mind, and on the degree of attention which it gives. For if we are
engaged in attention to any object, we are insensible of the
impressions made upon us by others, though they are sufficiently
strong to affect us at other times. Thus, when our attention is fixed
strongly upon any particular object, we become insensible of the
various noises that surround us, though these may be sometimes very
loud. On the contrary, if our attention be upon the watch, we can
perceive slight, and almost neglected impressions, while those of
greater magnitude become insensible. The ticking of a clock becomes
insensible to us from repetition, but if we attend to it, we become
easily sensible of it, though at the same time we become insensible
of much stronger impressions, such as the rattling of coaches in the
streets.
The attention depends in some degree on the will, but is generally
given to those impressions which are particularly strong, new,
pleasant, or disagreeable; in short, to those which particularly
affect the mind. Hence it is, that things which are new, produce the
most vivid impressions, which gradually grow fainter, and at last
become imperceptible.
There is one circumstance respecting sensation, which will probably
account for our only perceiving those impressions to which the mind
attends: and this is, that the mind is incapable of perceiving more
than one impression at a time: the more accurately we examine this,
the greater reason we shall have to think it true; but the mind can
turn its attention so quickly, from one object to another, that at
first sight, we are led to believe, that we are able to attend to
several at the same time.
But though the mind cannot perceive or attend to various sensations
at the same time, yet if two or more of these are capable of uniting
in such a manner as to produce a compound sensation, this may be
perceived by the mind.
This compound sensation may be produced either by impressions made at
the same instant, or succeeding each other so quickly, that the
second takes place before the first has vanished.
As an instance of the first, we may mention musical chords, or the
sounds produced by the union of two or more tones at the same time.
We have another instance likewise in odours or smells; if two or more
perfumes be mixed together, a compound odour will be perceived,
different from any of them.
As an instance of the latter, if a paper painted of various colours
be made to revolve rapidly in a circle, a compound colour, different
from any of them, will be perceived. These observations apply
particularly to the senses we have mentioned, and likewise to taste:
but the sensations afforded us by touch do not seem capable of being
compounded in this manner.
There are many things necessary to perfect sensation, besides those
that have been mentioned. The degree and perfection of sensation will
depend much on the mind, and will be continually altered by delirium,
torpor, sleep, and other circumstances; much likewise depends on the
state of the organs with respect to preceding impressions; for if any
organ of sense have been subjected to a strong impression, it will
become nearly insensible of those which are weaker.
Of this innumerable instances may be given: an eye which has been
subjected to a strong light, becomes insensible of a weaker: and on
the contrary, if the organs of sense have been deprived of their
accustomed impressions for some time, they are affected by very
slight ones. Hence it is, that when a person goes from daylight into
a darkened room, he can at first see nothing; by degrees however he
begins to have an imperfect perception of the different objects, and
if he remain long enough, he will see them with tolerable
distinctness, though the quantity of light be the same as when he
entered the room, when they were invisible to him.
Sensation often arises from internal causes, without any external
impulse. To this source may be referred consciousness, memory,
imagination, volition, and other affections of the mind. These are
called the internal senses. The senses, whether internal or external,
have never been accurately reduced to classes, orders, or genera; the
external indeed are generally referred to five orders; namely,
seeing, hearing, smelling, tasting, and feeling, or touch. With
respect to the four first, the few qualities of external bodies which
each perceives may be easily reduced to classes, each of which may be
referred to its peculiar organ of sensation, because each organ is so
constituted, that it can only be affected by one class of properties;
thus the eye can only be affected by light; the ear by the vibrations
of the air, and so of the rest.
The same organ, whatever be its state, or whatever be the degree of
impulse, always gives to the mind a similar sensation; nor is it
possible, by any means we are acquainted with, to communicate the
sensation peculiar to one organ by means of another. Thus we are
incapable, for instance, of hearing with our eyes, and seeing with
our ears: nor have we any reason to believe that similar impressions
produce dissimilar sensations in different people. The pleasure,
however, as well as the pain and disgust, accompanying different
sensations, differ very greatly in different persons, and even in the
same person at different times; for the sensations which sometimes
afford us pleasure, at other times produce disgust.
Habit has a powerful influence in modifying the pleasures of
sensation, without producing any change in the sensation itself, or
in the external qualities suggested by it. Habit, for instance, will
never cause a person to mistake gentian or quassia for sugar, but it
may induce an appetite or liking for what is bitter, and a disgust
for what is sweet. No person perhaps was originally delighted with
the taste of opium or tobacco, they must at first be highly
disgusting to most people; but custom not only reconciles the taste
to them, but they become grateful, and even necessary.
