distinguish such foods as are salutary; for we may in general
observe, that no kind of food which is healthy, and affords proper
nutriment to the body, is disagreeable to the taste; nor are any that
are ill tasted proper for our nourishment. Those substances,
therefore, which possess strong or disagreeable savours, and which,
in general, possess a power of producing great changes on our
constitution, are to be ranked as medicines, and only to be used when
the constitution is deranged; whereas, in general, those which are
pleasant, or mild tasted, are proper for nourishing the body. We are
therefore excited or prompted to receive nourishment by the pleasant
smell or taste of the food; but the avidity with which we take it
depends much on the state of the stomach, and likewise on a certain
inanition or emptiness; for the coarsest food is grateful to those
who are hungry, and whose digestion is good; whereas, to those who
have lately eaten, or whose digestive powers are impaired, the most
delicate food affords little pleasure. While we are eating, the
saliva flows into the mouth more copiously, which excites a more
acute sensation of taste. This flow of saliva is likewise frequently
excited by the smell or sight of substances agreeable to the taste,
which causes an appetite, or desire of eating, similar to that caused
by an accumulation of gastric juice in the stomach.
In brute animals, who have not, like ourselves, the advantage of
learning from each other by instruction, the faculty of taste is much
more acute, by which they are admonished to abstain from noxious or
unhealthy food. This sense, for the same reason, is more acute in
savages than in those who live in civiilsed society, which, whatever
perfection it gives to the reasoning faculties of man, certainly
diminishes the acuteness of all our senses, partly by affording fewer
inducements to exercise them, and partly by our manner of living, and
by the application of substances to the organs of sense, which tend
to vitiate them, and render them depraved.
Taste is modified by age, temperament, habit, and disease; and in
this it obeys the general laws of sensation. Children are pleased
with the taste of what is sweet, and little stimulating; as we
advance in years the taste of more stimulating substances becomes
agreeable to us; so that we are admonished by this sense to take into
the stomach the kind of nourishment fitted to each period of life. We
often, however, counteract this salutary monitor by depraving our
sense of taste, by the too free use of vinous or spirituous liquors,
which so far deadens the sense of taste, that sweet substances become
unpleasant, and nothing but acrid and stimulating things can make an
impression on our diminished and vitiated sense of taste.
This sense, as well as others, is liable to be diseased. In order
that the sense may be perfect, it is necessary that the membrane
which envelopes the nervous papillae of the tongue, and serves as a
cuticle, should neither be too thick nor too thin, too dry nor too
moist. It is necessary likewise that the qualities of the saliva be
natural; for alterations in the nature of this liquor affect very
much the sense of taste; if it is bitter, which sometimes happens in
bilious complaints, all kinds of food have a bitter taste; if it is
sweet, the food has a faint and unpleasant flavour; and if it is
acid, the food too tastes sour.
This sense is seldom observed to be too acute, unless from a vitiated
state of the cuticle, or membrane, which covers the tongue: if this
has been abraded or ulcerated, then the substances applied to the
tongue are more sensibly tasted; in many instances, however, they do
not produce an increased sensation of taste, but only of pain.
The sense of taste, as well as of touch, may become deficient, from
various affections of the brain and nerves; this, however, is not
often the case. Some persons have naturally a diminished sense of
taste, and this generally accompanies a diminished sense of smell.
This sense is frequently diminished in sensibility from a deficiency
of saliva, as well as of the proper moisture of the tongue. Hence, in
many diseases, it becomes defective, such as fevers, colds, and the
like; both from a want of the proper degree of moisture, and from
defect of appetite, which, as was before observed, is necessary to
the sense of taste.
The sense of taste is often diminished by a thickened mucous covering
of the tongue, which prevents the application of substances to its
nervous papillae. This mucous covering arises from a disordered state
of the stomach, as well as from several other affections of the body:
hence physicians inspect the tongue, that they may be able to judge
of the general state of the body; and next to the pulse, it is
undoubtedly the best criterion that we have, as it not only points
out the nature and degree of several fevers, but likewise, in many
instances, the degree of danger to be apprehended.
