others which act upon the body, many of which will hereafter be
considered: but all act according to this law; when their action has
been suspended or diminished, the excitability of the organ on which
they act becomes accumulated, or more easily affected by their
subsequent action; and, on the contrary, when their action has been
violent, or long continued, the excitability becomes exhausted, or
less fit to receive their actions.
Among the stimulants acting on the body, we may mention sound, which
has an extensive influence on human life. I need not mention here its
numerous natural, or artificial sources, as that has been fully done
in a preceding lecture. The effect of music, in stimulating and
producing a state of mind approaching to intoxication, is universally
known. Indeed the influence of certain sounds in stimulating, and
thereby increasing, the powers of life, cannot be denied. Fear
produces debility, which has a tendency to death. Sound obviates this
debility, and restores to the system its natural degree of
excitement. The schoolboy and the clown invigorate their trembling
limbs, by whistling, or singing, as they pass by a country
churchyard, and the soldier feels his departing courage recalled in
the onset of a battle, by the "spirit stirring drum."
Intoxication is generally attended with a higher degree of life or
excitement than is natural. Now sound will produce this effect with a
very moderate portion of fermented liquor; hence we find persons much
more easily intoxicated and highly excited at public entertainments,
where there is music and loud talking, than in private companies,
where no auxiliary stimulus is added to that of wine.
Persons who are destitute of hearing and seeing, possess life in a
more languid state than other people; which is, in a great degree,
owing to the want of the stimulus of light and noise.
Odours have likewise a very sensible effect in promoting animal life.
The effects of these will appear obvious in the sudden revival of
life, which they produce, in cases of fainting. The smell of a few
drops of hartshorn, or even a burnt feather, has frequently, in a few
minutes, restored the system from a state of weakness, bordering upon
death, to an equable and regular degree of excitement.
All these different stimuli undoubtedly produce the greatest effects
upon their proper organs; thus the effect of light is most powerful
on the eye; that of sound on the ear; that of food on the stomach,
&c. But their effects are not confined to these organs, but extended
over the whole body. The excitability exists, one and indivisible,
over the whole system; we may call it sensibility, or feeling, to
enable us to understand the subject. Every organ, or indeed the whole
body, is endowed with this property in a greater or less degree, so
that the effects produced by any stimulus, though they are more
powerful on the part where they are applied, affect the whole system:
odours afford an instance of this; and the prick of a pin in the
finger, produces excitement, or a stimulant effect, over the whole
body.
From what has been said, it must be evident that life is the effect
of a number of external powers, constantly acting on the body,
through the medium of that property which we call excitability; that
it cannot exist independent of the action of these stimuli; when they
are withdrawn, though the excitability does not instantly vanish,
there is no life, no motion, but the semblance of death. Life,
therefore, is constantly supported by, and depends constantly on, the
action of external powers on the excitability; without excitability
these stimulants would produce no effect, and whatever may be the
nature of the excitability, or however abundant it may be, still,
without the action of external powers, no life is produced.
From what has been said, we may see the reason why life is in a
languid state in the morning: It acquires vigour by the gradual and
successive application of stimuli in the forenoon: It is in its most
perfect state about midday, and remains stationary for some hours:
From the diminution or exhaustion of the excitability, it lessens in
the evening, and becomes more languid at bed time; when, from defect
of excitability, the usual exciting powers will no longer produce
their effect, a torpid state ensues, which we call sleep, during
which, the exciting powers cannot act upon us; and this diminution of
their action allows the excitability to accumulate; and, to use the
words of Dr. Armstrong,
"Ere morn the tonic, irritable nerves
Feel the fresh impulse, and awake the soul."
LECTURE X.
THE LAWS OF ANIMAL LIFE, CONTINUED.
In the last lecture I began to investigate the laws by which living
bodies are governed, and the effects produced by the different
exciting powers, which support life, upon the excitability, or vital
principle. The facts which we examined led us to two conclusions,
which, when properly applied, we shall find will explain most of the
phenomena of life, both in health, and in disease. The conclusions
alluded to, are these: when the exciting powers have acted more
feebly, or weakly, than usual, for some time; or when their action is
withdrawn, the excitability accumulates, and becomes more powerfully
affected by their subsequent action. And, on the contrary, when the
action of these powers has been exerted with violence, or for a
considerable time, the excitability becomes exhausted, and less fit
to receive their actions.
A number of facts were mentioned in proof of these conclusions, and a
great number more might have been brought forwards, could it have
served any other purpose than to have taken up our time, which I hope
may be better employed.
This exhaustion of the excitability, by stimulants, may either be
final, or temporary. We see animals, while the exciting powers
continue to act, at first appear in their greatest vigour, then
gradually decay, and at last come into that state, in which, from the
long continued action of the exciting powers, the excitability is
entirely exhausted, and death takes place.
