if it proceed from the second and third, it ought to be constant, and nearly equable.
Secondly, We are not to inquire, how the heart is first put into motion in the foetus, but only how its ordinary motions may be continued, by means of vibrations descending from the brain, after they are once begun.
Now, for this purpose, let us suppose, that the auricles, with the beginnings of the pulmonary artery and aorta, have finished their contractions; and that the ventricles and coronary vessels are full. It follows, that the ventricles will now be excited to contraction by three causes; viz. by the vibrations which have continued to descend freely into their fibres ever since their last contraction was finished, by their distention from fulness, and by the motion and impulse of the blood in the coronary vessels: but from the moment that the ventricles begin to contract, the auricles and arteries will begin to fill, and will be fullest at the instant when their contraction is finished. The auricles and arteries must therefore begin to contract at this instant, from descending vibrations and distention, and the ventricles to be relaxed and filled, the vibrations in their fibres being checked by their past contraction. Let these actions be completed, and the heart will be in the circumstances first supposed; i.e. the ventricles will be ready for a fresh contraction, and so on alternately, as long as life continues.
Thirdly, The vibrations which keep up the heart’s motion, are nearly allied to those arising from heat; for the second source of motory vibrations is the residue of all the vibrations excited in the various parts of the body; and the third arises chiefly from the heat of the blood. We may expect therefore, that the heart should contract either more frequently, or more strongly, or both, when the body is heated. And thus it is, as may appear from observations upon the pulse in fevers, in sleep, after eating, in pains or distempers attended with an increase of heat, &c. in which the motion of the blood is increased; whereas in nervous pains attended with coldness of the extremities, the pulse is low and slow.
Fourthly, In the declension of fevers the pulse is quick and feeble. It is feeble, because the whole body is so; and quick, partly from the new habit superinduced by the heat in the beginning of the fever; partly, because in fevers the heart is always kept nearly full, i.e. nearly at such a degree of distention as incites it to contraction, the principal causes of which are the weakness and inactivity of the body: hence in general the pulse is quick and feeble in persons of relaxed habits; the contrary in strong ones.
Fifthly, The pulse is quick in young and small animals; slow in old and large ones. For this, various reasons may be assigned; as first, that if the velocity wherewith the sides of the heart move towards each other be the same, the contraction must be sooner accomplished in small hearts than in large ones. Secondly, that the fibres in young animals are irritable, and soon excited to contraction, by distention, &c. Thirdly, that the contraction is performed slowly in old animals; and fourthly, that short fibres are perhaps sooner excited to contraction than long ones, it being necessary perhaps, that the vibrations should be reverberated from each end of the fibres, for many successions, before they can rise to a certain pitch. It agrees with this, that reciprocal motions are more frequent in general, and cæteris manentibus, as the animal is less; that the limbs have both long and short flexors and extensors, the first for great degrees of motion, the last for making a quick beginning; that the capsular ligaments of the joints have short muscular fibres inserted into them, in order to keep them from being pinched between the bones in the motions of the joints, as Winslow has observed; which they could not do, had not their contraction the start of the contractions which move the joint; and lastly, that the fibres which compose the heart, are all of equal lengths, according to Dr. Stewart’s analysis of them. See Phil. Trans. No. 460.
Sixthly, The heart may move incessantly without fatigue, if we only suppose the recruits to be sufficient, and the degree of motion to be within due limits. And it may be, that in labouring men the muscles of the limbs are as much exerted upon the whole as the heart. The warmth in which the heart is kept, and its receiving nerves from the eighth pair and intercostal, which seem to be particularly exempted from venal compression, deserve notice here: but the constant motion of the blood is principally to be considered, being the cause as well as the effect of the constant motion of the heart.
Seventhly, Since lying down prevents or alleviates the fainting which sometimes happens during bleeding, one may suppose that this fainting, and the consequent abatement or cessation of the motion of the heart arise, because a quantity of blood, sufficient to keep up the vibrations in the brain, could not ascend thither in an erect posture.
