Observations on Man (6th edition)/Part I/Chapter II/Section III
Section III
[edit]THE SENSE OF SMELL.
Smell may be distinguished into two sorts: first, that exquisite sensation, which odoriferous bodies impress upon the nose by means of alternate inspiration. This is smell, in the peculiar and most proper sense of the word; and it resides chiefly, or perhaps entirely, in that part of the pituitary membrane, which invests the cells of the ossa spongiosa. Secondly, that sensation or flavour, which most kinds of aliment and medicines impress upon the whole pituitary membrane during mastication, and just after deglutition. And this last makes a principal part of the pleasures and pains which are usually referred to the taste. For when a person has a cold, i.e. when the pituitary membrane is obstructed and loaded with mucus, meats lose their agreeable flavours; and the same thing happens in a polypus narium.
Besides this, it is to be observed, that the pituitary membrane has an exquisite sensibility, which may be referred to the head of feeling. For active powders, i.e. sternutatories, seem to irritate the membrane of the nose in the same way, as they do a part of the skin deprived of the cuticle, only in a greater degree, and more immediately. And thus smells themselves may be referred to the head of feeling; since strong smells are often observed to occasion sneezing.
It may also be remarked, that as the organ of feeling passes insensibly into that of taste, so the organ of taste does into that of smell. And these three senses have a much greater resemblance to one another, than any of them has to the sight, or to the hearing; or than the sight and hearing have to each other. However, the organ of feeling is distinguished from that of taste by its being covered with the hard cuticle, and the organ of taste from that of smell by the last’s being extended upon bones; so as to be much more sensible and irritable upon that account. To which we may add, that as a watery fluid is the proper menstruum for the dissolution of sapid particles, and conveyance of their tastes, so smells seem to make their impressions by means of air-particles.
Here we may observe, first, that since the smells of bodies diffuse themselves in general to great distances, and in some cases to immense ones, the odoriferous particles must repel each other; and consequently be easily susceptible of vibratory motions, for the same reasons as the particles of common air, or those of the æther. We may even suppose, that odoriferous particles are thrown off by vibratory motions in the body that emits them.
May not, however, the odoriferous particles be attracted by the body which emits them, after they have receded from it to a certain distance, and so follow it, in some measure, like an atmosphere? It is hard to account for the small or no diminution of weight in odoriferous bodies, after they have continued to emit smells for a long time, but upon some such supposition.
Secondly, Heat, friction, and effervescence, are all very apt to excite and increase smells; and have all a connexion with vibratory motions in the judgment of most philosophers.
Thirdly, Since heat and friction excite and increase smells, these may have some connexion with electricity; which is supposed by many philosophers to depend upon vibratory motions. And as air-particles are electrics per se, they may have, on this account, a peculiar fitness for conveying and impressing smells. May not air-particles, and odoriferous ones, repel each other?
Fourthly, It is usual, when we desire to receive a smell in full strength and perfection, to make quick, short, alternate inspirations and expirations. This corresponds to the rubbing the ends of the fingers upon the body to be examined by feeling, and the tongue against the palate in tasting. And all these three actions appear to be some presumption in favour of the doctrine of vibrations.
Fifthly, The greatness and quickness of the effect of odours upon the whole nervous system seem very suitable to the doctrine of vibrations. For this must be owing to the mere impression of some motion, there not being time for the absorption of particles sufficient for the effect produced. When sweet smells cause a sudden faintness, and deliquium animi, they may perhaps agitate the whole system of small medullary particles so much, as to make them attract each other with sufficient force to stop all vibratory motions; just as has been observed of the particles of muscular and membranous fibres. And the smells to which a person has an antipathy, may have been originally sweet, or lie so near the confines of pleasure, as to propagate their vibrations much farther than original fœtids can. For these seem to revive from fainting by making a vigorous impression on the nose, which yet is not propagated freely over the whole system; or, if it be, will occasion immediate sickness and fainting. Fœtids in this resemble other pains, which, if moderate, excite; if very violent, overpower.
If it be objected to this, that such fragrant smells as a person has an antipathy to, are disagreeable to him in the highest degree, and that upon the first perception; also that the smell of those fœtids which revive, as of asafœtida, spirit of hartshorn, &c. is agreeable to many; I answer, that these two opposite changes seem to arise merely from association. The faintness and revival, attending these smells respectively, must, by association, transfer the vestiges and miniatures of themselves upon the first perception of the smells, whose associates they are.
Sixthly, It is agreeable to the notion of vibrations, that spirituous liquors and opium should produce their appropriated effects by smell, as well as by being taken into the stomach, as they are found to do in fact. For, if these effects arise from specific vibrations, the mere impression of small active particles may be sufficient for the purpose of producing them. We must, however, suppose that the exhalations of odoriferous bodies are imbibed in some small degree by the absorbing vessels of the membrana schneideriana. We might shew by parity of reason, that the great subtlety of odoriferous effluvia favours the doctrine of vibrations.