We likewise see vegetables in the spring, while the exciting powers
have acted on them moderately, and for a short time, arrayed in their
verdant robes, and adorned with flowers of many mingling hues; but as
the exciting powers, which support their life, continue to be
applied, and some of them, for instance heat, as the summer advances,
become increased, they first lose their verdure, then grow brown, and
at the end of summer cease to live: because their excitability is
exhausted by the long continued action of the exciting powers: and
this does not happen merely in consequence of the heat of the summer
decreasing, for they grow brown, and die, even in a greater degree of
heat than that which in spring made them grow luxuriantly. In some of
the finest days of autumn, in which the sun acts with more power than
in the spring, the vegetable tribe droop, in consequence of this
exhausted state of their excitability, which renders them nearly
insensible of the action, even of a powerful stimulus.
These are examples of the final or irreparable exhaustion of the
excitability; but we find also that it may be exhausted for a time,
and accumulated again. Though the eye has been so dazzled by the
splendour of light, that it cannot see an object moderately
illuminated, yet if it be shut for some time, the excitability of the
optic nerve will accumulate again, and we shall again be capable of
seeing with an ordinary light.
We find also that we are not always equally capable of performing the
functions of life. When we have been engaged in any exertion, either
mental or corporeal, for some hours only, we find ourselves languid
and fatigued, and unfit to pursue our labours much longer.
If in this state several of the exciting powers are withdrawn,
particularly light and noise, and if we are laid in a posture which
does not require much muscular exertion, we soon fall into that state
which nature intended for the accumulation of the excitability, and
which we call sleep. In this state many of the exciting powers cannot
act upon us, unless applied with some violence, for we are insensible
to their moderate action. A moderate degree of light, or a moderate
noise, does not affect us, and the power of thinking, which very much
exhausts the excitability, is in a great measure suspended. When the
action of these powers has been suspended for six or eight hours, the
excitability is again capable of being acted on, and we rise fresh
and vigorous, and fit to engage in our occupations.
Sleep then is the method which nature has provided to repair the
exhausted constitution, and restore the vital energy. Without its
refreshing aid, our worn out habits would scarcely be able to drag on
a few days, or at most, a few weeks, before the vital spring would be
quite run down: how properly therefore has our great poet called
sleep "the chief nourisher in life's feast!"
From the internal sensations, often excited, it is natural to
conclude, that the nerves of sense are not torpid during sleep, but
that they are only precluded from the perception of external objects,
by the external organs being in some way or other rendered unfit to
transmit to them the impulses of bodies during the suspension of the
power of volition; thus the eyelids are closed, in sleep, to prevent
the impulse of light from acting on the optic nerve; and it is very
probable that the drum of the ear is not stretched; it seems likewise
reasonable to conclude, that something similar happens to the
external apparatus of all our organs of sense, which may make them
unfit for their office of perception during sleep.
The more violently the exciting powers have acted, the sooner is
sleep brought on, because the excitability is sooner exhausted, and
therefore sooner requires the means of renewing it: and, on the
contrary, the more weakly these powers have acted, the less are we
inclined to sleep. Instances of the first are, excess of exercise,
strong liquors, or study; and of the latter, an under or deficient
proportion of these.
A person who has been daily accustomed to much exercise, whether
mental or corporeal, if he omit it, will find little or no
inclination to sleep; this state may however be induced by taking
some diffusible stimulus, as a little spirits and water, or opium,
which seem to act entirely by exhausting the excitability, to that
degree which is compatible with sleep, and, when the stimulant effect
of these substances are over, the person soon falls into that state.
But though the excitability may have been sufficiently exhausted, and
the action of external powers considerably moderated, yet there are
some things within ourselves, which often stimulate violently, and
prevent sleep, such as pain, thirst, and strong passions and emotions
of the mind. These all tend to drive away sleep, by their vehement
stimulating effect, which still has power to rouse the excitability
to action, though it has been considerably exhausted. The best method
of inducing sleep, in these cases, is to endeavour to withdraw the
mind from these impressions, particularly from uneasy emotions, by
employing it on something that makes a less impression, and which
does not require much exertion, or produce too much commotion; such
as counting to a thousand, or counting drops of water which fall
slowly; by listening to the humming of bees, or the murmuring of a
rivulet. Virgil describes a situation fitted to induce sleep, most
beautifully, in the following words.
"Fortunate senex, hic inter flumina nota,
Et fontes sacros, frigus captabis opacum.
Hinc tibi, quae semper vicino ab limite sepes
Hyblaeis apibus florem depasta salicti,
Seape levi somnum suadebit inire susurro."
