Observations on Man (6th edition)/Part I/Chapter II/Section II
Section II[edit]
OF THE SENSE OF TASTE.
The taste may be distinguished into two kinds, as before observed of feeling; viz. the particular exquisite one, which resides in the tongue, and especially in the tip of it; and the general one, which extends itself to the insides of the lips and cheeks, to the palate, fauces, œsophagus, stomach, and whole alimentary duct, quite down to the anus; the sensibility growing perhaps less and less, perpetually, in going from the stomach to the rectum. The sensibility of the alimentary duct is probably of the same kind with, and not much greater in degree than, that of the internal surfaces of the gall-bladder, urinary bladder, pelvis, ureters, and, in general, of the secretory and excretory vessels, and of the receptacles belonging to the glands. But I refer the sensations of this duct to the taste, on account of their connexion with those of the tongue, in respect of their causes, uses, and effects.
As to the particular and superior powers of the tongue, they may, in part, be deduced from the number and largeness of its papillæ, and from their rising above the surface in living persons more remarkably than any other sentient papillæ in the whole body, so as to be extreme parts in an eminent degree. To which we ought perhaps to add, that the tip and sides, in which the taste is most exquisite, are also extreme parts. But there may be likewise a different peculiar distribution, and other causes of an exquisite sensibility, in the nerves of the tongue.
It deserves notice here, that the friction of the tongue against the palate is necessary, in order to excite the tastes of the aliments, which we masticate, in perfection. This practice is analogous to that of rubbing the ends of the fingers upon such bodies as we examine accurately by feeling; and both appear suitable to the notion of vibrations; also to that of the distention and erection of the sentient papillæ: which may even be seen in the papillæ of the tongue.
It has been observed, that bitters and acids applied to funguses of the brain, and even to issues upon the vertex, have sometimes occasioned the sensations of the tastes respectively arising from the same bitters and acids, when applied to the tongue. This may perhaps be solved by supposing, that the bitters and acids, when applied to the nerves of the fifth pair, in the funguses of the brain, and to those of the seventh, or perhaps of the fifth also, in the issues (for the fifth pair may transmit some branches to the external integuments from the dura mater, at the vertex), send up their own specific vibrations into those regions of the brain, which are the peculiar residence of tastes, i.e. to the regions which correspond to the fifth pair, according to some anatomists. And these sensations may even afford some evidence, that the fifth pair, not the ninth, supplies the tongue with sensory nerves.
Here I observe, first, that heating any sapid liquid increases its taste, especially if it be of the bitter, spirituous, or acrid kind; and, conversely, that the impression of such tastes generates a heat in the organ, which remains after the peculiar taste ceases. Now this connexion of certain tastes with heat is some presumption, that they arise from vibrations, provided we allow heat to arise from them, according to the common opinion.
Secondly, Since disagreeable tastes must, according to the account of pain given above, arise from such a violence in the vibrations excited, as produce the solution of continuity, and pleasant tastes, from more moderate degrees of vibrations, which, though they approach to the solution of continuity, yet fall short of it; the pains of taste must proceed from stronger vibrations than the pleasures. And, agreeably to this, bodies which impress very active and disagreeable tastes, manifest great activity in other trials for the most part.
Thirdly, It is very difficult to give any plausible account of the great variety of pleasant and unpleasant tastes from the doctrine of vibrations. However, the different frequency of vibrations, which belongs to the small particles of different sapid bodies, may be, in a good measure, the source of this great variety. For if the particles of the body A oscillate twice, while those of the body B oscillate only once, the sensations excited by them may be different, though both fall within the limits of pleasure, or both pass into those of pain. The differences of degree may also contribute; for it is observable, that different disagreeable tastes, in declining, leave agreeable ones, which approach to each other. This I have experienced in aloes, lime, and green tea not sweetened. It may therefore be, that the different disagreeable tastes were such rather on account of degree than kind. And, upon the whole, it may be, that the several combinations of the differences of kind with those of degree may be sufficient in number to account for all the varieties and specific differences of tastes.
Fourthly, Tastes appear to be more different than they are from the odours which accompany some of them. And this observation, by reducing the number of tastes properly so called, does somewhat lessen the difficulty of accounting for their number. But then it is also to be observed, that part of this difficulty is to be transferred to the head of odours.
Fifthly, The power of distinguishing tastes seems to depend upon sight, to a certain degree. And this consideration also lessens the number of tastes properly so called.
