circumstance that we observe, is its life, or its power of motion,
sensation, and thought: for though the structure of the different
parts which we have examined must excite our admiration and wonder,
each part being admirably fitted for the performance of its different
functions, yet without the breath of life, all these beautiful
contrivances would have been useless. We have seen that the structure
of the eye indicates in its contriver, the most consummate skill in
optics; and of the ear the most perfect knowledge of sounds; yet if
sensibility had not being given to the nerves which administer to
these organs, the pulses of the air might have been communicated to
the fluid in the labyrinth, and the rays of light might have formed
images in the retina, without our being, in the smallest degree,
conscious of their existence.
Though our efforts to discover the nature of life have hitherto been,
and perhaps always will be, unsuccessful, yet we can, by a careful
induction, or observation of facts, discover the laws by which it is
governed, with respect to the action of external objects. This is
what I shall now attempt to do.
The first observation which strikes us, is that of the very different
effects that are produced when inanimate bodies act on each other,
and when they exert their action on living matter.
When dead matter acts upon dead or inanimate matter, the only effects
we perceive are mechanical, or chemical; that is, either motion, or
the decomposition and new combination of their parts. If one ball
strikes another, it communicates to it a certain quantity of motion,
this is called mechanical action; and if a quantity of salt, or
sugar, be put into water, the particles of salt, or sugar, will
separate from each other, and join themselves to the particles of the
water; these substances in these instances are said to act chemically
on each other, and in all cases whatever, in which inanimate or dead
bodies act on each other, the effects produced are motion, or
chemical attraction; for though there may appear to be other species
of action which sometimes take place, such as electric and magnetic
attraction and repulsion, yet these are usually referred to the head
of mechanical action or attraction.
But when dead matter acts upon those bodies we call living, the
effects produced are much different. There are many animals which
pass the winter in a torpid state which has all the appearance of
death; and they would continue in that state, if deprived of the
influence of heat; now heat if applied to dead matter, will only
produce motion, or chemical combination: in fluids it produces
motions by occasioning a change in their specific gravity; and we
know that it is one of the most powerful agents in chemical
combination and decomposition; but these are the only effects it
produces when it acts upon dead matter. But let us examine its
effects when applied to living organized bodies. Bring a snake or
other torpid animal into a moderately warm room, and observe what
will be the consequence. After a short time the animal begins to
move, to open its eyes and mouth; and when it has been subject to the
action of heat for a longer time, it crawls about in search of food,
and performs all the functions of life.
Here then, dead matter, when applied to the living body, produces the
living functions, sense and motion: for if the heat had not been
applied, the animal would have continued senseless, and apparently
lifeless.
In more perfect animals, the effects produced by the action of dead
matter upon them, are more numerous, and are different in different
living systems; but are in general the following; sense and motion in
almost all animals, and in many the power of thinking, and other
affections of the mind.
The powers, or dead matters, which by their action produce these
functions, are chiefly heat, food, and air. The proof that these
powers do produce the living functions is in my opinion very
satisfactory, for when their action is suspended, the living
functions cease. If we take away, for instance, heat, air, and food,
from animals, they soon become dead matter. This is as strong a proof
that these matters are the cause of the functions, as that heat is
the cause of the expansion of bodies, when we find that by
withdrawing it the expansion ceases. Indeed it is not necessary that
an animal should be deprived of all these powers to put a stop to the
living functions; if any one of them is taken away, the body sooner
or later becomes dead matter: it is found by experience, that if a
man is deprived of air, he dies in about three or four minutes; for
instance, if he is immersed under water: if he is deprived of heat,
or in other words is exposed to a very severe degree of cold, he
likewise soon dies; or if he is deprived of food, his death is
equally certain, though more slow; it is sufficiently evident then
that the living functions are owing to the action of these external
powers upon the body.
What I have here said is not confined to animals, but the living
functions of vegetables are likewise caused by the action of dead
matter upon them. The powers, which by their actions produce the
living functions of vegetables, are principally heat, moisture,
light, and air.
From what has been said, it clearly follows, that living bodies must
have some property different from dead matter, which renders them
capable of being acted on by these external powers, so as to produce
the living functions; for if they had not, it is evident that the
only effects which these powers could produce, would be mechanical,
or chemical.
Though we know not exactly in what this property consists, or in what
manner it is acted on, yet we see that when bodies are possessed of
it, they become capable of being acted on by external powers, so as
to produce the living functions.