Though odoriferous particles are more subtle than the sapid ones, yet they are perhaps grosser than the rays of light. For the smoke of a tallow candle ceases to smell, when it begins to shine, i.e. when it is more attenuated by heat. Since therefore the vibrations from heat are probably smaller than those from light, we may range the vibrations of the medullary substance in the following order, in respect of subtlety; heat, light, smell, tastes, tangible impressions, and the vibrations of the air, from which sound arises. But it is to be observed, that these last may excite much more frequent vibrations in the auditory nerve, than those of the sounding body, to which they correspond; just as the vibrations from friction are much more numerous than the strokes of friction; and the tremors of the particles of an anvil much more numerous than the strokes of the hammer.
This proposition is analogous to the thirty-eighth, in which the agreement of the specific differences of tastes with the doctrine of vibrations is considered; and may be illustrated by it. One may say indeed, that taste and smell are so nearly allied to each other, that, if one be performed by vibrations, the other must also. I will repeat two principal observations.
First, If the varieties of kind in vibrations be combined with those of degree, we shall have a large fund for explaining the various fragrant and fœtid smells, notwithstanding that the first always agree in falling short of the solution of continuity, the last in going beyond it.
Secondly, the differences of kind in smells are not so many as may appear at first sight, a difference in degree often putting on the appearance of one in kind. Thus an onion cut fresh, and held close to the nose, smells very like asafœtida; and asafœtida, in an evanescent degree, like onion or garlick. Thus a dunghill at a distance has smelt like musk, and a dead dog like elder-flowers. And fœtids are said to enhance the flavour of fragrants. The three last instances shew, that pleasure and pain are very nearly allied to one another in this sense also.
It will be evident, upon a moderate attention, that the grateful smells, with which natural productions abound, have a great share in enlivening many of our ideas, and in the generation of our intellectual pleasures; which holds particularly in respect of those that arise from the view of rural objects and scenes, and from the representations of them by poetry and painting. This source of these pleasures may not indeed be easy to be traced up in all the particular cases; but that it is a source, follows necessarily from the power of association.
In like manner, the mental uneasiness, which attends shame, ideas of indecency, &c. arises, in a considerable degree, from the offensive smells of the excrementitious discharges of animal bodies. And it is remarkable in this view, that the pudenda are situated near the passages of the urine and fæces, the two most offensive of our excrements.
We may suppose the intellectual pleasures and pains, which are deducible from the flavours, grateful and ungrateful, that ascend behind the uvula into the nose during mastication, and just after deglutition, to have been considered in the last section under the head of taste, since these flavours are always esteemed a part of the tastes of aliments and medicines. And indeed the olfactory nerves seem to have as great a share in conveying to us both the original and derivative pleasures, which are referred to the taste, as the nerves of the tongue; which may help us to account for the largeness of those nerves in men, to whom smell, properly so called, is of far less consequence than any other of the senses, and taste of the greatest, while yet the nerves of taste are comparatively small.
We may add here, that the smell is a guide and guard placed before the taste, as that is before the stomach, in a great degree in men, but much more so in brutes, who have scarce any other means, than that of smell, whereby to distinguish what foods are proper for them. It is likewise probable, that the smell is a guard to the lungs; and that the grateful odours of flowers, fruits, and vegetable productions, in general, are an indication of the wholesomeness of country air; as the offensiveness of putrefaction, sulphureous fumes, &c. warn us beforehand of their mischievous effects upon the lungs. However, the rule is not universal in either case.
What has been delivered concerning the ideas of feeling and taste, may be applied to the smell. We cannot, by the power of our will or fancy, raise up any miniatures or ideas of particular smells, so as to perceive them evidently. However, the associated circumstances seem to have some power of affecting the organ of smell, and the corresponding part of the brain, in a particular manner; whence we are prepared to receive and distinguish the several smells more readily, and more accurately, on account of the previous influence of these associated circumstances. And, conversely, the actual smells of natural bodies enable us to determine them, though we do not see them, always negatively, and often positively, i.e. by suggesting their names, and visible appearances. And, when we are at a loss in the last respect, the name or visible appearance of the body will immediately revive the connexion.
These automatic motions are of three kinds; viz. the inspiration, by which young brute animals, especially quadrupeds, impress and increase the odours of their respective foods; the contraction of the fauces, and upper part of the gullet, which arises from those agreeable flavours, which ascend behind the uvula into the nose; and the action of sneezing.
As to the first; it is peculiar to brutes, children not using any methods of improving odours, till they are arrived at two or three years of age. The reasons of this difference may be, that the smell in many brutes is the leading sense; that their noses are long and large, and the ossa spongiosa hollowed by innumerable cells; whereas in young children the nose is depressed; the pituitary membrane loaded with mucus; and, when they grow up, the acuteness of their smell is far inferior to that of quadrupeds.
