so far as we are acquainted with it, and how its products are
conveyed into the blood. But to what purposes the blood is employed,
which is formed with so much care, we have yet to discover. It seems
to answer two purposes. The parts of which the body is composed,
namely, bones, muscles, ligaments, membranes, &c. are continually
changing: in youth they are increasing in size and strength, and in
mature age they are continually acting, and, consequently,
continually liable to waste and decay. They are often exposed to
accidents, which render them unfit for performing their various
functions; and even when no such accidents happen, it seems necessary
for the health of the system that they should be perpetually renewed.
Materials must therefore be provided for repairing, increasing, or
renewing all the various organs of the body. The bones require
phosphate of lime, and gelatine, the muscles fibrine, and the
cartilages and membranes albumen; and accordingly we find all these
substances contained in the blood, from whence they are drawn, as
from a storehouse, whenever they are wanted. The process by which
these different parts of the blood become various parts of the body
is called assimilation.
Over the nature of assimilation the thickest darkness still hangs;
all that we know for certain is, that there are some conditions
necessary to its action, without which it cannot take place. These
are, 1. A sound and uninterrupted state of the nerves. 2. A sound
state of the blood vessels. 3. A certain degree of tone or vigour in
the vessels of the part.
There remains yet to be noticed another set of vessels, connected
with the circulating and nutritive systems, called lymphatics. These
vessels are very minute, and filled with a transparent fluid: they
rise by open mouths in every cavity of the body, as well as from
every part of the surface, and the course of those from the lower
extremities, and indeed from most of the lower parts of the body, is
towards the thoracic duct, which they enter at the same time with the
lacteal vessels already described. They are furnished, like the
lacteals, with numerous valves, which prevent their contents from
returning towards their extremities.
The minute arteries in every part of the body exhale a colourless
fluid, for lubricating the different parts, and other important
purposes: and the lymphatic vessels absorb the superfluous quantity
of this fluid, and convey it back to the blood.
It must be evident therefore, that, if the lymphatics in any cavity
become debilitated, or by any other means be prevented from absorbing
this exhaled fluid, an accumulation of it will take place: the same
will happen, if the exhaling arteries be debilitated, so as to allow
a greater quantity of fluid to escape than the absorbents can take
up. When the balance between exhalation and absorption is destroyed,
by either or both of these means, a dropsy will be the consequence.
Before we finish the subject of digestion, I shall take a short view
of some of the morbid affections, attending this important function
of the animal economy.
A deficiency of appetite may arise, either from an affection of the
stomach, or a morbid state of the body: for there is such a sympathy
between the stomach and the rest of the system, that the first is
very seldom disordered, without communicating more or less disorder
to the system: nor can the system become deranged and the stomach
remain sound.
A want of appetite may arise from overloading the stomach, whereby
its digestive powers will be weakened. And this may be occasioned in
two ways. First, by taking food of the common quality in too great
quantity, which will certainly weaken the powers of the stomach. An
excellent rule, and one which if more attended to, would prevent the
dreadful consequences of indigestion, is always to rise from the
table with some remains of appetite. This is a rule applicable to
every constitution, but particularly to the sedentary and
debilitated.
The second way in which the stomach may be debilitated, is by taking
food too highly stimulating or seasoned; and this even produces much
worse effects than an over dose with respect to quantity. The tone of
the stomach is destroyed, and a crude unassimilated chyle is absorbed
by the lacteals, and carried into the blood, contaminating its whole
mass. Made dishes, enriched with hot sauces, stimulate infinitely
more than plain food, and bring on diseases of the worst kind: such
as gout, apoplexy, and paralysis. "For my part," says an elegant
writer, "when I behold a fashionable table set out in all its
magnificence, I fancy I see gouts, and dropsies, fevers, and
lethargies, with other innumerable distempers, lying in ambuscade
among the dishes."