Eighthly, Sudden and violent pains, such as those from wounds, increase the motions of the heart. This may be accounted for from the violent vibrations which ascend to the brain, and are thence propagated to the heart. But may not vibrations ascend also directly from the wounded part, along the course of the arteries and veins, to the heart?
Ninthly, All the passions of the mind increase the motion of the heart, as might be expected: but melancholy, when it makes men inactive, and uninfluenced by the impressions of objects, has a contrary effect, which is equally suitable to the foregoing theory.
Tenthly, In mortifications, also in the languid state that succeeds acute distempers, &c. the pulse sometimes intermits, suppose once in 5, 10, 20, &c. times, the interval being about double, and the strength of the pulse which succeeds the intermission about double also. Here the force of the heart seems to languish, and the time of contraction of the ventricle to be protracted, till a second contraction of the auricle intervenes, and protracts the ventricle’s contraction still farther, viz. to nearly twice the interval. The heart must therefore be surcharged, and contracting stronger from this respite, must send, as it were, a double quantity of blood into the aorta, i.e. make the pulse that succeeds much stronger. And this intermission may return at greater or less intervals, according to the circumstances, while this weakness of the heart continues: it may also remain, when once established, from habit, or association.
This intermission may perhaps rather be accounted for thus: in languid states the blood is accumulated in the beginning of the aorta. The aorta will not therefore receive much blood from the heart, though it does contract; i.e. no pulse will be felt; i.e. the interval will be double: the succeeding pulse will be strong, from the now greater emptiness of the aorta, and fulness of the heart: the intermission will also return when the accumulation in the aorta does. It agrees with this account, that these intermissions are usually felt in the region of the heart by the patients. May not intermissions arise from both causes in different circumstances?
Eleventhly, When the pulse flutters from flatulencies and spasms in the stomach and bowels, it appears that a sudden stop is put to the blood of the aorta, which by surcharging the left ventricle at once, makes it contract at a different time from the right, and not at a due interval from the contraction of the auricles: for this fluttering of the pulse attacks generally in an instant, and often upon a fruitless effort to expel wind from the stomach. When the contractions of the ventricles are once become asynchronous and inharmonious to those of the auricles, it may require some time to rectify this, and especially while there is an impediment to the blood’s motion through the aorta. This disorder may likewise continue through habit and association, and recur perpetually from less and less causes. The intermission of the pulse, considered in the two preceding paragraphs, is often caused by flatulencies, as well as the fluttering of it mentioned in this.
Twelfthly, The motions of the heart remain in their original automatic state more than any other in the body; the reasons of which are, that the common motions are equally associated with every thing, i.e. peculiarly so with nothing; and that very few impressions make changes in the motions of the heart so great and sudden, as to subject it to the ideas of these impressions: however, as great and sudden changes are sometimes brought on by pain, fear, surprise, joy, &c. we seem to have a semi-voluntary power to alter the motion of the heart immediately, by introducing strong ideas, our power of introducing these being semivoluntary: nay, it may be possible for persons in very particular circumstances to acquire still greater degrees of power over the motion of the heart.
The first observation to be made here, is, that common respiration is performed very feebly in new-born children. A slight disposition therefore to alternate action in the diaphragm, and other muscles of respiration, may be sufficient to account for that degree of respiration which takes place in new-born children. It is natural to expect, that respiration in them should be feeble, since they must lean in this, as well as in other things, to their preceding state in utero.
Secondly, That the vigorous impressions from the cold air, the hands of the midwife, &c. may excite the strong respiration and crying which take place upon birth ordinarily, appears from the methods used to make the new-born child respire freely, where it fails to do this in extraordinary cases; such as applying volatiles to the nose, pouring wine into the mouth, swinging it to and fro, &c. All these methods excite the muscles to contraction, by making strong impressions on the neighbouring sensory nerves. They are also very agreeable to the notion of vibrations.
