air tubes exhaust the fuel every day faster than it can be supplied,
then it will be necessary at night to stop up some of the tubes, so
that the expense of fuel may be less than its supply, in order to
make up for the deficiency. When this is made up, the tubes may in
the morning be opened, and the combustion carried on during the day
as usual. This will illustrate the nature of sleep. In speaking of
this subject, it was observed, that the more violently the exciting
powers have acted, the sooner is sleep brought on; because the
excitability is sooner exhausted. In the same way the more the air
rushes through the tubes, the sooner will the fuel be consumed, and
want replenishing. When the exciting powers have acted feebly, a
person feels no inclination to sleep, because the excitability is not
exhausted to the proper degree, and therefore does not want
accumulating. But any diffusible stimulus, as spirits, or opium, will
soon exhaust it to the proper degree.
In the same way, if the air have not passed rapidly through the
tubes, the fuel will not be exhausted: but it may be brought to a
proper degree of exhaustion by the application of oxygen gas.
When the air which nourishes the flame is so regulated, that it
consumes the fuel as it is supplied, but no faster, a clear and
steady flame will be kept up, which will go on as long as the fuel
lasts, or the grate resists the action of the fire: but at last when
the fuel, which we do not suppose inexhaustible, is burnt out, the
fire must cease.
In the same manner, if the different exciting powers which support
life were properly regulated, all the functions of the body would be
properly performed, and we should pass our life in a state of health,
seldom known to any but savages, and brute animals not under the
dominion of man, who regulate these powers merely by the necessities
of nature.
When air is applied in too great quantity, and especially if some of
the tubes convey oxygen gas, then a violent combustion and flame is
excited, which will, in all probability, consume or burn out the
furnace or grate, or if it do not, it will burn out the fuel, and
thus exhaust itself.
In like manner, if the stimulants which support life be made to act
too powerfully, and particularly if any powerful stimulus, not
natural to the body, such as wine or spirits, be taken in too great
quantity, a violent inflammatory action will be the consequence,
which may destroy the human machine: but if it do not, it will
exhaust the excitability, and thus bring on great debility.
This analogy might be pursued further, but my intention was solely to
illustrate some of the outlines of our theory, by a comparison which
may facilitate the conception of the manner in which external powers
act on living bodies. The different powers which support life, and
without whose action we are unable to exist, such as heat, food, air,
&c. have been very improperly called nonnaturals, a term which is
much more applicable to those substances which we are daily in the
habit of receiving into the system, which excite it to undue actions,
and which nature never intended we should receive; such as spirituous
and fermented liquors, and high seasoned foods. In the preceding
illustration, I have spoken of a tube, as constantly pouring in fresh
fuel, because it was not easy otherwise to convey a familiar idea of
the power which all living systems possess of renewing their
excitability, when exhausted. The excitability is an unknown
somewhat, subject to peculiar laws, some of which we have examined,
but whose different states we are obliged to describe, though,
perhaps, inaccurately, by terms borrowed from the qualities of
material substances.
Though Dr. Brown very properly declined entering into the
consideration of the nature of excitability, or the manner in which
it is produced, the discoveries which have been made in chemistry
since his time, have thrown great light on the subject, and it is now
rendered highly probable that the excitability or vital principle, is
communicated to the body by the circulation, and is intimately
connected with the process of oxidation.
Many circumstances would tend to show, that a strict connexion exists
between the reception of oxygen into the body, and the vital
principle.
When an animal has been killed by depriving it of oxygen gas, the
heart and other muscles, and indeed the whole system, will be found
completely to have lost its excitability. This is not the case when
an animal is killed in a different manner. When an animal is shot, or
killed in the common manner, by bleeding to death, if the heart be
taken out, it will contract for some hours, on the application of
stimulants. But this is not the case with an animal that has been
drowned, or killed by immersion in carbonic acid, azotic, or
hydrogenous gases; in these last instances, the heart either does not
contract at all, or very feebly, on the application of the strongest
stimulants.