We may call this property, with Haller, irritability, or, with Brown,
excitability; or we may use vital principle, or any other term, could
we find one more appropriate. I shall use the term excitability, as
perhaps the least liable to exception, and in using this term, it is
necessary to mention that I mean only to express a fact, without the
smallest intention of pointing out the nature of that property which
distinguishes living from dead matter; and in this we have the
illustrious example of Newton, who called that property which causes
bodies in certain situations to approach each other, gravitation,
without in the least hinting at its nature. Yet though he knew not
what gravitation was, he investigated the laws by which bodies were
acted on by it, and thus solved a number of phenomena which were
before inexplicable: in the same manner, though we are ignorant of
the nature of excitability, or of the property which distinguishes
living from dead matter, we can investigate the laws by which dead
matter acts upon living bodies through this medium. We know not what
magnetic attraction is, yet we can investigate its laws: the same may
be observed with respect to electricity. If ever we should obtain a
knowledge of the nature of this property, it would make no alteration
in the laws which we had before discovered.
Before we proceed to the investigation of the laws by which the
living principle or excitability is acted on, it will be first
necessary to define some terms, which I shall have occasion to use,
to avoid circumlocution: and here it may not be improper to observe,
that most of our errors in reasoning have arisen from want of strict
attention to this circumstance, the accurate definition of those
terms which we use in our reasoning. We may use what terms we please,
provided we accurately define them, and adhere strictly to the
definition. On this depends the excellence and certainty of the
mathematical sciences. The terms are few, and accurately defined; and
in their different chains of reasoning mathematicians adhere with the
most scrupulous strictness to the original definition of the terms.
If the same method were made use of in reasoning on other subjects,
they would approach to the mathematics in simplicity and in truth,
and the science of medicine in particular would be stripped of the
heaps of learned rubbish which now encumber it, and would appear in
true and native simplicity. Such is the method I propose to follow: I
am certain of the rectitude of the plan; of the success of the
reasoning it does not become me to judge.
When the excitability is in such a state as to be very susceptible of
the action of external powers, I shall call it abundant or
accumulated; but when it is found in a state not very capable of
receiving their action, I say it is deficient or exhausted. Let no
one however suppose that by these terms I mean to hint in the least
at the nature of the excitability. I do not mean by them that it is
really at one time increased in quantity or magnitude, and at another
time diminished: its abstract nature is by no means attempted to be
investigated. These or similar terms the poverty or imperfection of
language obliges us to use. We know nothing of the nature of the
excitability or vital principle, and by the terms here used I mean
only to say, that the excitability is sometimes easily acted on by
the external powers, and then I call it abundant or accumulated; at
other times the living body is with more difficulty excited, and then
I say the vital principle or excitability, whatever it may be, is
deficient or exhausted.
On examination we shall find the laws by which external powers act on
living bodies to be the following.
First, when the powerful action of the exciting powers ceases for
some time, the excitability accumulates, or becomes more capable of
receiving their action, and is more powerfully affected by them.
If we examine separately the different exciting powers which act on
the body, we shall find abundant confirmation of this law. Besides
the exciting powers which act on the body, which I mentioned; viz.
heat, food, and air, there are several others, such as light, sound,
odorous substances, &c. which will be examined in their proper
places. These powers, acting by a certain impulse, and producing a
vigorous action of the body, are called stimulants, and life we shall
find to be the effect of these and other stimulants acting on the
excitability.
The stimulus of light, though its influence in this respect is
feeble, when compared with some other external powers, yet has its
proportion of force. This stimulus acts upon the body through the
medium of the organ of vision. Its influence on the animal spirits
strongly demonstrates its connexion with animal life, and hence we
find a cheerful and depressed state of mind in many people, and more
especially in invalids, to be intimately connected with the presence
or absence of the sun. Indeed to be convinced of the effects of light
we have only to examine its influence on vegetables. Some of them
lose their colour when deprived of it, many of them discover a
partiality to it in the direction of their flowers; and all of them
perspire oxygen gas only when exposed to it; nay it would seem that
organization, sensation, spontaneous motion, and life, exist only at
the surface of the earth, and in places exposed to light. Without
light nature is lifeless, inanimate, and torpid.
Let us now examine if the action of light upon the body is subject to
the law that has been mentioned. If a person be kept in darkness for
some time, and then be brought into a room in which there is only an
ordinary degree of light, it will be almost too oppressive for him,
and will appear excessively bright; and if he have been kept for a
considerable time in a very dark place, the sensation will be very
painful. In this case, while the retina or optic nerve was deprived
of light, its excitability accumulated, or became more easily
affected by light: for if a person go out of one room into another,
which has an equal degree of light, he will perceive no effect.
You may convince yourselves of the truth of this law, by a very
simple experiment; shut your eyes, and cover them for a minute or two
with your hand, and endeavour not to think of the light, or what you
are doing; then open them, and the daylight will for a short time
appear brighter.
If you look attentively at a window for about two minutes, then cast
your eyes upon a sheet of white paper, the shape of the window frames
will be perfectly visible upon the paper; those parts which express
the wood work appearing brighter than the other parts. The parts of
the optic nerve on which the image of the frame falls, are covered by
the wood work from the action of the light; the excitability of these
parts will therefore accumulate; and the parts of the paper which
fall upon them must of course appear brighter.