Having examined the sense of taste, I shall now proceed to consider
that of smell; the use of which, like taste, is to enable us to
distinguish unwholesome from salutary food; indeed, by this sense, we
are taught to avoid what is prejudicial before it reaches the sense
of taste, to which it might be very injurious; and thus we are
enabled to avoid any thing which has a putrid tendency, which, if
received into the stomach, would taint the whole mass of fluids, and
bring on speedy dissolution.
The seat of this sense is a soft pulpy membrane, full of pores, and
small vessels, which lines the whole internal cavity of the nose. On
this membrane are distributed abundance of soft nerves, which arise
chiefly from an expansion of the first pair of nerves coming from the
brain. This membrane is likewise plentifully supplied with arteries;
so that by means of this nervous and arterial apparatus, this
membrane is possessed of very great sensibility; but the nerves of
the nose being almost naked, require a defence from the air, which is
continually drawn through the nostrils into the lungs, and forced out
again by respiration. Nature has therefore supplied this part with a
thick insipid mucus, very fluid at its first separation, but
gradually thickening, as it combines with oxygen, into a dry crust,
approaching often to a membranous matter. This mucus is poured out,
or exhaled, by the numerous minute arteries of the nostrils, and
serves to keep the nervous apparatus moist, and in a proper state for
receiving impressions, as well as to prevent the violent effects
which might arise from the stimulus of the air and other bodies. The
sense of smell is the most acute about the middle of the septum of
the nose, where the nervous membrane which I have described is
thicker and softer, than in the cavities more deeply situated, where
it is less nervous and vascular. These parts are not however
destitute of the sense.
As taste proceeds from the action of the soluble parts of bodies on
the nervous papillae of the tongue, so smell is occasioned by minute
and volatile particles flying off from bodies, which become mixed
with the air, and drawn up with it into the nostrils, where these
small particles stimulate or act upon the nerves before described,
and produce the sensation which we call smelling.
The air therefore, being loaded with the subtile and invisible
effluvia of bodies, is, by the powers of respiration, drawn through
the nose, so as to apply these particles to the almost naked
olfactory nerves, which, as was before observed, excites the sense of
smelling. When we wish to smell accurately, we shut the mouth, open
the nostrils as wide as possible, and making a strong inhalation,
draw up a greater number of these volatile particles, than could be
drawn up by the common action of respiration, by which means the
olfactory nerves are more stimulated, and produce a stronger
sensation.
In order that this sense may be enjoyed in perfection, it is
necessary that the organ of smell be in a proper state or condition
to receive impressions, and that the odorous bodies be likewise in a
proper state. With respect to the first, it is necessary that the
state of the nerves be sound, and particularly that they be kept in a
proper state with respect to moisture.
With regard to the odorous bodies, it is necessary, first, that their
minute particles should be disengaged, either by heat, friction,
fermentation, or other means capable of decomposing those bodies
which are the subjects of smell: secondly, that they may be capable
of assuming the vaporous or gaseous state, by combining with caloric,
or at any rate, that they should remain for a certain time dissolved
or suspended in the air: thirdly, that they should not meet with any
substance in their way to the nostrils, which is capable of
neutralising them, or altering their properties by its chemical
action.
Notwithstanding all the pains which physiologists have taken to
detect the nature of odorous bodies, they have met with but little
success. They are so extremely minute as to escape the other senses,
and we can only say that they must be composed of particles in an
extreme state of division and subtilty, because very small quantities
of odorous matter exhale a sufficient quantity of particles to fill a
large space. A grain of camphor, musk, or amber exhales an odour
which penetrates every part of a large apartment, and which remains
for a long time.
There is perhaps no substance in nature which is absolutely incapable
of being changed from a solid state into that of a fluid or gas, by
combining with caloric; though different substances require very
different quantities of heat to produce those effects. Those which
are with difficulty converted into fluids or gases, are termed fixed,
while those which are easily changed are called volatile; though
these are only terms of comparison, for there is probably no body
which is absolutely fixed, or incapable of being reduced to vapour by
the application of a sufficient degree of heat.