We have already seen that oxygen unites with the blood in the lungs,
during respiration: by the circulation of the blood it is distributed
to every part of the system, and we shall find, that in proportion to
its abundance is the excitability of the body. In proof of this, I
shall relate some facts and experiments.
Dr. Girtanner injected a quantity of very pure oxygen gas into the
jugular vein of a dog: the animal raised terrible outcries, breathed
very quickly, and with great difficulty: by little and little his
limbs became hard and stiff, he fell asleep, and died in the course
of a few minutes afterwards. It ought here to be observed, that any
of the gases, or almost any fluid, however mild, when thus suddenly
introduced into the circulating system, generally, and speedily,
occasions death.
On opening the chest, the heart was found more irritable than
ordinary, and its external contractions and dilatations continued for
more than an hour: the right auricle of the heart, which usually
contains black venous blood, contained, as well as the right
ventricle, a quantity of blood of a bright vermilion colour; and all
the muscles of the body were found to be more than usually irritable.
This experiment not only proves that the vermilion colour of the
blood proceeds from oxygen, but likewise seems to show, that oxygen
is the cause of excitability.
A quantity of azotic gas, which had been exposed for some time to the
contact of lime water, in order to separate any carbonic acid gas it
might contain, was injected into the jugular vein of a dog. The
animal died in twenty seconds. Upon opening the chest, the heart was
found filled with black and coagulated blood: this organ, and most of
the muscles had nearly lost the whole of their irritability, for they
contracted but very weakly, on the application of the strongest
stimulants.
A quantity of carbonic acid gas was injected into the jugular vein of
a dog: the animal became sleepy, and died in about a quarter of an
hour: the heart was found filled with black and coagulated blood, and
had lost the whole of its irritability; neither it, nor any of the
muscles producing any contractions, upon the application of
stimulants.
Humboldt likewise mentions a curious fact, which tends strongly to
confirm this idea. When the excitability of the limb of a frog had
been so far exhausted, by the application of zinc and silver, that it
would produce no more contractions, on moistening it with oxygenated
muriatic acid, the contractions were renewed.
After the excitability of the sensitive plant (mimosa pudica) had
been so far exhausted, by irritation, that it ceased to contract,
when further irritated, I restored this excitability, and brought it
to a very high degree of irritability, by moistening the earth in
which it grew with oxygenated muriatic acid. Seeds likewise vegetate
more quickly when moistened with this acid, than when they are not.
In short, we shall find, first, that every thing which increases the
quantity of oxygen in organized bodies, increases at the same time
their excitability.
Secondly, That whatever diminishes the quantity of oxygen, diminishes
the excitability.
The excitability of animals, made to breathe oxygen gas, or to take
the oxygenated muriate of potash, or acid fruits, is very much
increased.
On the contrary, when persons have inspired carbonic acid, or azotic
gas, or have taken into the system substances which have a strong
affinity for oxygen, and therefore tend to abstract it, such as
hydrogen, and spirits, the excitability becomes very much diminished.
When we sleep, in consequence of the excitability being exhausted,
the breathing becomes free, and a great quantity of oxygen is
received by the lungs, and combined with the blood, while very little
of it becomes exhausted by the actions of the body, for none,
excepting those which are called involuntary motions, are carried on
during sound sleep: so that in a few hours the body recovers the
excitability which it had lost: it is again sensible of the
impressions of external objects, and with the return of light we
wake.
These facts afford satisfactory proofs that the excitability of the
body is proportioned to the oxygen which it receives: but in what
manner it produces this state of susceptibility, and how it is
exhausted by stimulants, we have yet to learn.
The following theory may perhaps throw some light upon the subject. I
propose it, however, merely as an hypothesis, for we have no direct
proofs of it, but it seems to account for many phenomena.
It is now well known, that while the limb of an animal possesses
excitability, the smallest quantity of electricity sent along the
principal nerve leading to it, produces contractions similar to those
produced by the will. This is instanced in the common galvanic
experiment with the limb of a frog, which I had formerly occasion to
show.