Sixthly, If we suppose the sapid body to consist of particles, that excite vibrations of different frequencies, which may be the case of many bodies in their natural state, and probably must be with such as are compounded by art, compound medicines for instance, a great variety of tastes may arise, some resembling the tastes of simple bodies, others totally unlike these: just as some of the colours reflected by natural and artificial bodies resemble one or other of the simple primary ones, whilst others are colours that can scarce be referred to any of these. And we may farther suppose, in both cases, that where the vibrations approach so near in frequency as to overrule each other, and produce one species only, there results a taste, or a colour, that resembles a primary one; whereas, if the vibrations differ so much in frequency, as that two or more principal species keep their own frequency, the taste, or colour generated from them, cannot be likened to any primary one.
Seventhly, That there are different species of vibrations, which yet all constitute sweet or bitter, I conjecture not only from the foregoing general reasoning, but also because there are both sweets and bitters in all the three kingdoms, animal, vegetable, and mineral. Thus milk, sugar, and saccharum Saturni, all yield a sweet taste; gall, aloes, and crystals of silver, all a bitter one.
Eighthly, Some differences of taste may arise from the different time required for the solution, and consequent activity, of the sapid particles, also from the different local effect which they may have upon the papillæ, when absorbed by their venous vessels, &c.
Ninthly, Very nauseous and stupefacient tastes may perhaps arise from violent and irregular vibrations, and bear some analogy to the sensation, or want of it, impressed by the stroke of the torpedo.
Tenthly, It seems to deserve particular consideration here, that milk, and the flesh of certain domestic animals, yield tastes, which are naturally and originally pleasant, to a considerable degree, and yet not in excess, as sugar, and other very sweet bodies, are. For it is reasonable to conclude, that the particles of milk, and common flesh-meats, must agree very much in the strength and frequency of vibrations with the particles of our own solids and fluids. They may therefore just moderately increase the natural vibrations of the organ, when applied thereto, and enforced by suction, mastication, and friction of the tongue against the palate. For the same reasons we may guess, that the common diet of animals does not undergo very great changes, in respect of the vibratory motions of its particles, from its circulation, and consequent assimilation. However, there are some eminent instances to the contrary, especially in poisons. These last observations may be extended to vegetables, salutary and poisonous, respectively.
Eleventhly, Some acrid tastes, that of mustard for instance, affect the tip of the uvula, and the edge of the soft palate, in a particular manner. Now this may a little confirm the conjectures above made concerning the sensibility and irritability of extreme parts.
Twelfthly, It is easy to conceive, upon the principles of these papers, how sweets and bitters of an inferior degree should render those of a superior one less affecting, respectively, as they are found to do.
The foregoing articles are only imperfect conjectures, and do not even approach to a satisfactory solution. They may just serve to shew, that the doctrine of vibrations is as suitable to the phænomena of tastes, as any other hypothesis yet proposed. The following methods may perhaps be of some use for the analysis of tastes.
First, To make trials upon bodies whose particles seem similar to each other. Such are perhaps distilled spirits, acid, alkaline, and fermented; also salts and oils; but they must all be sufficiently purified by repeated distillations, solutions, and such-like chemical operations; else we are sure, that their component particles are heterogeneous.
Secondly, To note the changes of taste in chemical operations, and compare them with the changes of colour; which last, by discovering the sizes of the particles, may determine many things relating to their mutual actions. The solutions of metals in acids, by affording many singular and vivid tastes, and sometimes colours, seem to deserve especial notice here.
Thirdly, There are many regular changes in natural bodies, which, by comparison with other phænomena, may be of use. Thus it is remarkable, that the juice of many or most fruits is first acid, i.e. whilst unripe, then sweet, then vinous, after the first fermentation, then acid again, after the second fermentation.
This inquiry is of great importance in medicine and philosophy. And the theory of tastes appears capable of becoming a principal guide in discovering the mutual actions of the small particles of bodies. The difficulty is to make a beginning. This theory may not perhaps be more complex than that of colours; one may, at least, affirm, that the theory of colours appeared as complex and intricate before Sir Isaac Newton’s time, as that of tastes does now; which is some encouragement to make an attempt.
It will easily be conceived, that if tastes, properly so called, (of which under the last proposition) favour, or suit with, the doctrine of vibrations, the sensations of the stomach and bowels may likewise. But a particular examination of these sensations, and comparison of them with tastes, will make this more evident.
First, then, we may observe, that the stomach is less sensible than the tongue, the bowels in general than the stomach, and the inferior bowels than the superior. Thus opium, and bitters, and sometimes spirituous liquors, are disagreeable to the tongue, but fall within the limits of pleasure in the stomach. Thus bile is extremely nauseous in the mouth, and offends even the stomach; but it cannot be disagreeable to the duodenum, which it first enters, or the bowels, through which it passes. Thus also the fæces seem to be equally suited to the several bowels along which they descend, though they grow perpetually more putrid and acrimonious in their descent; i.e. there is an abatement of sensibility in the bowels, which corresponds to the increase of acrimony in the fæces.