The odorous property is probably as general as that of being
convertible into gas. There is perhaps no body so hard, compact, and
apparently inodorous, as to be absolutely incapable of exciting smell
by proper methods: two pieces of flint rubbed together, produce a
very perceptible smell. Metals which appear nearly inodorous, excite
a sensation of smell by friction, particularly lead, tin, iron, and
copper. Even gold, antimony, bismuth, and arsenic, under particular
circumstances, give out peculiar and powerful odours. The odour of
arsenic in its metallic state, and in a state of vapour, resembles
that of garlic. The chief means of developing the odorous principles
are friction, heat, electricity, fermentation, solution, and mixture.
The effect of mixture is very remarkable in the case of lime and
muriate of ammoniac, neither of which, before mixture, has any
perceptible odour.
There is perhaps then no body which is perfectly inodorous, or
entirely destitute of smell: for those which have been generally
accounted such, may be rendered odorous by some of the methods I have
mentioned.
Several naturalists and physiologists, such as Haller, Linneus, and
Lorri, have attempted to reduce the different kinds of odours to
classes, but without any great success; for we are by no means so
well acquainted with the physical nature of the odorous particles, as
we are with that of light, sound, and the objects of touch; and till
we do obtain a knowledge of these circumstances, which perhaps we
never shall, it will be in vain to attempt any accurate
classification. The division of them into odours peculiar to the
different kingdoms, is very inaccurate; for the odour of musk, which
is thought to be peculiarly an animal odour, is developed in the
solution of gold by some mineral solvents; it is perceptible in the
leaves of the geranium moschatum, and some other vegetables. The
smell of garlic is possessed by many vegetables, by arsenic, and by
toads. The violet smell is perceived in some salts, and in the urine
of persons who have taken turpentine. The same may be observed with
respect to several other odours.
As taste keeps guard, or watches over the passage by which food
enters the body, so smell is placed as a sentinel at the entrance of
the air passage, and prevents any thing noxious from being received
into the lungs by this passage, which is always open. Besides, by
this sense, we are invited or induced, to eat salutary food, and to
avoid such as is corrupted, putrid, or rancid. The influence of the
sense of smell on the animal machine is still more extensive: when a
substance which powerfully affects the olfactory nerves is applied to
the nostrils, it excites, in a wonderful manner, the whole nervous
system, and produces greater effects in an instant, than the most
powerful cordials or stimulants received by the mouth would produce
in a considerable space of time. Hence in syncope or fainting, in
order to restore the action of the body, we apply volatile alkali, or
other strong odorous substances, to the nostrils, and with the
greatest effect. It may indeed for some time supply the place, and
produce the effects, of solid nutriment usually received into the
stomach We are told that Democritus supported his expiring life,
and retarded, for three days, the hour of death, by inhaling the
smell of hot bread, when he could not take nutriment by the stomach.
Bacon likewise gives us an account of a man who lived a considerable
time without meat or drink, and who appeared to be nourished by the
odour of different plants, among which were garlic, onions, and
others which had a powerful smell. In short, the stimulus which
active and pleasant odours give to the nerves, seems to animate the
whole frame; and to increase all the senses, internal and external.
The perfection of the organ of smell is different in different
animals; some possessing it very acutely; others on the contrary
having scarcely any sense of smell. We may in general observe that
this sense is much more acute in many quadrupeds than in man: an in
them the organ is much more extensive: in man, from the shape of the
head, little opportunity is given for extending this organ, without
greatly disfiguring the face. In the dog, the horse, and many other
quadrupeds, the upper jaw being large, and full of cavities, much
more extension is given to the membrane which is the organ of smell,
which in some animals is beautifully plaited, in order to give it
more surface. Hence a dog is capable of following game, or of tracing
his master in a crowd, or in a road where it could not be done by the
mere track. Nay, we are told of a pickpocket being discovered in a
crowd, by a dog who was seeking its master, and who was directed to
the man by the pocket handkerchief of his master, which the
pickpocket had stolen. In dogs the sense of smell must be uncommonly
delicate, to enable them to distinguish the way their master has gone
in a crowded city.