If a person be brought out of a dark room where he has been confined,
into a field covered with snow, when the sun shines, it has been
known to affect him so much as to deprive him of sight altogether.
This law is well exemplified when we come into a dark room in the day
time. At first we can see nothing; but with the absence of light the
excitability accumulates, and we begin to have an imperfect glimpse
of the objects around us; after a while the excitability of the
retina is so far accumulated, and we become so sensible of the feeble
light reflected from the surfaces of bodies, that we can discern
their shapes, and sometimes even their colours.
Let us next consider what happens with respect to heat, which is a
uniform and active stimulus in promoting life. The extensive
influence of heat upon animal life is evident from its decay and
suspension during winter, in certain animals, and from its revival
upon the approach and action of the vernal sun.
If this stimulus is for some time abstracted from the whole body, or
from any part, the excitability accumulates, or, in other words, if
the body has been for some time exposed to cold, it is more liable to
be affected by heat afterwards applied. Of this also you may be
convinced by an easy experiment. Put one of your hands into cold
water, and then put both into water which is considerably warm: the
hand which has been in the cold water will feel much warmer than the
other. If you handle some snow in one hand while you keep the other
in the bosom, that it may be of the same heat with the body, and then
bring both within the same distance of the fire, the heat will affect
the cold hand infinitely more than the warm one. This is a
circumstance of the utmost importance, and ought always to be
carefully attended to. When a person has been exposed to a severe
degree of cold for some time, he ought to be cautious how he comes
near a fire, for his excitability will be so much accumulated that
the heat will act very violently, often producing a great degree of
inflammation, and even sometimes of mortification. This is a very
common cause of chilblains, and other similar inflammations. When the
hands, or any other parts of the body, have been exposed to a violent
cold, they ought first to be put in cold water, or even rubbed with
snow, and exposed to warmth in the gentlest manner possible.
The same law regulates the action of food, or matters taken into the
stomach: if a person have for some time been deprived of food, or
have taken it in small quantity, whether it be meat or drink, or if
he have taken it of a less stimulating quality, he will find that
when he returns to his ordinary mode of life it will have more effect
upon him than before he lived abstemiously.
Persons who have been shut up in a coal work, from the falling in of
the pit, and have had nothing to eat for two or three days, have been
as much intoxicated by a bason of broth, as a person in common
circumstances with two or three bottles of wine.
This circumstance was particularly evident among the poor sailors who
were in the boat with Captain Bligh after the mutiny. The Captain was
sent by government to convey some plants of the bread fruit tree from
Otaheite to the West Indies: soon after he left Otaheite the crew
mutinied, and put the captain and most of the officers, with some of
the men, on board the ship's boat, with a very short allowance of
provisions, and particularly of liquors, for they had only six quarts
of rum, and six bottles of wine, for nineteen people, who were driven
by storms about the south sea, exposed to wet and cold all the time,
for nearly a month; each man was allowed only a teaspoonful of rum a
day, but this teaspoonful refreshed the poor men, benumbed as they
were with cold, and faint with hunger, more than twenty times the
quantity would have done those who were warm and well fed; and had it
not been for the spirit having such power to act upon men in their
condition, they never could have outlived the hardships they
experienced. All these facts, and many others which might be brought
forward, establish, beyond dispute, the truth of the law I mentioned;
viz. that when the powerful action of the exciting powers ceases for
some time, the excitability accumulates, or becomes more capable of
receiving their actions, and is more powerfully affected by them.
When the legs or arms have for some time been exposed to cold, the
slightest exertion, or even the stimulus of a gentle heat, throws the
muscles into an inordinate action or cramp. The glow of the skin, in
coming out of a cold bath, may be explained on the same principle.
The heat of the skin is diminished by the conducting power of the
water, in consequence of which the excitability of the cutaneous
vessels accumulates; and the same degree of heat afterwards applied,
excites these now more irritable vessels to a great degree of action.
On this principle depends the supposed stimulant or tonic powers of
cold, the nature of whose action has been much mistaken by physicians
and physiologists. Heat is allowed to be a very powerful stimulus;
but cold is only a diminution of heat; how then can cold act as a
stimulus? In my opinion it never does; but its effects may be
explained by the general law which we have been investigating. When a
lesser stimulus than usual has been applied to the body, the
excitability accumulates, and is then affected by a stimulus even
less than that which, before this accumulation, produced no effect
whatever. The cold only renders the body more subject to the action
of heat afterwards applied, by allowing the excitability to be
accumulated. No person, I believe, ever brought on an inflammation,
or inflammatory complaint, by exposure to cold, however long might
have been that exposure, or however great the cold; but if a person
have been out in the cold air, and afterwards come into a warm room,
an inflammatory complaint will most probably be the consequence.