If it be objected here, that honey, mercurius dulcis, &c. offend the stomach and bowels often, though pleasant or insipid in the mouth, I answer, that such bodies require time, heat, solution, &c. before the whole of their qualities can be exerted.
Secondly, The particular manner in which opiates, fermented liquors, grateful aliments, and narcotics, may act first upon the stomach and bowels, and afterwards upon the whole body, agreeably to the doctrine of vibrations, has been given above in treating of sleep.
Thirdly, The action of vomits, purges, and acrid poisons, such as corrosive sublimate, is very reconcileable to this doctrine, by only supposing, that they excite very vigorous vibrations, and that these are communicated to the muscular coats of the stomach and bowels, to the muscles of the abdomen, and, in violent cases, to the whole muscular and nervous system. I shall consider the automatic motions, which arise from these causes, below, under a particular proposition. It may serve to shew the analogy of the sensations, and the general nature of active medicines, to observe, that these will often operate in several ways, viz. as vomits, purges, diuretics, diaphoretics, sternutatories, vesicatories, and corrosives, by a change of application and circumstances.
Fourthly, Since the meats, to which particular persons have an antipathy, and from which they receive violent ill effects, are, in general, highly grateful to others, one may perhaps conjecture, that the vibrations excited by these meats in the stomachs of those who have an antipathy to them, do but just pass the limits of pleasure; so as to diffuse themselves much farther, and more powerfully, than if the first impression was very painful.
Lastly, The connexion between the sensations of the tongue and stomach, and consequently between the manners of explaining them, may be inferred from the office of the taste, as a guide and guard to the organs of digestion; which is very evident, in general, in all animals, notwithstanding a few exceptions, more especially in men.
Natural hunger may be reckoned a pleasure in its first rise, and to pass into a pain only by increase and continuance. We may suppose therefore, that during hunger the nerves of the stomach are so irritable, as that the common motions of the muscular coat, and the impressions made by its contents in consequence of these motions, excite such vigorous vibrations, as first lie within the limits of pleasure, and afterwards pass into the limits of pain. And when the sight of food, or any other associated circumstance, increases the sense of hunger instantaneously, it may be conjectured to do so, in great measure, by increasing the contractions of the muscular coat of the stomach.
But the sensibility and irritability of the nerves of the tongue are also increased by hunger; for common aliments yield a very different taste, according as the person is satiated or hungry. And it is probable farther, that the nerves of the upper part of the bowels sympathize with those of the stomach in hunger; and increase the uneasy sensation, in violent degrees of it.
Let us now consider in what way food may be supposed to lessen this sensibility and irritability of the nerves of the tongue, stomach, and bowels; and how abstinence, bitters, wines, &c. may increase them, upon the principles of these papers.
First then, As the small absorbing vessels in the mouth, stomach, and bowels, must, after eating for some time, be saturated with alimentary particles, those that are now applied will scarce make any impression for want of a sufficient attraction.
Secondly, Such as are attracted cannot make any considerable difference of vibrations, because the vibrations which they are qualified to excite, do already take place. And these two remarks, put together, shew, that a person may relish a second kind of food after being satiated with a first.
Thirdly, The actions of mastication, deglutition, and digestion, exhaust the neighbouring glands and glandular receptacles of their liquids, and the neighbouring muscular fibres of their ready power of contracting: these parts are therefore no longer susceptible of a pleasurable state, or only in a low degree.
Fourthly, Abstinence reverses all these steps; in which, however, the perpetual affusion, dilution, and ablution of the saliva has a considerable share. And thus after a proper interval the organs return to a state of great sensibility and irritability.
Fifthly, Bitters and acids exhibited in a moderate degree seem gently to increase the vibratory motions, and raise them before the due time to the degree that corresponds to hunger. A small quantity of food has the same effect, also agreeable motions of mind, fresh air, exercise, and many other things. But if the bitters, acids, &c. be carried beyond a certain degree, they occasion pain or sickness, which is very agreeable to the doctrine of vibrations, as laid down in these papers.
Sixthly, In fevers, the mouth, fauces, œsophagus, and stomach, are hot, dry, inflamed, and incrusted. They are therefore preoccupied by vibrations of a kind quite different from those which attend hunger, and therefore exclude this state.
It may not be amiss to observe here, that the sensation of hunger is a guide and guard to the organs of digestion in a still more eminent degree, than the tastes of the several aliments.
Thirst is opposite to hunger, and is a general attendant upon fevers. It follows also, in an evident manner, from all considerable degrees of heat in the fauces. The nerves therefore of the mouth, fauces, œsophagus, and stomach, are, during thirst, preoccupied by disagreeable vibrations, of the inflammatory kind, as above observed. And as the pleasures of taste may be said to resemble those of titillation, so thirst seems allied to itching.