Almost every species of sensation becomes grateful or otherwise,
according to the force of the impression; for there is no sensation
so pleasant, but, that, by increasing its intensity, it will become
ungrateful, and at length intolerable. And, on the contrary, there
are many which on account of their force are naturally unpleasant,
but become, when diminished, highly pleasant. The softest and
sweetest sounds may be increased to such a degree as to be extremely
unpleasant: and when we are in the steeple of a church, the noise of
a peal of bells stuns and confounds our senses, while at a distance
their effect is very pleasant. The smell of musk likewise at a
distance, and in small quantity, is pleasant; but when brought near,
or in large quantity, it becomes highly disagreable. The same may be
observed with respect to the objects of the other senses.
For a similar reason, many sensations which are at first pleasing,
cease to delight by frequent repetition; though the impression
remains the same. This is so well known that illustrations are
unnecessary. Those who are economical of their pleasures, or who wish
them to be permanent, must not repeat them too frequently. In music,
a constant repetition of the sweetest and fullest chords, cloys the
ear; while a judicious mixture of them with tones less harmonious
will be long relished. Those who are best acquainted with the human
heart need not be told, that this observation is not confined to
music.
On the same principle likewise we can account for the pleasure
afforded by objects that are new; and why variety is the source of so
many pleasures; why we gradually wish for an increase in the force of
the impression in proportion to its continuance.
The pleasures of the senses are confined within narrow limits, and
can neither be much increased nor too often repeated, without being
destructive of themselves; thus we are admonished by nature, that our
constitutions were not formed to bear the continual pleasures of
sense; for the too free use of any of them, is not only destructive
of itself, but induces those painful and languid sensations so often
complained of by the voluptuary, and which not unfrequently produce a
state of mind that prompts to suicide.
As the transition from pleasure to pain is natural, so the remission
of pain, particularly if it is great, becomes a source of pleasure.
There is much truth, therefore, in the beautiful allegory of
Socrates, who tells us, that Pleasure and Pain were sisters, who,
however, met with a very different reception by mankind on their visit
to the earth; the former being universally courted, while the latter
was carefully avoided: on this account, Pain petitioned Jupiter, who
decreed that they should not be parted; and that whoever embraced the
one, obtained also the other.
There is a great diversity with respect to the duration of the
pleasures of the different senses: some of the senses become soon
fatigued, and lose the power of distinguishing accurately their
different objects: others, on the contrary, remain perfect a long
time. Thus smell and taste are soon satiated; hearing more slowly;
while, of all the external senses, the objects of sight please us the
longest. We may, however, prolong the pleasures of sense by varying
them properly, and by a proper mixture of objects or circumstances
which are indifferent, and afford less delight. But the very
constitution of our nature limits our enjoyments, and points out the
impossibility of perpetual pleasures in this state of our existence.
To a person who is thirsty, water is delicious nectar; to one who is
hungry, every kind of food is agreeable, and even its smell pleasant;
to a person who is hot and feverish, the cool air is highly
refreshing. But to the same persons in different circumstances, the
same things are not only indifferent, but even disgusting; for
instance, a person cannot bear the sight or smell of food, after
having satiated himself with it, and perpetual feasting will cloy the
appetite of the keenest epicure.
I shall conclude this account of the general laws of sensation, by a
short recapitulation of those laws.
And, in the first place, it may be observed, that the energy or force
of any sensation, is proportioned to the degree of attention given by
the mind to the external object which causes it.
Secondly, A repetition of sensations diminishes their energy, and at
last nearly destroys it; but this energy is restored by rest, or the
absence of these sensations.
Thirdly, The mind cannot attend to two impressions at the same time:
so that two sensations never act with the same force at the same
instant; the stronger generally overcoming the weaker. The mind,
however, can attend to the weaker sensation, in such a manner, as to
overpower the stronger, or to render it insensible.
Having fully considered the general laws of sensation, I shall now
proceed to examine those proper to each sense; and in this
examination, two objects will engage our attention. 1. The structure
of the organ which receives and transmits the impulse to the mind. 2.
The qualities or properties of external bodies, particularly those by
which they are fitted to excite sensation.
The first sense that we shall examine is touch, which, of all the
external senses, is the most simple, as well as the most generally
diffused. By means of this sense, we are capable of perceiving
various qualities and properties of bodies, such as hardness,
softness, roughness, smoothness, temperature, magnitude, figure,
distance, pressure, and weight; this sense is seldom depraved;
because the bodies, whose properties are examined by it, are applied
immediately to the extremities of the nerves, without the
intervention of any medium liable to be deranged, as is the case with
the eye, and ear.