All times of the day are not equally fitted for the reception of
nourishment. That digestion may be well performed, the functions of
the stomach and of the body must be in full vigour. The early part of
the day therefore is the proper time for taking nutriment; and, in my
opinion, the principal meal should not be taken after two or three
o'clock, and there should always be a sufficient time between each
meal to enable the stomach to digest its contents. I need not remark
how very different this is from the common practice of jumbling two
or three meals together, and at a time of the day likewise when the
system is overloaded. The breakfast at sunrise, the noontide repast
and the twilight pillow, which distinguished the days of Elizabeth,
are now changed for the evening breakfast, and the midnight dinner.
The evening is by no means the proper time to take much nourishment:
for the powers of the system, and particularly of the stomach, are
then almost exhausted, and the food will be but half digested.
Besides, the addition of fresh chyle to the blood, together with the
stimulus of food acting on the stomach, always prevents sleep, or
renders it confused and disturbed, and instead of having our worn out
spirits recruited, by what is emphatically called by Shakespeare,
"the chief nourisher in life's feast," and rising in the morning
fresh and vigorous, we become heavy and stupid, and feel the whole
system relaxed.
It is by no means uncommon, for a physician to have patients, chiefly
among people of fashion and fortune, who complain of being hot and
restless all night, and having a bad taste in the mouth in the
morning. On examination, I have found that, at least in nineteen
cases out of twenty, this has arisen from their having overloaded
their stomachs, and particularly from eating hot suppers; nor do I
recollect a single instance of a complaint of this kind in any person
not in the habit of eating such suppers.
The immoderate use of spirituous and fermented liquors, is still more
destructive of the digestive powers of the stomach; but this will be
better understood, when we have examined the laws by which external
powers act upon the body. The remarks I have made could not, however,
I think, have come in better, than immediately after our examination
of the structure of the digestive organs, as the impropriety of
intemperance, with respect to food, is thus rendered more evident.
The appetite becomes deficient from want of exercise, independently
of the other causes that have been mentioned. Of all the various
modes of preserving health, and preventing diseases, there is none
more efficacious than exercise; it quickens the motion of the fluids,
strengthens the solids, causes a more perfect sanguification in the
lungs, and, in short, gives strength and vigour to every function of
the body. Hence it is, that the Author of nature has made exercise
absolutely necessary to the greater part of mankind for obtaining
means of existence. Had not exercise been absolutely necessary for
our well being, says the elegant Addison, nature would not have made
the body so proper for it, by giving such an activity to the limbs,
and such a pliancy to every part, as necessarily produce those
compressions, extensions, contortions, dilatations, and all other
kinds of motion, as are necessary for the preservation of such a
system of tubes and glands.
We may, indeed, observe, that nature has never given limbs to any
animal, without intending that they should be used. To fish she has
given fins, and to the fowls of the air wings, which are incessantly
used in swimming and flying; and if she had destined mankind to be
eternally dragged about by horses, her provident economy would surely
have denied them legs.
The appetite becomes deficient on the commencement of many diseases,
but this is to be looked upon here rather as a salutary than as a
morbid symptom, and as a proof that nature refuses the load, which
she can neither digest nor bear with impunity.
In healthy people the appetite is various, some requiring more food
than others; but it sometimes becomes praeternaturally great, and
then may be regarded as a morbid symptom. The appetite may be
praeternaturally increased, either by an unusual secretion of the
gastric juice, which acts upon the coats of the stomach, or by any
acrimony, either generated in, or received into the stomach, or,
lastly, by habit, for people undoubtedly may gradually accustom
themselves to take more food than is necessary.
The appetite sometimes becomes depraved, and a person thus affected,
feels a desire to eat substances that are by no means nutritious, or
even esculent: this often depends on a debilitated state of the whole
system. There are some instances, however, in which this depravity of
the appetite is salutary; for example, the great desire which some
persons, whose stomachs abound with acid, have for eating chalk, and
other absorbent earths: likewise, the desire which scorbutic patients
have for grass, and other fresh vegetables. Appetites of this kind,
if moderately indulged in, are salutary, rather than hurtful.