It is agreeable to this account of thirst, that liquors actually cold afford immediate relief; also that warm diluents, which soften the parts, and wash off acrimonious particles, do it after some time. As the cause of thirst is of a permanent nature in fevers, it must return again and again, till the fever be removed. Gentle acids yield a pleasure in thirst, which seems to correspond to that which scratching excites, in parts that itch previously.
Some of these changes are,
That sweets generally grow less and less agreeable, and sometimes even disagreeable, or nauseous at last.
That astringent, acid, and spirituous liquids, which displease at first, afterwards become highly grateful.
That even bitters and acrids first lose their offensive qualities, and after a sufficient repetition give a relish to our aliment.
And that many particular foods and medicines become either extremely pleasant or disgusting, from associations, with fashion, joy, hope of advantage, hunger, the pleasures of cheerful conversation, &c. or with sickness, vomitings, gripings, fear, sorrow, &c.
Now, in order to account for these changes, we may consider the following things.
First, That the organs become less and less sensible by age, from the growing callosity and rigidity of all the parts of animal bodies. The pleasant savours may therefore be expected to become less pleasant, and the moderately disagreeable ones to fall down within the limits of pleasure, upon this account.
Secondly, The disposition to vibrations in the organ and corresponding part of the brain must also receive some alteration by the frequent repetition of impressions. For though this returns, at a proper distance from each meal, to its former state, within an indefinite distance, as one may say, yet some difference there probably is, upon the whole, which in a sufficient length of time amounts to a perceptible one. However, we must also suppose on the other hand, that the make of the nervous system sets some bounds to this gradual alteration in the disposition to vibrate; else the taste would be much more variable than it is, and continue to change more after adult age, than it is found to do in fact. It may perhaps change faster in the use of a high diet than of a low one; which would be an evidence of the reality of the cause here assigned.
Thirdly, The pleasant and painful impressions which particular foods and medicines make upon the stomach, always either accompany the taste, or follow it in a short time; and by this means an association is formed, whereby the direct pleasantness or nauseousness of the taste is enhanced, if the impressions upon the tongue and stomach be of the same kind; or diminished, and perhaps overpowered, and even converted into its opposite, if they be of different kinds. For if the two impressions A and B, made upon the tongue and stomach respectively, be repeated together for a sufficient number of times, b will always attend A upon the first moment of its being made. If therefore B be of such a magnitude as to leave a trace b sufficiently great, the addition of this trace b to A, the impression made upon the tongue, may produce all the changes in it above-mentioned, according to their several natures and proportions. This follows from the doctrine of association, as it takes place in general; but here the free propagation of vibrations from the stomach to the mouth, along the surface of the membranes, adds a particular force. In like manner a disagreeable taste, by being often mixed with a pleasant one, may at last become pleasant alone, and vice versâ: hunger and satiety may also, by being joined with particular tastes, contribute greatly to augment or abate their relish. And I believe it is by the methods of this third kind, that the chief and most usual changes in the taste are made.
Fourthly, The changes which are made by associations with mental pleasures and pains, or bodily ones not belonging to this organ, as with fine colours, music, &c. receive a like explication as the last-mentioned instances of associations. Here the pleasure excited in the eye or ear overrules the taste at first: afterwards we may suppose the organ to be so altered by degrees, in respect of the disagreeable taste, from its frequent impression, or other cause, as to have the solution of continuity no longer occasioned by its action. It is probable also, that the evanescent pleasures of sight and hearing, at least pleasant vibrations in the parts of the brain corresponding to these two organs, accompany these tastes ever afterwards.
It may be observed here, that the desire of particular foods and liquors is much more influenced by the associated circumstances, than their tastes, it being very common for these circumstances, particularly the sight or smell of the food or liquor, to prevail against men’s better judgment, directing them to forbear, and warning them of the mischiefs likely to arise from self-indulgence.
Here we must lay down previously, that the uterus is in a state of distention during pregnancy; and that it propagates sympathetic influences by means of nervous communications to the stomach, so as to put it into a state of great sensibility and irritability. All this will be easily acknowledged.
It follows therefore, since the limits of pleasure and pain are contiguous, that the stomach during pregnancy may at some times have an eager appetite for food, as well as a nausea at others; that this appetite may be the more eager, because it borders upon a nausea; and that it will no more answer to the usual exigencies and circumstances of the body, than the nausea does. The same eager appetite will bring up the ideas of various aliments from prior associations; and if a new association of it, when particularly eager, happen to be made with this or that food or liquor, the sympathetic eager appetite will ever after bring in the idea of that food or liquor, and adhere inseparably to it. The same eager appetite may also be transferred upon something that is not properly a food, from its exorbitant nature, prior nauseas in respect of common food, and accidental joint appearance. And, upon the whole, the usual circumstances attending the longings of pregnant women are deducible from association, and are as agreeable to the doctrine of vibrations, as to any other yet proposed; or even more so.