Thirdly, If we suppose the diaphragm to have finished its contraction, and consequently the thorax to be raised, and the abdomen to be distended, expiration must succeed, for three reasons; viz. because the contraction of the diaphragm has checked the vibrations in its fibres, and therefore has disposed them to relaxation; because the elasticity of the cartilages of the ribs tends to restore the thorax to a more depressed state; and because the fibres of the abdominal muscles are excited to vibrations and contraction by their distention. Suppose now expiration to be completed, and the vibrations, which descend into the diaphragm, from the second and third source of motory vibrations, will renew its contraction, and consequently the action of inspiration; and so on alternately.
Fourthly, The diaphragm is the more disposed to receive vivid vibrations in its fibres, from its nearness to the heart, and the warmth in which it is always kept. It deserves notice here, that the diaphragm receives its nerves from the cervical pairs, not from the eighth pair and intercostal, as the neighbouring parts, the heart, stomach, &c. do. Perhaps it was requisite, that it should derive its nerves from the spinal marrow, since the other muscles of respiration do. The nerves of the diaphragm are elongated during its contraction, the contrary to which happens in the nerves of other muscles. They are also kept in constant motion by the constant motion of the thorax, through which they pass; and this perhaps may contribute to the perpetual activity of the diaphragm.
Fifthly, The intercostal muscles are disposed to alternate contractions for the same reasons as the diaphragm, though these reasons do not hold in the same degree; however, since the contraction of the diaphragm, and consequent distention of the abdomen, must raise the thorax, the contraction of the intercostal muscles must from the first be synchronous to that of the diaphragm; and association will soon generate a permanent disposition to this synchronism. It follows according to this theory from the vicinity of the internal intercostal muscles to the external ones, that they ought to have the same action. And the same conclusion appears to result from anatomical considerations.
Sixthly, It may however be supposed, that the diaphragm and intercostal muscles are, as it were, fatigued by constant action; and that they incline to sympathize in inactivity with the rest of the muscles, especially during sleep. But then the accumulation of blood in the lungs, which the intermission of respiration must occasion, after it has once had a free passage through the lungs by means of respiration, will produce so much uneasiness, i.e. such vigorous vibrations, in the lungs, and particularly in their external membrane, which is continuous to the pleura, as will renew respiration.
It is agreeable to this, that if very young children sleep upon a bed, they often fall into a great uneasiness from the stoppage of respiration, and recover from it by crying; and that rocking, or any other gentle motion, will prevent this. The last circumstance is particularly favourable to the notion of vibrations.
The night-mare in adults appears to be a disorder of the same kind. A full supper, and lying upon the back, dispose to it, by hindering the free motion of the diaphragm. Motion, and especially when attended with quick respiration, laughter, &c. prevent it, by giving activity to the diaphragm. Shaking the person affected, calling to him, &c. rouse from it, by exciting vibrations. The uneasiness from accumulated blood does the same thing at last.
It is easy to see, that if a like disposition to inaction should arise in the hearts of new-born children, or adults, the motion and distending force of the venal blood must renew their action for similar reasons. It is remarkably coincident with this, that forcing the venal blood upon the heart has in some cases been found to renew its action even after death.
Seventhly, The actions of sighing, crying, coughing, sneezing, &c. by recurring frequently, and making all the muscles, which can in any degree contribute to respiration, unite therein, enlarge the range of respiration by degrees, and make this action be performed with greater strength perpetually in the young child. The increase of strength in the whole muscular system must however be considered likewise.
Eighthly, The same actions have also a great share in begetting voluntary respiration. For more muscles are put into action in voluntary respiration, than in that which is ordinary.
Ninthly, It is agreeable to the foregoing theory, and to parallel observations on the heart, that respiration should be stronger in fevers, in sleep, and in general wherever the heat of the blood is increased, than in ordinary cases. For respiration depends chiefly on the second and third sources of motory vibrations. By this means the force of the respirations answers in a general way, as it ought to do, to the force with which the blood is moved, or to the force of the heart.
I now come to the particular consideration of the actions of sighing, coughing, laughter, and sobbing, and shedding tears from grief.