The appetite for liquids as well as solids is sometimes observed to
be deficient, and sometimes too great. The former can scarcely be
considered as a morbid symptom, provided the digestion and health be
otherwise good. But when along with diminished thirst, the fauces and
tongue are dry, this deficiency may be regarded as a morbid and
dangerous symptom.
A more common morbid symptom, however, is too great thirst, which may
arise from a deficiency of fluids in the body, produced by violent
exercise, perspiration, too great a flow of urine, or too great an
evacuation of the intestines. A praeternatural thirst may likewise
arise from any acrid substance received into the stomach, which our
provident mother, nature, teaches us to correct by dilution; this is
the case with respect to salted meats, or those highly seasoned with
pepper. It may arise also from the stomach being overloaded with
unconcocted aliment, or from a suppressed or diminished secretion of
the salivary liquors in the mouth, which may arise from fever, spasm,
or affections of the mind; an increased thirst may likewise take
place, from a derivation or determination of the fluids to other
parts of the body; of this, dropsy affords an example. Indeed,
various causes may concur to increase the thirst; this is the case in
most fevers, where great thirst is occasioned by the dissipation of
the fluids of the body by heat, as well as by the diminished
secretion of the salivary humours which should moisten the mouth; to
which may be added, the heat and diminished concoctive powers of the
stomach.
From what has been said, we can easily understand, why praeternatural
thirst may sometimes be a necessary instinct of nature, at other
times, an unnecessary craving; why acids, acescent fruits, and weak
fermented liquors quench thirst more powerfully than pure water; and
lastly, why thirst, in some instances, may be relieved by emetics,
when it has resisted other remedies.
There is no organ of the body whose functions are so easily deranged
as those of the stomach; and these derangements prove a very fertile
source of disease; they ought, therefore, carefully to be guarded
against; and it is fortunate for us that we have this generally in
our power, if we would but avail ourselves of it: for most of the
derangements proceed from the improper use of food and drink, and a
neglect of exercise. Indeed, when we examine, we shall find but a
short list in the long catalogue of human diseases, which it is not
in our power to guard against and prevent: and which surely will be
guarded against, when their causes are known, and consequences
understood.
Among the diseases arising from a disordered state of the stomach and
indigestion, may be enumerated the following: great oppression and
anxiety, pain in the region of the stomach, with acid eructations,
nausea, vomiting, the bowels sometimes costive, sometimes too loose,
but seldom regular, depression of spirits, and all the long list,
commonly, but very improperly, termed nervous complaints, deficient
nutrition, and consequently general weakness, a relaxed state of the
solids, too great a tenuity of the fluids, headach, vertigo, and
many other complaints, too numerous to mention here.
The greatest misfortune, and which indeed arises from a want of
physiological knowledge, is, that people labouring under these
disorders, imagine they may be cured by the reception of drugs into
the stomach, and thus they are induced to receive into that organ,
half the contents of an apothecary's shop. There is no doubt that
these complaints may oftentimes be alleviated, and the cure assisted,
by medicines: thus, when the stomach is overloaded, this may be
removed by an emetic; the same complaint of the bowels may be removed
by a cathartic; and when the stomach is debilitated, we are
acquainted with some substances which will give it vigour, such as
iron, the Peruvian bark, and several kinds of bitters. These however,
when used alone, afford but temporary relief; and unless the cause
which induced the disease be removed, it will afterwards return with
redoubled violence. When the stomach, for instance, is debilitated by
want of exercise, I would ask, is there an article in the whole
materia medica, that can cure the complaints of sedentary people,
unless proper exercise at the same time be taken? With exercise tonic
remedies will undoubtedly accelerate the cure, but without it, they
will only make bad worse.
Again, when the stomach is debilitated by the use of improper food,
or the abuse of fermented or spirituous liquors, I would say to any
one who pretended to cure me of these complaints, without my making a
total change in the manner of living, that he either was ignorant of
the matter, or intended to deceive me.