It may illustrate this account to observe, that in the usual cases of melancholy madness, an uneasy state seems to be introduced into the white medullary substance of the brain by the degeneration of the humours, or other such-like mechanical cause, which carries the vibrations within the limits of pain, and raises an inflammation sui generis in the infinitesimal vessels of the medullary substance; that ideas of objects of fear, sorrow, &c. are raised, in consequence of this, by means of prior associations; and that after some time, some one of these, by happening to be presented oftener than the rest, by falling more in with the bodily indisposition, &c. overpowers all the rest, excites and is excited by the bodily state of fear, sorrow, &c. till at last the person becomes quite irrational in respect of this one idea, and its immediate and close associates, however rational he may be in other respects. And a like account may be given of the violent particular desire towards a person of a different sex, where this desire rests chiefly in the sensual gratification, and the beauty of the person. And all these three instances seem to me to favour the doctrine of vibrations a little, as well as that of association very much.
The pleasures of the taste, considered as extending itself from the mouth through the whole alimentary duct, are very considerable, and frequently repeated; they must therefore be one chief means, by which pleasurable states are introduced into the brain, and nervous system. These pleasurable states must, after some time, leave miniatures of themselves, sufficiently strong to be called up upon slight occasions, viz. from a variety of associations with the common visible and audible objects, and to illuminate these, and their ideas. When groups of these miniatures have been long and closely connected with particular objects, they coalesce into one complex idea, appearing, however, to be a simple one; and so begin to be transferred upon other objects, and even upon tastes back again, and so on without limits. And from this way of reasoning it may now appear, that a great part of our intellectual pleasures are ultimately deducible from those of taste; and that one principal final cause of the greatness and constant recurrency of these pleasures, from our first infancy to the extremity of old age, is to introduce and keep up pleasurable states in the brain, and to connect them with foreign objects.
The social pleasures seem, in a particular manner, to be derived from this source; since it has been customary in all ages and nations, and is, in a manner, necessary, that we should enjoy the pleasures of taste in conjunction with our relatives, friends, and neighbours.
In like manner nauseous tastes, and painful impressions upon the alimentary duct, give rise and strength to mental pains. The most common of these painful impressions is that from excess, and the consequent indigestion. This excites and supports those uneasy states, which attend upon melancholy, fear, and sorrow.
It appears also to me, that these states are introduced, in a great degree, during sleep, during the frightful dreams, agitations, and oppressions, that excess in diet occasions in the night. These dreams and disorders are often forgotten; but the uneasy states of body, which then happen, leave vestiges of themselves, which increase in number and strength every day from the continuance of the cause, till at last they are ready to be called up in crowds upon slight occasions, and the unhappy person is unexpectedly, and at once, as it were, seized with a great degree of the hypochondriac distemper, the obvious cause appearing no ways proportionable to the effect. And thus it may appear, that there ought to be a great reciprocal influence between the mind and alimentary duct, agreeably to common observation; which is farther confirmed by the very large number of nerves distributed there.
As the pleasures of taste are in general greater than those of feeling, and the pains in general less, it follows that the ideas which are affixed to the several words expressing the several pleasant and unpleasant tastes, will be of a middle nature in respect of the ideas generated by tangible impressions; and lie between the ideas of the pains of feeling, and those of its pleasures.
Agreeably to this, it seems very difficult, or even impossible, to excite a genuine vivid miniature of an acid, sweet, salt, or bitter taste, by the mere force of imagination. However, the vibrations peculiar to each of these leave such vestiges of themselves, such an effect in the tongue, and corresponding parts of the brain, as, upon tasting the qualities themselves, at once to bring up the names whereby they are expressed, with many other associated circumstances, particularly the visible appearances of the bodies endued with these qualities. And these vestiges may be called ideas. Analogy leads us also to conclude, as before observed under feeling, that some faint vestiges or ideas must be raised in the parts of the brain corresponding to the tongue, upon the mere passage of each word, that expresses a remarkable taste, over the ear. And, when the imagination is assisted by the actual sight or smell of a highly grateful food, we seem able to raise an idea of a perceptible magnitude. This is confirmed by the manifest effect exerted upon the mouth, and its glands, in such cases.
The sight of what we eat or drink seems also, in several instances, to enable us to judge more accurately of the taste and flavour; which ought to be effected, according to this theory, by raising small ideas of the taste and flavour, and magnifying the real impression in consequence thereof. For an actual impression must excite vibrations considerably different, according to the difference in the previous ones; and where the previous ones are of the same kind with those impressed, the last must be magnified.