Sighing is an extraordinary contraction of the diaphragm. It arises not only from all causes, which accumulate the blood in the lungs, and thus excite the diaphragm to an extraordinary contraction; but also from such compressions of the abdomen as hinder the free motion of the diaphragm for a time. For thus a disposition to vibrate is accumulated in the fibres of the diaphragm. It is however to be observed, that this compression of the abdomen does also accumulate blood in the lungs. The abdomen seems to be compressed in a state of sorrow, attention, &c. The frequent recurrency of sighing makes it voluntary early in youth. But it is not performed voluntarily with the same force, as when from a pressing uneasiness in the lungs.
Coughing arises from an uneasy sensation in the wind-pipe fixing in the point of the epiglottis, as the sensation which causes sneezing does in the tip of the nose. This ought also to become voluntary, and to be weaker when voluntary, than when excited by a strong irritation.
Hawking is a voluntary action, derived from coughing, as blowing the nose is from sneezing. Spitting is nearly related to these actions. It tallies perfectly with the foregoing theory, that children cannot hawk, spit, or blow the nose, for some years.
Laughter is a nascent cry, recurring again and again, as has been observed before, Prop. XXVI. By degrees it puts on a certain type, and recurs again and again according to that type, just as other actions. And it is excited in young children not only by the sensation of tickling, which lies, as it were, between pleasure and pain, but by the apprehension of this, or any other apprehension sufficiently moderate, by every surprise, and every mental emotion that lies between pleasure and pain, and by all the associates of these, as particularly by seeing others laugh. And thus children laugh more and more, and get a power of performing the action of laughter at pleasure, though with less force than when it is excited by its proper cause in full vigour.
It is remarkable, that young brute creatures, in their sportings with each other, make such noises, as bear the same analogy to their violent cries, which laughter in us bears to crying from pain.
Bodily pain is attended with violent and irregular respiration on account of the violent and irregular vibrations, which, in this case, first ascend to the brain, and then descend into the diaphragm. Hence mental pain, which is the offspring of bodily pain, is attended also with violent and irregular respiration, i.e. with sobbing. The crying which used to attend bodily pain in childhood is often checked in the mental pains of adults by fear, shame, &c. i.e. by a voluntary or semi-voluntary power; and this seems to make the respiration so much the more irregular.
It is more difficult to account for the shedding tears from grief; for very young children are not apt to shed tears when they cry. It seems to me, that so great and general a disorder in the brain, as that which takes place in violent grief, must affect the fifth pair of nerves in a particular manner, so as to influence the lachrymal glands both directly, and also indirectly, viz. by the strong convulsions produced in the muscles of the eyes and face. The membranes of the fauces and nose are likewise affected in grief, as is evident from the sensations in the fauces, and tip of the nose; and thus vibrations may run to the lachrymal glands through the ductus ad nasum, and lachrymal points, as observed before in irritations from sternutatories, &c. Young children may not shed tears freely, because very great and general disorders of the brain, and its influence over the fifth pair of nerves, do not take place till intellectual aggregates are formed. And the like reason may hold in respect of brutes.
The actions of sobbing and weeping are therefore, in part, deducible from association; i.e. are not merely automatic, in the first sense of that word. Agreeably to which, they are in certain cases manifestly subject to the voluntary power. Thus, some persons can, by introducing imaginary scenes of compassion and sorrow, so far agitate the brain, as to bring on the actions of sobbing and weeping, though not in the same degree, as when they arise from a strong real mental cause. They may likewise be caught by infection, from others, as laughter, and most of our other semi-voluntary and voluntary actions, are; which is another argument of their dependence on association.
Since strong vibrations must, according to the foregoing theory, descend at once into the whole muscular system in general convulsions, we must seek for a cause of sufficient extent for this purpose. Now there seem to be three kinds of vibrations, which may answer this condition: first, violent vibrations in the brain. Secondly, violent ones at the skin, suddenly checked. Thirdly, violent ones in the bowels or uterus, suddenly checked also, and thence running instantaneously over the whole nervous system by means of the intercostal, or, as Winslow justly calls it, the great sympathetic nerve.