In infancy much sleep is required; the excitability, being then
extremely abundant, is soon exhausted by external stimulants, and
therefore soon requires renewing or accumulating; on this account,
during the first five or six months of their life, children require
this mode of renewing their exhausted excitability several times in
the day; as they advance in years, and as this excess of excitability
is exhausted by the application of stimulants, less sleep is
required: in the prime of life least of all is necessary. There is
great difference however, in this respect, in different
constitutions. Some persons are sufficiently refreshed by three or
four hours sleep, while others require eight or ten hours. More
however depends, in my opinion, on the mode of living. Those who
indulge in the use of spirituous or fermented liquors, which exhaust
the excitability to a great degree, require much more sleep than
those who are content with the crystal stream. The latter never feel
themselves stupid or heavy after dinner, but are immediately fit to
engage in study or business. As age advances, more sleep is again
required; and the excitability at last becomes so far exhausted, and
the system so torpid, that the greatest portion of gradually expiring
life is spent in sleep.
Temperance and exercise are the most conducive to sound healthy
sleep, hence the peasant is rewarded, for his toil and frugal mode of
life, with a blessing, which is seldom enjoyed by those whom wealth
renders indolent and luxurious. The poor in the country enjoy sound
and sweet sleep: forced by necessity to labour, their excitability
becomes exhausted in a proper and natural manner, and they retire to
rest early in the evening. Their sleep is generally sound, and early
in the morning they find themselves recruited, and in a state fit to
resume their daily labour. The blooming complexion, strength, and
activity, of these hardy children of labour, who recruit their
wearied limbs on pallets of straw, form a striking contrast with the
pallid and sickly visage, and debilitated constitution of the
luxurious and wealthy, who convert night into day, and court repose
in vain on beds of down. Nature undoubtedly intended that we should
be awake, and follow our occupations, whether of pleasure or
business, during the cheering light of day, and take repose when the
sun withdraws his rays. All other animals, and even vegetables, obey
the command of nature: man alone is refractory; but nature's laws are
never violated with impunity. Dr. Mackenzie very properly observes,
that those who sleep long in the morning, and sit up all the night,
injure the constitution without gaining time: and those who do this
merely in compliance with fashion, ought not to repine at a
fashionable state of bad health.
From what has been said, it is evident that, in order to enjoy sound
sleep, our chambers should be free from noise, dark, and moderately
cold; because the stimulant effects of noise, light, and heat,
prevent the accumulation of excitability: and as we shall afterwards
see that this accumulation depends on free respiration, and the
introduction of oxygen by that means into the system, our bed rooms
ought to be large and airy, and, in general, the beds should not be
surrounded by curtains. We may from this likewise see the reason why
it is so desirable to sleep in the country, even though we are
obliged to spend the day in town.
These observations on sleep have however led me a little from the
direct road; but I thought they could not be better introduced than
here. I shall now return to the subject of our more immediate
inquiry.
By induction we have discovered two of the principal laws by which
living bodies are governed: the first is, that when the ordinary
powers which support life have been suspended, or their action has
been lessened for a time, the excitability, or vital principle,
accumulates, or becomes more fit to receive their actions; and
secondly, when these powers have acted violently, or for a
considerable time, the excitability is exhausted, or becomes less fit
to receive their actions. There are therefore three states in which
living bodies exist. First, a state of accumulated excitability.
Secondly, a state of exhausted excitability. Thirdly, when the
excitability is in such a state as to produce the strongest and most
healthy actions, when acted upon by the external powers.
From what has been said, it must be evident that life depends
continually on the action of external powers on the excitability, and
that by their continued action, if they be properly regulated, the
excitability will be gradually, and insensibly exhausted, and life
will be resigned into the hands of him who gave it, without a
struggle, and without a groan.
We see then that nature operates in supporting the living part of the
creation, by laws as simple and beautiful as those by which the
animated world is governed. In the latter we see the order and
harmony which is observed by the planets, and their satellites, in
their revolution round the great source of heat and light;
"---------- all combin'd
And ruled unerring, by that Single Power,
Which draws the stone projected, to the ground.
In the animated part of the creation, we observe those beautiful
phenomena which are exhibited by an almost infinite variety of
individuals; all depending upon, and produced by one simple law; the
acting of external powers upon their excitability.
I cannot express my admiration of the wisdom of the Creator better
than in the words of Thomson.
"O unprofuse magnificence divine!
O wisdom truly perfect! thus to call
From a few causes such a scheme of life;
Effects so various, beautiful, and great."
Life then, or those functions which we call living, are the effects
of certain exciting powers acting on the excitability, or property
distinguishing living from dead matter. When these effects, viz. the
functions, flow easily, pleasantly, and completely, from the action
of these powers, they indicate that state which we call health.