The motions dependent on the sensations of the tongue, and alimentary duct, may be thus enumerated: suction, mastication, deglutition, the distortion of the mouth and face in consequence of nauseous tastes, the peristaltic motion of the stomach and bowels, vomiting, ructus, hiccough, spasms, and violent motions in the bowels, the motions which empty the neighbouring glands, and the expulsion of the fæces.
First, then, Suction in new-born children appears to depend chiefly on the sensations of the lips and tongue. I say chiefly, because some predisposition thereto may be generated in utero, or otherwise impressed, and the great aptness of new-born children in sucking seems to favour this. However, when we consider, that the impressions of the cold air upon the lips and mouth in its passage to the lungs, of the nipple upon the lips, and of the milk upon the tongue, ought to excite motions in the neighbouring muscles of the lips, and lower jaw; that the motions which concur to the action of suction, are such as might be expected from these causes; and even that the motions of the head and neck, by which the child indicates the want of a breast, may flow from the great sensibility and irritability of these parts, when the child is hungry; a presumption arises, that the whole action of suction, with all its circumstances, is excited by the impressions mechanically or automatically; and that by the running of vibrations from the sensory nerves into the neighbouring motory ones.
Secondly, The first rudiments of the action of mastication are derived from that of suction, i.e. from the alternate motion of the lower jaw necessary to squeeze out the milk. After this action has been excited for some time by the taste of the milk, it will return with sufficient facility from the impressions made by solid food; and the same impressions may excite other motions in the muscles of the tongue and cheeks, viz. those which concur to make the action of mastication in its imperfect and automatic state.
Thirdly, It may appear in like manner, that the pleasurable impressions of the milk upon the tongue, mouth, and fauces, of new-born children, may excite those motions of the muscles of the tongue, os hyoides, soft palate, and fauces, which make the action of deglutition; and consequently, that this is deducible from sensation automatically.
It confirms this position, that, according to the theory of these papers, the soft palate ought to be drawn down by the impressions made on the tongue and mouth, not drawn up; since this last would be to suppose the sensory vibrations to pass over muscles that are near, and run to those at a distance, which is absurd. For Albinus has proved, both from anatomy, and the observation of the fact, that the soft palate is drawn down in swallowing; not up, according to the opinion of Boerhaave.
It confirms it also, that nauseous liquids are immediately and mechanically rejected by young children; the impressions arising from them producing such a contraction as shuts the passage.
It confirms it still farther, that young children do not swallow their saliva. For this makes no impression sufficient to generate the action of deglutition in an automatic way.
We may conjecture here, that the common vibrations, excited in the membrane of the mouth and fauces, grow particularly strong at the tip of the uvula; and that a greater power of contraction is transmitted to the neighbouring muscles upon this account.
Fourthly, It may be observed, that nauseous tastes distort the mouth and face automatically, not only in young children, but even in adults. And, for the same reason, pleasant ones ought to have a less effect, of the same kind; as they seem to have. And I conjecture, that the distortions of the face, which attend grief, also the gentle, smiling motions, which attend joy, are, in part, deducible from this source.
I conjecture also, that the risus sardonius, and the tendency to laughter, which some persons observe in themselves in going to sleep, have a relation to the fore-mentioned motions of the face. As the muscles here considered are, in great measure, cutaneous, they will, on this account, be more subject to vibrations excited in the mouth, or which run up to it from the stomach.
Fifthly, It may easily be conceived, that the impressions, which the aliment and fæces make upon the stomach and bowels, may excite the peristaltic motion in their muscular coats. It only remains to shew, why this should tend downwards. Now, for this, we may assign the following reasons. First, that the action of swallowing determines that of the stomach to move the same way with itself, i.e. downwards; and that this determination may, in common cases, carry its influence as far as the great guts. Secondly, that the contraction of the upper orifice of the stomach may stop the waves that sometimes come upwards in the stomach, and return them back, so as to force open the pylorus where that is less contracted; as on the other hand, where the pylorus is more contracted than the upper orifice, the motion of the stomach is inverted, and there arises a disposition to ructus or vomiting. Thirdly, that when waves ascend in the lower bowels, a gentle contraction in the pylorus may be sufficient to stop and return them. Fourthly, that one principal use of the cæcum, and appendicula vermiformis, which last is an extreme and pointed part, seems to be, to return the waves, which the constriction of the anus may send upwards. And the effects of glysters and suppositories in procuring stools, i.e. in putting the whole colon into motion, agree well with this use of the cæcum, and appendicula vermiformis. It agrees also with all the reasoning of this paragraph, that when a stoppage is made any where in the bowels by an inflammation, spasm, strangulation from a rupture, &c. the peristaltic motion is inverted.