Convulsions from compressions and inflammations of the brain, and most of those which are termed epileptic, seem to be of the first kind. In epilepsies the irregular vibrations, excited in the medullary substance of the brain, are perhaps so violent, as first to make the small particles attract each other, and thus, by checking themselves, to extinguish all sense and motion. However, they may return after a short time, and descend into the whole muscular system.
The stretchings and yawnings which happen in ague-fits, in going to sleep and waking, the startings to which some persons are subject in going to sleep, and the convulsive tremors and rigidity in ague-fits, seem to be of a second kind, or to arise from a sudden check of vibrations at the skin. For in agues the surface is chilled, as it is also by the least motions in going to sleep, or waking. Agreeably to this, it may be observed, that, upon stepping into a cold bed, one is disposed to general convulsions, like those of stretching. Yawning may also depend in part upon a like check of violent vibrations in the mouth and fauces; for it is a motion excited in the neighbourhood, and is observed to accompany sickness.
The startings and convulsions which happen to children from gripes, to women from disorders of the uterus, and to all persons in general from certain poisons, seem to be of the third kind, or to arise from vigorous vibrations in the abdomen, suddenly checked, and running into the whole system by means of the intercostal nerve.
It has been observed already, that convulsive motions are apt to return from less and less causes perpetually, on account of the vestiges which they leave of themselves, and the power of associated circumstances. I will add here, that seeing a person in convulsions is apt to occasion them in such as are of nervous and irritable frames; and that there is reason to believe, that some persons, who have been enthusiasts or impostors, have been able to throw themselves into convulsions by a semi-voluntary power, and particularly, as it seems, by introducing strong ideas, and internal feelings.
It is commonly observed, that yawning is apt to infect a whole company, after one person in it has set the example; which is a manifest instance of the influence of association over motions originally automatic.
It was necessary to deliver many things which properly relate to this proposition under the twenty-first, in order to make the derivation of voluntary motion from automatic, by means of association, in some measure intelligible to the reader. I will now resume the subject, and add what I am able for the full explication and establishment of the theory proposed.
Walking is the most simple of the three kinds of voluntary motion here mentioned, being common to the brute creation with man, whereas handling and speaking are, in a manner, peculiar to him. His superiority in this respect, when compared with the superiority of his mental faculties, agrees well with the hypothesis here advanced concerning handling and speaking, viz. their dependence on ideas, and the power of association.
The new-born child is unable to walk on account of the want of strength to support his body, as well as of complex and decomplex motory vibratiuncles, generated by association, and depending upon sensations and ideas by association also. As he gets strength, he advances likewise in the number and variety of compound motions of the limbs, their species being determined by the nature of the articulations, the position of the muscles, the automatic motions excited by friction, accidental flexures and extensions made by the nurse, &c. When he is tolerably perfect in these rudiments of walking, the view of a favourite plaything will excite various motions in the limbs; and thus if he be set upon his legs, and his body carried forward by the nurse, an imperfect attempt to walk follows of course. It is made more perfect gradually by his improvements in the rudiments, by the nurse’s moving his legs alternately in the proper manner, by his desire of going up to persons, playthings, &c. and thence repeating the process which has succeeded (for he makes innumerable trials, both successful and unsuccessful;) and by his seeing others walk, and endeavouring to imitate them.
It deserves notice here, that in the limbs, where the motions are most perfectly voluntary, all the muscles have antagonists, and often such as are of nearly equal strength with themselves; also, that the muscles of the limbs are not much influenced at first by common impressions made on the skin, and scarce at all when the child is so far advanced as to get a voluntary power over them. For these things facilitate the generation of the voluntary power, by making the muscles of the limbs chiefly dependent on the vibrations which descend from the brain, and also disposing them to act from a small balance in favour of this or that set of antagonists.
When the child can walk up to an object that he desires to walk up to, the action may be termed voluntary, i.e. the use of language will then justify this appellation. But it appears from the reasoning here used, that this kind and degree of voluntary power over his motions is generated by proper combinations and associations of the automatic motions, agreeably to the corollaries of the twentieth proposition. Voluntary powers may therefore result from association, as is asserted in these papers.