The habit of living in society, however, deadens this sense in man as
well as taste; for we have the advantage of learning the properties
of bodies from each other by instruction, and have therefore less
occasion to exercise this sense; and the less any sense is exercised,
the less acute will it become; hence it is, that those whom necessity
does not oblige to to exercise their senses and mental faculties, and
who have nothing to do but lounge about, and consume the fruits of
the earth, become half blind, half deaf, and, in general, have great
deficiency in the sense of smell. The use of spirituous liquors, and
particularly of tobacco in the form of snuff, serves likewise in a
remarkable manner to deaden this sense.
Savages, however, who are continually obliged to exercise all their
senses, have this, as well as others, in very great perfection. Their
smell is so delicate and perfect, that it approaches to that of dogs.
Soemmering and Blumenbach indeed assert, that in Africans and
Americans the nostrils are more extended, and the cavities in the
bones lined with the olfactory membrane much larger than in
Europeans.
I have already observed the powerful effects which some odours have
upon the nervous system. There are some which agreeably excite it,
and produce a pleasant and active state of the mind, while others, on
the contrary, produce the most terrible convulsions, and even
fainting. Those particular antipathies with respect to smells, arise
sometimes from something in the original constitution of the body,
with which we are unacquainted, but generally from the senses having
been powerfully and unpleasantly affected by certain odours at an
early period of life. The latter may often be cured by resolution and
perseverance, but the former cannot.
The sense of smell sometimes becomes too acute, either from a
vitiated state of the organ itself, which is not often the case; or
from an increased sensibility or irritability of the whole nervous
system, which is observed in hysteria, phrenitis, and some fevers.
This sense is however more often found deficient; and this may arise
from a fault in the brain or nerves, which may either proceed from
external violence, or from internal causes. A defect of smell often
arises from a vitiated state of the organ itself; for instance, if
the nervous membrane is too dry, or covered with a thick mucus; of
both of which we have an example in catarrh or common cold, where, at
the beginning, the nostrils feel unusually dry, but as the disease
advances, the pituitary membrane becomes covered with a thick mucus:
in both states, the sense of smell is in general deficient, and
sometimes nearly abolished.
This sense is sometimes depraved, and smells are perceived when no
odorous substance is present; or odours are perceived to arise from
substances, which are very different from those which we perceive in
a sound state.
There are many diseases likewise of the nose, and neighbouring parts,
which cause a depraved sensation; such as ulcers, cancer, caries; a
diseased state of the mouth, teeth, throat, or lungs; or a vitiated
state of the stomach, which sometimes exhales a vapour similar to
that of sulphureted hydrogen. This sense likewise sometimes becomes
depraved from a diseased state of the brain and nerves.
LECTURE VII.
SOUND AND HEARING.
Having in the last lecture examined the senses of taste and smell, I
now proceed to that of hearing. As the sense of smell enables us to
distinguish the small particles of matter which fly off from the
surfaces of bodies, and float in the air, so that of hearing makes us
acquainted with the elastic tremors or impulses of the air itself.
The sense of hearing opens to us a wide field of pleasure, and though
it is less extensive in its range than that of sight, yet it
frequently surmounts obstacles that are impervious to the eye, and
communicates information of the utmost importance, which would
otherwise escape from and be lost to the mind.
Sound arises from a vibratory or tremulous motion produced by a
stroke on a sounding body, which motion that body communicates to the
surrounding medium, which carries the impression forwards to the ear,
and there produces its sensation. In other words, sound is the
sensation arising from the impression made by a sonorous body upon
the air or some other medium, and carried along by either fluid to
the ear.
Three things are necessary to the production of sound; first, a
sonorous body to give the impression; secondly, a medium or vehicle
to convey this impression; thirdly, an organ of sense or ear to
perceive it. Each of these I shall separately examine.