In many cases the change of food must be strictly observed and
persevered in for a long time before a cure can be effected. In some
instances where the powers of the stomach were too weak to prevent
the food from undergoing perhaps both a vinous and acetous
fermentation, and where, in consequence of the disengagement of gas
and the formation of acid, the most excruciating pains were felt, the
most dreadful sickness experienced, and all the symptoms of
indigestion present in the most aggravated state; after almost every
article in the materia medica, generally employed, had been tried
without success, I have cured the patient merely by prohibiting food
subject to fermentation, such as vegetables, and enjoining a strict
use of animal food alone.
In short, wherever the cause of a disease can be ascertained, the
grand and simple secret in the cure, is the careful removal of that
cause.
LECTURE V.
OF THE SENSES IN GENERAL.
In this lecture, I propose to take a view of the connexion of man
with the external world, and shall endeavour to point out the manner
in which he becomes acquainted with external objects, by means of the
faculties called senses.
A human creature is an animal endowed with understanding, and reason;
a being composed of an organized body, and a rational mind.
With respect to his body, he is pretty similar to other animals,
having similar organs, powers, and wants. All animals have a body
composed of several parts, and, though these may differ from the
structure of the human body in some circumstances, to accommodate it
to peculiar habits and wants of the animal, still there is a great
similarity in the general structure.
The human body is feeble at its commencement, increases gradually in
its progress by the help of nourishment and exercise, till it arrives
at a certain period, when it appears in full vigour; from this time
it insensibly declines to old age, which conducts it at length to
dissolution. This is the ordinary course of human life, unless it
happens to be abridged either by disease or accident.
With regard to his reasoning faculties, or mind, man is eminently
distinguished from other animals. It is by this noble part that he
thinks, and is capable of forming just ideas of the different objects
that surround him: of comparing them together; of inferring from
known principles unknown truths; of passing a solid judgment on the
mutual agreement of things, as well as on the relations they bear to
him; of deliberating on what is proper or improper to be done; and of
determining how to act. The mind recollects what is past, joins it
with the present, and extends its views to futurity. It is capable of
penetrating into the causes of events, and discovering the connexion
that exists between them.
Governed by invariable laws, which connect him with all the beings,
whether animate or inanimate, among which he exists, man has certain
relations of convenience, and inconvenience, arising from the
particular constitution of the surrounding objects, as well as of his
own body. These external objects possess qualities which may be
useful or prejudicial to him; and his interest requires, that he
should be capable of ascertaining and appreciating these properties.
It is by sensation, or feeling, that the knowledge of external
objects is obtained. The faculty of feeling, modified in every organ,
perceives those qualities for which the peculiar structure of the
organ is fitted; and all the various sensations of sound, colour,
taste, smell, resistance, and temperature, find appropriate organs by
which they are perceived, without mixing with, or confounding each
other. External objects, therefore, act upon the parts of the body
endowed with feeling, and their action is diversified in such a
manner, as to give us a great number of sensations, which appear to
have no resemblance to each other, and which make us acquainted with
the various properties of surrounding objects.
It would not, however, have been sufficient for man, merely to have
possessed this power of perceiving the different properties of the
objects which surround him: it was necessary likewise, that he should
be possessed of motion, that he might be able to approach or avoid
them, to seize or repulse them, as it suited his convenience or
advantage. By the extreme mobility of his limbs, he is able to move
his body, and transport it from place to place; to bring external
objects nearer to him, to remove them to a greater distance, and to
place them in such situations and such circumstances, as may enable
them to act on each other, and produce the changes which he wishes.
The human body, therefore, may be regarded as a machine composed
(besides the moving parts which have formerly been noticed) of divers
organs upon which external objects act, and produce those impressions
which convince us of their presence, and make us acquainted with
their properties. These impressions are transmitted to the sentient
principle, or mind; and the faculty we possess of perceiving these
impressions has been called by physiologists, sensibility.