When he is arrived at such a perfection in walking, as to walk readily upon being desired by another person, the action is esteemed still more voluntary. One reason of which is, that the child, in some cases, does not walk when desired, whilst yet the circumstances are apparently the same as when he does. For here the unapparent cause of walking, or not walking, is will. However, it follows from this theory, that all this is still owing to association, or to something equally suitable to the foregoing theory; e.g. to the then present strength or weakness of the association of the words of the command with the action of walking, to its proceeding from this or that person, in this or that manner, to the child’s being in an active or inactive state, attentive or inattentive, disposed by other circumstances to move as directed, or to move in a different way, &c. a careful observation of the fact will always shew, as far as is reasonably to be expected in so nice a matter, that when children do different things, the real circumstances, natural or associated, are proportionably different, and that the state of mind called will depends upon this difference. This degree of voluntary power is therefore, in like manner, of an acquired nature.
Suppose an adult to walk in order to shew his perfectly voluntary power; still his selecting this instance is owing to one association, and his performing the action to another, viz. to the introduction of the audible idea of the word, the visible one of the action, &c.
Walking passes into the secondarily automatic state more perfectly perhaps than any other action; for adults seldom exert any degree of volition here, sufficient to affect the power of consciousness or memory for the least perceptible moment of time. Now this transition of walking, from its voluntary to its secondarily automatic state, must be acknowledged by all to proceed merely from association. And it seems to follow by parity of reason, that the transition of primarily automatic actions into voluntary ones, may be merely from association also, since it is evident, that association has at least a very great and extensive influence there.
The complex artificial motions of the lower limbs, used in the several kinds of dancing, bear nearly the same relation to the common motions used in walking forwards, backwards, upwards, downwards, and sideways, as these common motions do to the simple rudiments above mentioned, such as the flexion and extension of the ancle or knee. Since therefore the voluntary and secondarily automatic power of dancing are plainly the result of association, why may we not suppose the same of the common motions in walking, both in their voluntary and secondarily automatic state? In learning to dance, the scholar desires to look at his feet and legs, in order to judge by seeing when they are in a proper position. By degrees he learns to judge of this by feeling; but the visible idea left partly by the view of his master’s motions, partly by that of his own, seems to be the chief associated circumstance, that introduces the proper motions. By farther degrees these are connected with each other, with the music, and with other more and more remote circumstances.
I have already shewn in what manner children learn the voluntary and secondarily automatic power of grasping. How they learn the various complex motions, by which they feed and dress themselves, &c. also how children and adults learn to write, to practise manual arts, &c. and in what senses and degrees all these actions are voluntary, and secondarily automatic, and yet still remain as purely mechanical as the primarily automatic actions are, may now be understood from what has been already delivered under this proposition. The method of playing upon musical instruments has also been explained, so as to concur in establishing the same conclusions.
In like manner, the account given of the action of speaking might now be completed, and extended to all the modes of it, vulgar and artificial; and to singing, with its modes. I will add a few words concerning stammering and the loss of speech by palsies.
Stammering seems generally to arise from fear, eagerness, or some violent passion, which prevents the child’s articulating rightly, by the confusion which it makes in the vibrations that descend into the muscular system; so that, finding himself wrong, he attempts again and again, till he hits upon the true sound. It does not begin therefore in general, till children are of an age to distinguish right from wrong in respect of pronunciation, and to articulate with tolerable propriety. A nervous disorder of the muscles of speech may have a like effect. When the trick of stammering has once begun to take place in a few words, it will extend itself to more and more from very slight resemblances, and particularly to all the first words of sentences, because there the organs pass in an instant from inactivity to action; whereas the subsequent parts of words and sentences may follow the foregoing from association; just as, in repeating memoriter, one is most apt to hesitate at the first word in each sentence.
A defect of memory from passion, natural weakness, &c. so that the proper word does not occur readily, occasions stammering also. And, like all other modes of speaking, it is caught, in some cases, by imitation.