We may therefore, as we before hinted, distinguish three states of
the irritable fibre, or three different degrees of excitability, of
which the living body is susceptible.
1. The state of health which is peculiar to each individual, and
which has been called by Haller, and other physiologists, the tone of
the fibre. This is produced by a middle degree of stimulus acting
upon a middle degree of excitability: and the effect produced by this
action, we call excitement.
2. The state of accumulation, produced by the absence or diminished
action of the accustomed stimuli.
3. The state of exhaustion, produced by the too powerful action of
stimuli; and this may be produced either by the too powerful, or long
continued action of the common stimulants which support life, such as
food, air, heat, and exercise; or it may be caused by an application
of stimulants, which act more powerfully on the excitability, and
which exhaust it more quickly, such as wine, spirits, and opium,
musk, camphor, and various other articles used in medicines.
The state of health, or tone, if we use that term, consists therefore
in a certain quantity or energy of excitability necessary to its
preservation. To maintain this state, the action of the stimuli
should be strong enough to carry off from the body the surplus of
this irritable principle. To obtain this end, a certain equilibrium
is necessary between the excitability and the stimuli applied, or the
sum of all the stimuli acting upon it must be always nearly equal,
and sufficient to prevent an excess of excitability, but not so
strong as to carry off more than this excess. It is in this
equilibrium between the acting stimuli and the excitability, that the
health, or tone of the living body consists.
When the sum of the stimuli, acting on the body, is so small, as not
to carry off the excess of excitability, it accumulates, and diseases
of irritability are produced. Of this nature are those diseases to
which the poor are often subject, and which will be particularly
considered hereafter.
When the sum of the stimuli acting on the body, is too great, it is
deprived not only of the excess of excitability, but also of some
portion of the irritable principle necessary for the tone of the
body: or, to speak more distinctly, the body loses more excitability
than it receives, and of course must, in a short time, be in a state
of exhaustion. This gives rise to diseases which afflict drinkers, or
those who indulge in any kind of intemperance, or persons born in
climates where the temperature is moderate, but who emigrate to those
which are much warmer.
Thus we have endeavoured, after the example of Dr. Brown, to
ascertain the cause of the healthy state, before the causes of
diseases were investigated; and though this is contrary to the
general practice, yet it must be evident to every one, that unless we
are acquainted with the causes of good health, it will be impossible
for us to form any estimate of those variations from that state,
called diseases: hence it is that a number of diseases, which have
been brought on merely by the undue action of the exciting powers,
such as gout, rheumatism, and the numerous trains of nervous
complaints, which were by no means understood, may be easily and
satisfactorily explained, and as easily cured, by restoring the
proper action of these powers, and bringing the excitability to its
proper state. As this theory, therefore, is so important, not only in
respect to the preservation of health, which nearly concerns every
individual, but to the cure of diseases, which is the province of the
physician, I have endeavoured to explain it as fully and minutely as
possible; to make it still plainer we may perhaps make use of the
following illustration.
Suppose a fire to be made in a grate or furnace, filled with a kind
of fuel not very combustible, and which could only be kept burning by
means of a machine, containing several tubes placed before it, and
constantly pouring streams of air into it. Suppose also a pipe to be
fixed in the back of the chimney, through which a constant supply of
fresh fuel is gradually let down into the grate, to repair the waste
occasioned by the combustion kept up by the air machine.
The grate will represent the human body; the fuel in it the life or
excitability, and the tube behind, supplying fresh fuel, will denote
the power of all living systems, constantly to regenerate or produce
excitability; the air machine, consisting of several tubes, may
denote the various stimuli applied to the excitability of the body;
the flame produced in consequence of that application, represents
life; the product of the exciting powers acting upon the
excitability.
Here we see, that flame, like life, is drawn forth from fuel by the
constant application of streams of air, poured into it from the
different tubes of the machine. When the quantity of air poured in
through these different tubes is sufficient to consume the fuel as it
is supplied, a constant and regular flame will be produced: but if we
suppose that some of them are stopped, or that they do not supply a
sufficient quantity of air, then the fuel will accumulate, and the
flame will be languid and smothered, but liable to break out with
violence, when the usual quantity of air is supplied.
On the contrary, if we suppose a greater quantity of air to rush
through the tubes, then the fuel will be consumed or exhausted faster
than it is supplied; and in order therefore to reduce the combustion
to the proper degree, the quantity of air supplied must be
diminished, and the quantity of fuel increased.
If we suppose one of the tubes, instead of common air, to supply
oxygen gas, it will represent the action of wine, spirits, ether,
opium, and other powerful stimulants upon the body: a bright and
vivid flame will be produced, which however will only be of short
duration, for the fuel will be consumed faster than it is supplied,
and a state of exhaustion will take place.