I have been informed, that in a person who had some inches of the ilium hanging out of his body, so that the peristaltic motion might be viewed, the least touch of a foreign body would stop this motion at once. It agrees with this, that when rabbits are opened alive, the peristaltic motion does not take place till after some time, viz. because the handling of the bowels has checked it. May we not hence suspect, that the fibres of the muscular coat of the bowels are contracted by an electrical virtue, which passes off, and disappears for a time, upon the touch of non-electrics? Or may we suppose, that such touches stop subtle vibrations in the small parts of the fibres?
Sixthly, Since vomiting is excited by disagreeable and painful impressions in the stomach, and requires the contraction of the diaphragm, and abdominal muscles, it agrees well with the notion, that sensory vibrations run into the neighbouring muscles for contracting them. I suppose also, that both orifices of the stomach are strongly contracted, previously to vomiting; and that the upper orifice, being most sensible, is contracted most strongly. Hence its power of contraction may be soon exhausted, and consequently it may open of itself in the action of vomiting. However, it may, in some cases, require to be forced open by the superior action of the diaphragm, and abdominal muscles. Almost all great pains and disorders in the lower belly occasion vomiting; which is very agreeable to the foregoing notion.
The nose itches, the mouth flows with water, the lower lip trembles, both are pale, and the person yawns, previously to vomiting, in many cases; all which things favour the notion of vibrations running freely along the surfaces of membranes.
Ructus, or the expulsion of wind from the stomach, is nearly related to vomiting, differing rather in degree than kind. Its suitableness therefore to the theory of these papers must be judged of from what has been advanced concerning vomiting.
The hiccough is also related to vomiting. It is supposed to proceed from an irritation at the upper orifice of the stomach, causing a sudden contraction of the diaphragm, so as to pull down the pharynx and larynx after it. May it not rather be a sudden contraction of the inferior or small muscle of the diaphragm only? This is particularly near the supposed seat of irritation; and upon this supposition, sneezing, surprise, and all other methods of making the whole diaphragm act together strongly, would remove it, as is observed in fact.
Seventhly, Permanent spasms, and violent motions, in the bowels, arise in consequence of uneasy and painful impressions there from indigested aliment, acrid fæces, irritating purges, poisons, &c. They are generally attended with the fermentation of the contents of the bowels, and the consequent generation of air; which, when confined by a spasm on each hand, distends the intermediate part of the bowel often to an excessive degree, causing a proportional degree of painful vibrations. If we suppose these vibrations to check themselves all at once, by occasioning a sudden contraction in the affected membrane, they may be propagated over the whole nervous system instantaneously, and give rise to the convulsion-fits, which happen to young children from gripes, and distention of the stomach and bowels, and to adults, from poisons, &c. This is upon supposition, that neither spasm gives way: for, if either does, the pain goes off, for a time at least, without farther ill symptoms. Such pains in the bowels resemble those in the bladder, when the detrusor and sphincter are both contracted violently at the same time, by the irritation of a stone. The stomach, the gall-bladder, and rectum, all seem capable of like contractions in muscular fibres, that have opposite actions. The causes of all these spasms and motions are evidently the impressions in the neighbourhood, and their circumstances are, at first sight, agreeable to the theory of these papers.
Eighthly, The glands belonging to the mouth, and alimentary duct, appear to me to be emptied, not only by the compression, which the neighbouring muscles and muscular fibres make upon them, but also by the sensory vibrations which run up their excretory ducts, into the folliculi, and receptacles where there are such, and even into the secretory ducts; by which the peristaltic motion of all these is increased, so as both to receive more freely from the blood during their state of relaxation, and to squeeze more strongly through the excretory ducts during their state of contraction. Thus tobacco, pyrethrum, and other acrids, solicit a profuse discharge from the salival glands, by being barely kept in the mouth, i.e. though the neighbouring muscles do not squeeze the glands by the action of mastication. Thus likewise vomits and purges increase both the secretions of all the glands of the intestines, and those of the liver and pancreas. It may be also, that the vibrations which run up the gall-duct are sometimes so strong as to occasion a spasm there; in which case, if the patient vomit at the same time, a symptomatic and temporary jaundice may follow.
Ninthly, The expulsion of the fæces in new-born children is perfectly automatic, and seems to follow even from very gentle compressions of the abdominal muscles, when the rectum is full, inasmuch as the sphincter ani has in them scarce any force. The same may be said of the expulsion of the urine, the sphincter vesicæ being also very weak in new-born children. To which we may add, that the least irritation from fulness or acrimony in the rectum or bladder throws the abdominal muscles into contraction in young children, both on account of the extreme sensibility and irritability of their whole nervous systems, and because they have, as yet, no associated influences over the muscles of the abdomen, whereby to restrain their contractions.
As the sphincters of the rectum and bladder gain strength, more force is required to expel the fæces and urine. However, it appears, that these muscles usually exhaust themselves, previously to the instant of expulsion, thus giving free scope to their antagonists. For, according to theory, they ought to be contracted sooner and stronger than their antagonists, as being nearer to the seat of irritation.