A palsy of the organs of speech may be occasioned in the same manner as any other palsy; and yet the muscles of the lips, cheeks, tongue, and fauces, may still continue to perform the actions of mastication and deglutition sufficiently well, because these actions are simpler than that of speech, and are also excited by sensations, which have an original influence over them.
A defect of memory may also destroy the power of speaking, in great measure, though the organs be not much affected in a paralytical way. Thus a person who plays well upon a harpsichord, may by some years’ disuse become unable to play at all, though the muscles of his hands be in a perfect state, merely because his memory, and the associations of the motions of his fingers, with the sight of notes, with the ideas of sounds, or with one another, are obliterated by distance of time, and disuse.
The suspension of an action may be performed two ways, as before-mentioned; viz. either by putting the muscles concerned in it into a languid inactive state, or by making the antagonists act with vigour. In the first case, the whole limb is put into a state of relaxation, and extreme flexibility; in the last, into a state of rigidity. The voluntary power of the first kind is obtained by associations with the languor that arises from fatigue, heat, sleepiness, &c. that of the last from the general tension of the muscles, which happens in pain, and violent emotions of mind. Children improve in both these kinds of voluntary power by repeated trials, as occasion requires, by imitation, desire, &c. But they are both difficult for some time. Thus we may observe, that children cannot let their heads or eyelids fall from their mere weight, nor stop themselves in running or striking, till a considerable time after they can raise the head, or bend it, open the eyes, or shut them, run or strike, by a voluntary power.
Imitation is a great source of the voluntary power, and makes all the several modes of walking, handling, and speaking, conformable to those of the age and nation in which a person lives in general, and to those of the persons with whom he converses in particular. Besides the two sources already mentioned, Prop. XXI., viz. the sight of the child’s own actions, and the sound of his own words, it has many others. Some of these are the resemblance which children observe between their own bodies, with all the functions of them, and those of others; the pleasures which they experience in and by means of all motions, i.e. imitations; the directions and encouragements given to them upon this head; the high opinions which they form of the power and happiness of adults; and their consequent desire to resemble them in these, and in all their associates. Imitation begins in the several kinds of voluntary actions about the same time, and increases not only by the sources alleged, but also by the mutual influences of every instance of it over every other, so that the velocity of its growth is greatly accelerated for some time. It is of the highest use to children in their attainment of accomplishments, bodily and mental. And thus every thing, to which mankind have a natural tendency, is learnt much sooner in society, than the mere natural tendency would beget it; and many things are learnt so early, and fixed so deeply, as to appear parts of our nature, though they be mere derivatives and acquisitions.
It is remarkable that apes, whose bodies resemble the human body more than those of any other brute creature, and whose intellects also approach nearer to ours, which last circumstance may, I suppose, have some connexion with the first, should likewise resemble us so much in the faculty of imitation. Their aptness in handling is plainly the result of the shape and make of their fore legs, and their intellects together, as in us. Their peculiar chattering may perhaps be some attempt towards speech, to which they cannot attain, partly from the defect in the organs, partly, and that chiefly, from the narrowness of their memories, apprehensions, and associations; for they seem not to understand words to any considerable degree. Or may not their chattering be an imitation of laughter?
Parrots appear to have far less intellect than apes, but a more distinguishing ear, and like other birds, a much greater command of the muscles of the throat. Their talk seems to be almost devoid of all proper connexion with ideas. However, in respect of sounds, they imitate as much as children, or as apes in respect of other actions. And indeed the talk of children, by out-running their understandings in many things, very much resembles that of parrots.
As we express our inward sentiments by words, so we do also by gestures, and particularly by the muscles of the face. Here, again, association and imitation display themselves. This dumb show prevails more in the hotter climates, where the passions are more impetuous, than in these northern ones. It is also probable, that the narrowness and imperfection of the ancient languages made it more necessary and prevalent in ancient times. Deaf persons have an extraordinary aptness both in learning and decyphering this, as might well be expected. The imitation of manners and characters by dumb show is often more striking than any verbal description of them.