Sensation has generally been defined by metaphysicians to be a change
in the mind, of which we are conscious, caused by a correspondent
change in the state of the body. This definition, however, leaves the
matter where they found it, and throws no light whatever on the
nature of sensation; nor can we say any thing more concerning it,
than that, when the organs are in a sound state, certain sensations
are perceived, which force us to believe in the existence of external
objects, though there is no similarity whatever, nor any necessary
connexion, that we can perceive, between the sensation and the object
which caused it.
All the different degrees of sensation may be reduced to two kinds:
pleasant and painful. The nature of these two primitive modes of
sensation, is as little known to us as their different species: all
that can be said, is, that the general laws by which the body is
governed, are such, that pleasure is generally connected with those
impressions which tend to its preservation, and pain with those which
cause its destruction.
In a general point of view, sensibility may be regarded as an
essential property of every part of the living body, disposing each
part to perform those functions, the object of which is to preserve
the life of the animal. Sensibility presides over the most necessary
functions, and watches carefully over the health of the body: she
directs the choice of the air proper for respiration, and also of
alimentary substances; the mechanism of the secretions is likewise
placed under her power; and in the same way that the eye perceives
colours, and the ear sounds, so every animated and living part is
fitted to receive impressions from the objects appropriated to it.
That every part of the animal is endowed with sensibility, is evident
from a variety of facts, particularly from the action which follows
when a muscle taken out of the animal body is irritated by any
stimulus: this is evident, by a variety of facts mentioned by Whytt,
Boerhaave, and others, which show, that parts recently taken from the
animal body retain a portion of sensibility, which continues to
animate them, and render them capable of action for a considerable
time.
The primary organ of sensation appears to be the brain, its
continuation in the form of medulla oblongata and spinal marrow, and
the various nerves proceeding from these; and it seems now generally
agreed, that unless there be a free communication of nerves between
the part where the impression is made, and the brain, no sensation
will take place; for instance, if the nerves be cut or compressed.
In a sound body, sensation is caused, whenever a change takes place
in the state of the nervous power, whether that change be produced by
an external, or an internal cause. The former kind of sensation is
said to arise from impression or impulse, the latter from
consciousness.
Every impression or impulse is not, however, equally calculated to
produce sensation; for this purpose, a middle degree of impulse
appears the best. An impulse considerably less produces no sensation,
and one more violent may cause pain, but no proper sensation denoting
the presence or properties of external objects. Thus too small a
degree of light makes no impression on the optic nerve; and if the
object be too strongly illuminated, the eye is pained, but has no
proper idea of the figure or colour of the object. In the same way,
if the vibrations which give us an idea of sound, be either too quick
or too slow, we shall not obtain this idea. When the vibration is too
quick, a very disagreeable and irritating sensation is perceived, as
for instance, in the whetting of a saw: and on the other hand, when
the vibrations are too slow, they will not produce a tone or sound.
This might be proved of all the senses, and shows, that a certain
degree of impression is necessary to produce perfect sensation.
There is another circumstance likewise requisite to produce
sensation: it is not enough, that the impression should be of the
proper strength; it is necessary likewise, that it should remain for
some time, otherwise no sensation will be produced. There are many
bodies whose magnitude is amply sufficient to be perceived by the
eye; yet, by reason of their great velocity, the impulse they make on
any part of the retina is so short, that they are not visible. This
is proved by our not perceiving the motions of cannon and musket
balls, and many other kinds of motion. On this principle depends the
art of conjuration, or legerdemain; the fundamental maxim of those
who practise them, is, that the motion is too quick for sight.
If the impulse be of a proper degree, and be continued for a
sufficient length of time, the impression made by it will not
immediately vanish with the impulse which caused it, but will remain
for a time proportioned to the strength of the impulse. This, with
respect to sight, is proved by whirling a firebrand in a circular
manner, by which the impression of a circle is caused, instead of a
moving point: and, with respect to hearing, it may be observed, that
when children run with a stick quickly along railing, or when a drum
is beaten quickly, the idea of a continued sound is produced, because
the impression remains some time: for it is evident, that the sounds
produced in succession are perfectly distinct and insulated.