The actions of vomiting, and expulsion of the fæces, are very nearly related to one another in their automatic state. However, it seems to me, that an irritation in the stomach produces only a gentle contraction in the sphincter ani, viz. such a one as does not exhaust its power, and which therefore tends to confine the fæces. In like manner, an irritation in the rectum may gently contract the upper orifice of the stomach. It deserves notice here, that the sphincter ani lies out of the peritonæum; and consequently, that vibrations cannot run from it to the orifice of the stomach along the peritonæum, nor vice versâ. The same observation holds in respect of the sphincter of the bladder.
The circumstances attending the exclusion of the fœtus, which continues automatic perfectly or nearly, may be much illustrated by what is here delivered concerning the expulsion of the fæces.
When young children continue to suck, or masticate a tasteless body put between their lips, or into their mouths, we may conceive, that the actions of suction and mastication begin to pass from their automatic towards their voluntary state. Drinking out of a vessel, so as to draw up the liquid, is learnt, in part, from sucking the breast, in most cases; but it may be learnt without, as is evident in those children that are dry-nursed. Mastication, when it approaches to a perfect state in children, is chiefly voluntary, the first rudiments receiving perpetual changes, so as to fall in more and more with pleasure and convenience. See Prop. XXII. Cor. I. In adults, both suction and mastication follow the command of the will with entire readiness and facility. The manner in which this is effected has been already explained in treating of the voluntary power of grasping.
Deglutition of insipid liquors becomes voluntary early. But it is difficult even for some adults, to swallow pills and boluses, though tasteless; and very nauseous liquids are sometimes rejected by them automatically, as well as by young children. The action of deglutition affords manifest evidences of the gradual transition of automatic motions into voluntary ones, as well as of voluntary ones into such as are secondarily automatic. For, in common cases, we swallow without the least express intention.
When the face of a child or adult is distorted upon the sight of a nauseous medicine, which has before produced distortions automatically, i.e. from the impressions made on the mouth and fauces, we see an evident instance of the power of associated circumstances; and may have the conception of voluntary powers, derived from a succession of such associations, made easy to the imagination.
The peristaltic motion of the stomach and bowels remains automatic to the last, depending partly on the vibrations descending from the brain, partly on the impressions made on the villous coat. It cannot depend on associated circumstances in its common state, because, being perpetual, it is equally associated with every thing, i.e. particularly so with nothing. However, as grateful aliments increase it, the sight of them may do the same by association. Could we see our stomachs and bowels, it is probable, that we should get some degree of voluntary power over them.
Vomiting is sometimes, and a nausea often, excited by associated circumstances; and there have been instances of persons who could vomit at pleasure by first introducing some of these. But, I suppose, this action never follows the mere command of the will, without the intervention of some strong associated circumstance. We have, in like manner, a semi-voluntary power of restraining vomiting, for a time at least, by means of ideas of decency, shame, fear, &c.
Some persons have a power of expelling flatulencies from the stomach in a manner which is almost voluntary; and many imitate an automatic hiccough very exactly. It facilitates these powers, that both the motions here considered are very frequent especially during childhood. Those who can hiccough voluntarily, attain to it by repeated trials, as in other cases of voluntary actions.
The spasms, and violent motions of the bowels, cannot be expected to become voluntary. They do, however, seem to return in many cases, from less and less bodily causes perpetually, on account of associated circumstances, as has been already remarked.
In like manner, the vibrations which run up the excretory ducts of the glands, must be supposed to remain totally under the influence of their original causes; unless we except the contraction of the gall-duct, which happens sometimes in violent fits of anger. This may perhaps arise from vibrations excited by associated circumstances.
Both the power of expelling the fæces and urine, and that of checking this expulsion, are under the influence of many associated circumstances, and voluntary to a considerable degree. And it will easily appear, from the principles of this theory, that they ought to be so. The filling the chest with air by the contraction of the muscles of inspiration, is a circumstance which never attends these actions in their purely automatic state. Young children learn it by the same steps as they do other methods of exerting the greatest force, and to the greatest advantage. See Prop. XXII. Cor. I.
It deserves notice here, that the action of the muscular coat of the stomach and intestines is far less subject to the power of the will, than that of the great fleshy muscles of the trunk and limbs. The efficient cause of this is the great and immediate dependence which the action of the muscular coat has upon the sensations of the villous, on account of the exquisiteness of these sensations, their constant recurrency, and the contiguity of the coats. And there is a perfect agreement of the final cause with the efficient here, as in other cases. For any great degree of voluntary power over the muscular coat of the bowels would much disturb the digestion of the aliment, as those nervous persons experience, who are so unhappy as to be exceptions to the general rule, through the influence of associated circumstances.