The organ of touch is seated chiefly in the skin, but different parts
of this covering possess different degrees of sensibility. The skin
consists of three parts. 1. The cutis vera, or true skin, which
covers the greatest part of the surface of the body. When the skin is
examined by a microscope, we find it composed of an infinite number
of papillae, or small eminencies, which seem to be the extremities of
nerves, each of which is accompanied by an artery and a vein, so that
when the vessels of the skin are injected, the whole appears red. 2.
Immediately over the true skin, and filling up its various
inequalities, lies a mucous reticulated substance, which has been
called by Malpighi, who first described it, rete mucosum. The real
skin is white in the inhabitants of every climate; but the rete
mucosum is of various colours, being white in Europeans, olive in
Asiatics, black in Africans, and copper coloured in Americans. This
variety depends chiefly on the degree of light and heat; for, if we
were to take a globe, and paint a portion of it with the colour of
the inhabitants of corresponding latitudes, we should have an uniform
gradation of shade, deepening from the pole to the equator.
The diversity of colour depends upon the bleaching power of the
oxygen, which, in temperate climates, combines more completely with
the carbonaceous matter deposited in the rete mucosum; while, in
hotter climates, the oxygen is kept in a gaseous state by the heat
and light, and has less tendency to unite with the carbonaceous
matter. In proof of this, the skins of Africans may be rendered white
by exposure to the oxygneated muriatic acid.
Over the rete mucosum is spread a fine transparent membrane, called
the cuticle, or scarf skin, which defends the organ of feeling from
the action of the air, and other things which would irritate it too
powerfully. In some parts of the body this membrane is very thick, as
in the soles of the feet, and palms of the hands; and this thickness
is much increased by use and pressure.
In general, the thinner the cuticle is, the more acute is the sense
of touch. This sense is very acute and delicate about the ends of the
fingers, where we have the most need of it; but in the lips, mouth,
and tongue, it is still more delicate; a galvanic or electrical shock
being felt by the tongue, when it is impossible for us to perceive it
by the fingers.
This sense, like the others, becomes more exquisite when its organ is
defended from the action of external bodies; it is on this account
that the cuticle becomes so sensible under the end of the nail, which
defends it from the action of external objects; and when part of the
nail is taken away, we can scarcely bear to touch any thing with this
newly exposed part of the skin.
When we place our fingers upon the surface of any body, the first
sensation we experience is that of resistance, after which the other
properties are perceived in a natural order; such as heat or cold,
moisture or dryness, motion or rest, distance, and figure or shape.
With respect to the diseases of this sense, it is very seldom that it
becomes too acute over the whole body; though it frequently does so
in particular parts, which may arise from the cuticle being too thin
or abraded, or from an inflamed state of the part.
It however becomes sometimes obtuse, and indeed almost abolished over
the whole body; and this takes place from compression of the brain,
and various affections of the nervous power. This diminution is
called anaesthesia. The touch becomes deficient, and indeed almost
abolished, when the cuticle is injured by the frequent application of
hot bodies, or acrid substances: thus the cuticle of the hands of
blacksmiths and glassblowers is generally so hard and horny, that
they can take up and grasp in their hand pieces of redhot iron with
impunity.
We generally refer pain to this sense, though it may arise from too
violent an impression made upon any of the organs of sense.
Pain is an unpleasant sensation, which the mind refers to some part
of the body, and very accurately, if any part of the surface is
affected, but less so, if it arises from the affection of an internal
part. The sensation of pain may arise from any thing which tends to
injure the structure of the body, whether that be internal or
external; so that it serves as a monitor to put us on our guard, and
to induce us to remove any thing which might be injurious to us. This
sensation is produced by any thing which punctures, cuts, tears,
distends, compresses, bruises, corrodes, burns, or violently
stimulates any part of the body.
A moderate degree of pain in any part excites the action of the whole
body; a greater quantity of blood and nervous energy is determined to
the part. A still greater degree of pain brings on inflammation and
its consequences, and if it be intense, it will bring on fever,
convulsions, delirium, fainting, and even death.
The endurance of pain depends much on the strength of mind possessed
by the patient, which, in some instances, is such, that the most
violent pains are patiently endured; while in other instances, the
slightest can scarcely be born.
It is a curious circumstance, that a moderate degree of pain, when
unaccompanied by fever, often tends to render the understanding more
clear, lively, and active. This is confirmed by the experience of
people labouring under gout. We have an account of a man who
possessed very ordinary powers of understanding, but who exhibited
the strongest marks of intelligence and genius in consequence of a
severe blow on the head; but that he lost these powers when he
recovered from the effects of the blow. Pechlin mentions a young man,
who during a complaint originating from worms, possessed an
astonishing memory and lively imagination, both of which he nearly
lost by being cured. Haller mentions a man who was able to see in the
night, while his eyes were inflamed, but lost this power as he got
well. All these facts show, that a certain action or energy is
necessary for the performance of any of the functions of the body or
mind; and whatever increases this action will, within certain limits,
increase those functions.