Strictly speaking, sonorous bodies are those whose sounds are
distinct, of some duration, and which may be compared with each
other, such as those of a bell or a musical string, and not such as
give a confused noise, like that made by a stone falling on the
pavement. To be sonorous, a body must be elastic, so that the tremors
exerted by it in the air may be continued for some time: it must be a
body whose parts are capable of a vibratory motion when forcibly
struck.
All hard bodies, when struck return more or less of a sound; but
those which are destitute of elasticity, give no repetition of the
sound; the noise is at once produced and dies; while other bodies,
which are more elastic and capable of vibration, repeat the sounds
produced several times successively. These last are said to have a
tone; the others are not allowed to have any. If we wish to give
nonelastic bodies a tone, it will be necessary to make them continue
their sound, by repeating our blows quickly upon them. This will
effectually give them a tone; and an unmusical instrument has often
by this means a fine effect in concerts. The effects of a drum depend
upon this principle. Gold, silver, copper, and iron, which are
elastic metals, are sonorous; but lead, which possesses scarcely any
elasticity, produces little or no tone. Tin, which in itself has very
little more sound than lead, highly improves the tone of copper when
mixed with it. Bell metal is formed of ten parts of copper, and one
of tin. Each of these is ductile when separate, though tin is only so
in a small degree, yet they form when united a substance almost as
brittle as glass, and highly elastic. So curious is the power of tin
in this respect, that even the vapour of it, when in fusion, will
give brittleness to gold and silver, the most ductile of all metals.
Sonorous bodies may be divided into three classes; first, bells of
various figures and magnitudes: of these such as are formed of glass
have the most pure and elegant tones, glass being very elastic, and
its sound very powerful; secondly, pipes of wood or metal; thirdly,
strings formed either of metallic or animal substances. The sounds
given by strings are more grave or more acute according to the
thickness, length, and tension of the strings.
Air is universally allowed to be the ordinary medium of sound, or the
medium by which sounds are propagated from sonorous bodies, and
communicated to the ear. This may be shown by an experiment with the
air pump; also with the condenser.
But though air is the general vehicle of sound, yet sound will go
where no air can convey it; thus the scratching of a pin at the end
of a long piece of timber may be heard by an ear applied at the other
end, though it could not be heard at the same distance through the
air. On this account it is that sentinels are accustomed to lay their
ears to the ground, by which means they can often discover the
approach of cavalry, at a much greater distance than they can see
them.
For the same reason two stones being struck together under water, may
be heard at a much greater distance by an ear placed under water
likewise, than it can be heard through the air. Dr. Franklin, who
several times made this experiment, thinks that he has heard it at a
greater distance than a mile. This shows that water is better adapted
to convey sound than air.
When an elastic body is struck, that body, or some part of it, is
made to vibrate. This is evident to sense in the string of a violin
or harpsichord, for we may perceive by the eye, or feel by the hand,
the trembling of the strings, when by striking they are made to
sound. If a bell be struck by a clapper on the inside, the bell is
made to vibrate. The base, of the bell, is a circle, but it has been
found that by striking any part of this circle on the inside, that
part flies out, so that the diameter which passes through this part
of the base will be longer than the other diameter. The base, by the
stroke, is changed into an ellipse or oval, whose longer axis passes
through the part against which the clapper is struck. The elasticity
of the bell restores the figure of the base, and makes that part
which was forced out of its place, return back to its former
situation, from which the same principle throws it out again; so that
the circular figure of the bell will be again changed to an ellipse,
only now the shorter axis will pass through the part which was first
struck.
The same stroke, which makes the bell vibrate, occasions the sound,
and as the vibrations decay, the sound grows weaker. We may be
convinced by our senses that the parts of the bell are in a vibratory
motion while it sounds. If we lay the hand gently on it, we shall
easily feel this tremulous motion, and even be able to stop it, or if
small pieces of paper be put upon the bell, its vibrations will put
them in motion.