Indeed coming out of the cold air into a moderately warm room
generally produces a lively and continued warmth in the parts that
have been exposed.
The second general law is, that when the exciting powers have acted
with violence for a considerable time, the excitability becomes
exhausted, or less fit to be acted on; and this we shall be able to
prove by a similar induction.
Let us first examine the effects of light upon the eye: when it has
acted violently for some time on the optic nerve, it diminishes the
excitability of that nerve, and renders it incapable of being
affected by a quantity of light, that would at other times affect it.
When we have been walking out in the snow, if we come into a room, we
shall scarcely be able to see any thing for some minutes.
If you look stedfastly at a candle for a minute or two, you will with
difficulty discern the letters of a book which you were before
reading distinctly. When our eyes have been exposed to the dazzling
blaze of phosphorus in oxygen gas, we can scarcely see any thing for
some time afterwards, and if we look at the sun, the excitability of
the optic nerve is so overpowered by the strong stimulus of his
light, that nothing can be seen distinctly for a considerable time.
If we look at the setting sun, or any other luminous object of a
small size, so as not greatly to fatigue the eye, this part of the
retina becomes less sensible to smaller quantities of light; hence
when the eyes are turned on other less luminous parts of the sky, a
dark spot is seen resembling the shape of the sun, or other luminous
object on which our eyes have been fixed.
On this account it is that we are some time before we can distinguish
objects in an obscure room, after coming from broad daylight, as I
observed before.
We shall next consider the action of heat. Suppose water to be heated
to 90 degrees, if one hand be put into it, it will appear warm; but
if the other hand be immersed in water heated to 120 degrees, and
then put into the water heated to 90 degrees, that water will appear
cold, though it will still feel warm to the other hand: for the
excitability of the hand has been exhausted, by the greater stimulus
of heat, to such a degree as to be insensible of a less stimulus.
Before we go into a warm bath, the temperature of the air may seem
very warm and pleasant to the body, even though exposed naked to it;
but after we have remained for some time in the warm bath, we feel
the air, when we come out, very cool and chilling, though it is of
the same temperature as before; for the hot water exhausts the
excitability of the vessels of the skin, and renders them less
capable of being affected by a smaller degree of heat. Thus we see
that the effects of the hot and cold bath are different and opposite;
the one debilitates by stimulating, and the other produces stimulant
or tonic effects by debilitating. This seeming paradox may, however,
be easily explained by the principles we have laid down; and though
the hot and cold bath produce such different effects, yet it is only
the same fluid, with a small variation in the degree of temperature;
but these effects depend on the temperature of our body being such,
that a small decrease of it will produce an accumulation of
excitability, while a small increase will exhaust it.
I shall next proceed to examine the effects of the substances taken
into the stomach; and as the effects of spirituous and vinous liquors
are a little more remarkable than those of food, I shall first begin
with them.
A person who is not accustomed to take these liquors, will be
intoxicated by a quantity that will produce no effect upon one who
has been some time accustomed to take them; and when a person has
used himself to these stimulants for some time, the ordinary powers
which in common support life, will not have their proper effects upon
him, because his excitability has been, in some measure, exhausted by
these stimulants.
The same holds good with respect to tobacco and opium; a person
accustomed to take opium, or smoke tobacco, will not be affected by a
quantity that would completely intoxicate one not used to them,
because the excitability has been so far exhausted by the use of
those stimulants, that it cannot be acted on by a smaller quantity.
That tobacco or opium act in the same manner as wine or spirits,
scarce needs any illustration. In Turkey they intoxicate themselves
with opium, in the same way that people in this country do with wine
and spirits; and those who have been accustomed to take this drug for
a considerable time, feel languid and depressed when they are
deprived of it for some time; they repair to the opium houses, as our
dram drinkers do to the gin shops in the morning, sullen, dejected,
and silent; in an hour or two, however, they are all hilarity. This
shows the effects of opium to be stimulant. Tobacco intoxicates those
who are not accustomed to it, and in those who are, it produces a
serene and composed state of mind by its stimulating effects. Like
opium and fermented liquors it exhausts the excitability, and leaves
the person dejected, and all his senses blunted, when its stimulant
effects are over.
That what is more properly called food acts in the same way as the
substances I have just examined, is evident from the fact which I
mentioned some time ago, that persons whose excitability has been
accumulated, by their being deprived of food for some days, have been
intoxicated by a bason of broth.
These facts, with innumerable others which will easily suggest
themselves, prove, beyond doubt, the truth of the second law, namely,
that when the exciting powers have acted violently, or for a
considerable time, the excitability is exhausted, or less fit to be
acted on.