Feeling is by far the most useful, extensive, and important of the
senses, and may be said indeed to be the basis of them all. Vision
would be of very little use to us, if it were not aided by the sense
of feeling; we shall afterwards see that the same observation may be
applied to the other senses. In short, it is to this sense that we
are indebted, either immediately or indirectly, for by far the
greatest part of our knowledge; for without it we should not be able
to procure any idea with respect to the magnitude, distance, shape,
heat, hardness or softness, asperity or smoothness of bodies; indeed,
if we were deprived of this sense, it is difficult to say whether we
should have any idea of the existence of any external bodies; on the
contrary, it seems probable that we should not.
LECTURE VI.
TASTE AND SMELL.
From the sense of touch we proceed naturally to that of taste, for
there seems to be less difference between these two senses than
between any of the others. The sense of taste appears to be seated
chiefly in the tongue; for any sweet substance, such as sugar,
applied to any other part of the mouth, scarcely excites the least
sensation of taste. The same may be observed with respect to any
other sapid body, which, unless it is strongly acrid or irritating,
produces no effect on any other part than the tongue; but if it is
possessed of much acrimony, it then not only affects the palate, and
uvula, but even the oesophagus.
The tongue is a muscular substance, placed in the mouth, connected by
one end with the adjacent bones and cartilages, while the other end
remains free, and easily moveable. The tongue is furnished,
particularly on its upper surface, with innumerable nervous papillae,
which are much larger than those I described as belonging to the
skin. These papillae are of a conical figure, and extremely sensible,
forming, without doubt, the true organ of taste; other papillae are
found between them, which are partly conical, and partly cylindrical.
Over the papillae of the tongue is spread a single mucous, and
semipellucid covering, which adheres firmly to them, and serves the
purpose of a cuticle.
Under these papillae are spread the muscles which make up the fleshy
part of the tongue: these are extremely numerous, and by their means
the tongue possesses the power of performing a great variety of
motions with surprising velocity.
The arteries leading to the tongue are extremely numerous; and, when
injected with a red fluid, the whole substance appears of a beautiful
red.
The tongue is likewise furnished with a large supply of nerves, some
of which undoubtedly serve to supply its muscles with nervous energy,
while others terminate in the papillae, and form the proper organ of
taste: this office seems to be performed by the third branch of the
fifth pair of nerves. The papillae, before described, are formed or
composed of a number of small nerves, arteries, and veins, firmly
united together by cellular substance. These papillae are excited to
action by the application of any sapid body; in consequence of which
they receive a greater supply of blood, become enlarged, and vastly
more sensible.
The structure of the tongue differs in different animals, which
likewise possess corresponding differences with respect to taste. In
those quadrupeds, in which it is armed with sharp points, the sense
of taste is by no means acute. The same is the case with birds and
reptiles, whose tongues are very dry and rough.
In a former lecture I took notice of a liquor which is secreted by
the glands of the mouth and neighbouring parts, which is called
saliva. This liquor acts an important part in the production of
taste; it does not differ much from water, excepting by containing a
quantity of mucilage; and nothing is sapid, or capable of affecting
the sense of taste, unless it is in some degree soluble in this
liquor. Hence earthy substances, which are nearly insoluble, have
little or no taste.
It is not, however, sufficient that the substance be possessed of
solubility alone; it is necessary likewise that it should be
possessed of saline properties, or, at least, of a kind of acrimony,
which renders it capable of stimulating the nervous papillae. Hence
it is that those substances which are less saline, and less acrid
than the saliva, have no taste.
We are capable of distinguishing various kinds of taste, but some of
them with less accuracy than others. Among the different kinds of
taste, the following have been considered by Haller, and some other
physiologists, as primitive: sweet, sour, bitter, and saline. The
others have been thought to be compounded of these; for the sense of
taste, as well as sight and hearing, is capable of perceiving
compound impressions. To these primitive tastes, Boerhaave added
alkaline, spirituous, aromatic, and some others. Of these, in
different proportions, all the varieties of tastes, which are
extremely numerous, are composed.
Some tastes are pleasant and agreeable, others disagreeable, and
scarcely tolerable: there is, however, a great diversity in this
respect experienced by different persons; for the same taste, which
is highly grateful to some, is extremely unpleasant to others.
But the most pleasant tastes, agreeably to the general laws of
sensation, which I described in the last lecture, become gradually
less pleasant, and at last disgusting; while, on the contrary, the
most disagreeable savours, such as tobacco, opium, and assafoetida,
become, by custom, not only tolerable, but highly agreeable.