If it be said, that this action is not automatic in brutes, but an instinct, which they bring into the world with them; I answer, that the nearness of the muscles affected, viz. those which dilate the nose, larynx, and lungs, to the seat of the impression, makes it probable, that the motion depends upon the sensation, as in other instances mentioned in these papers, some of which are allowed by all.
It may be, that something of the same kind takes place in young children, as soon as their smell begins to be sufficiently acute. But it is so mixed with, and modelled by, voluntary motions, as to be separately indiscernible.
The second motion, or the contraction of the fauces, and upper part of the œsophagus, from the grateful flavours which ascend up into the nostrils behind the uvula, is part of the action of deglutition; but it could not properly be mentioned in the last Section, because it arises from a sensation referred to this.
Ungrateful flavours have often a contrary effect, and extend their influence so far as to preclude the passage through the gullet, and even throw back the ungrateful liquid or morsel with violence. And we may observe, that in many other cases also, when the pleasure passes into pain, the automatic motion thereon depending passes into one of an opposite nature; just as in algebra, when an affirmative quantity in the data is changed into a negative one, a like change is to be made sometimes, and yet not always, in the conclusion.
It deserves notice here, that pinching the nose prevents the perception of these flavours, as it seems, by checking the vibrations, which would run along the pituitary membrane. When the flavours are very pungent, they fix in the tip of the nose; or, if this be hindered by pinching the nose, they fly to the uvula, which is the nearest extreme part to this.
In like manner, pinching the nose, or pressing the lachrymal bag, whose membrane is continuous to the pituitary one, checks the sensation that gives rise to sneezing. And when looking at a strong light excites this action, or acrid vapours make the eyes water, we may conjecture, that vibrations pass through the lachrymal duct from the eye to the nose in the first case, and from the nose to the eye in the last. The watering of the eyes from drinking hastily, especially pungent liquors, from plucking a hair out of the nostrils, and from sternutatories, admit of a like explication.
And these instances may help to explain the sensations in the fauces, uvula, and tip of the nose, also the flowing of tears from the eyes, which attend grief. I conjecture that the stomach is particularly affected in grief; and that it sends up vibrations along the common membrane, to the fauces, uvula, tip of the nose, and eyes. However, the disorder of the medullary substance is great and general in great mental uneasinesses.
As to sneezing; no one can doubt its being automatic. And it is reasonable to expect, that the muscles actually concerned in it, viz. those of inspiration, and the erectors of the head and neck, should be affected by vivid sensations in the pituitary membrane. It seems also to me, that the muscles which stop the passage through the nose, ought to be contracted first, i.e. during the inspiration, as being nearer to the seat of irritation; and afterwards relaxed during expiration, partly by their having exhausted their own power, partly by the contraction of their antagonists, which are irritated also. The contrary happens, but for the same general reasons, in the action of deglutition, as has been already observed. And there is a remarkable coincidence of the efficient and final causes in both these instances.
In speaking of the sources of motory vibrations above, Prop. XVIII. I supposed, that just before the motory vibrations excited by the irritation of membranes took place, the sensory ones in them were checked by the general contraction of their fibres, in all their directions. And I mentioned sneezing, as affording an instance of this. For the sensation, which causes it, disappears the instant before the inspiration; and if this be not strong enough, i.e. if the muscles do not receive the vibrations from the pituitary membrane with sufficient freedom, it returns again and again, being increased by this reciprocation, till at last it causes sneezing. It seems agreeable to this account, that the passage of air, cold absolutely or relatively, through the nose, will often occasion sneezing; and through the mouth, yawning. For cold air must contract the membranes along whose surfaces it passes.
When sneezing rouses from a stupor, it may be supposed to excite the usual degree and kind of vibrations in the medullary substance of the brain, by such a moderate concussion of it, as lies within the limits of nature and health.
The short, quick, alternate inspirations and expirations, by which we distinguish smells in perfection, are in men, totally or nearly, a voluntary action, derived partly from common respiration, partly from sneezing, the prospect of pleasure and convenience concurring to it, and modelling it, as in other cases. It seems also, that in brutes this action must pass from its pure automatic state to some degree of a voluntary one.
In what manner and degree deglutition is voluntary, has been considered already.
Sneezing is checked for a time by attention, surprise, and all strong mental emotions. It may also be performed voluntarily; but then the force is much inferior to that of automatic sneezing. The same may be observed of hiccough, coughing, yawning, stretching, &c. and is very agreeable to the derivative nature of these motions, when voluntary, i.e. when performed by motory vibratiuncles. The action of sneezing is differently modelled by voluntary and semi-voluntary powers in different persons.