From the effects produced, when a stream of electricity is sent
through water, I think it not improbable that hydrogen and
electricity may be identical. When a piece of zinc and silver are
connected together, and the zinc is put in a situation to decompose
water, and oxidate, a current of hydrogen gas will separate from the
silver wire, provided this be immersed under water; but when it is
not, a current of electricity passes, which is sensible to the
electrometer.
Now there appears no greater improbability in the supposition that
hydrogen, in a certain state, may be capable of passing through
metals, and animal substances, in the form of electricity, and that
when it comes in contact with water, which is not so good a
conductor, it may combine with caloric, and form hydrogen gas, in
which state it becomes incapable of passing through the conductors of
electricity: I say there appears no greater improbability in this,
than that caloric should sometimes be in such a state, that it will
pass through metals, and animal substances, which conduct it, and at
other times, as when combined with oxygen or hydrogen, it should form
gases, and be then incapable of passing through these conductors of
heat. Galvanic effects may be produced by the oxidation of fresh
muscular fibre without the aid of metals, and contractions have been
thus produced in the limb of an animal; and we have already noticed,
that when this contraction ceases, it may be restored, by moistening
the limb with oxygenated muriatic acid.
The excitability of the body may, most probably, be conveyed by
respiration, and the circulation of the blood, which tend continually
to oxidate the different parts: and hydrogen or electricity may be
secreted by the brain, and sent along the nerves, which are such good
conductors of it, and by uniting with the oxygen of the muscle, may
cause it to contract; but as the oxygen will, by this union, be
diminished, if the contractions be often repeated, the excitability
will thus be expended faster than it can be supplied by the
circulation, and will become exhausted. But will facts bear us out in
this explanation? To see this, we must examine the chemical nature of
the substances which produce the greatest action, and the greatest
exhaustion of the vital principle: namely, those which produce
intoxication.
Fermented liquors differ from water, in containing carbon and more
hydrogen: these produce intoxication: but pure spirits, which contain
still more hydrogen, produce a still higher degree of intoxication,
and consequent exhaustion of the excitability. Ether, which appears
to be little more than condensed hydrogen, probably kept in a liquid
state by union with a small quantity of carbon, and which easily
expands by caloric into a gas, which very much resembles hydrogen
gas, produces a still greater degree of intoxication: so that we see
the action produced by different substances, as well as the
exhaustion of excitability which follows, is proportioned to the
quantity of hydrogen they contain.
There is another circumstance which seems to strengthen this idea.
The intoxicating powers of spirits are diminished by the addition of
vegetable acids, or substances which contain oxygen, which will
counteract the effects of the hydrogen. Thus it is known that the
same quantity of spirit, made into punch, will not produce either the
same ebriety, or the same subsequent exhaustion, as when simply mixed
with water.
Recollect however that I propose this only as a hypothesis: its truth
may be confirmed by future observations and experiments, or it may be
refuted by them: but it is certainly capable of explaining many of
the phenomena, which is one of the conditions required by Newton's
first rule of philosophizing.
Heat, and light, and other stimuli, may perhaps exhaust the
excitability, by facilitating the combination of oxygen in the fibres
with the hydrogen and carbon in the blood.
There are several substances which cause a diminution or exhaustion
of the excitability, without producing any previous increased
excitement. These substances have by physicians been called
sedatives: and though the existence of such bodies is denied by Dr.
Brown, yet we are constrained to admit them; nor do their effects
seem incapable of being explained on the principle laid down,
especially if we call in the aid of chemistry.
Any substance which is capable of combining rapidly with oxygen, and
diminishing its quantity, will be a sedative. But the action of some
of the animal and vegetable poisons is difficult to explain in the
present state of our knowledge; such very minute portions of these
produce great exhaustion of the excitability, and even death, that we
can scarcely explain their action on the supposition that they
combine with the oxygen. They may perhaps act as ferments, and
occasion throughout the whole system a new and rapid combination of
oxygen with the hydrogenous, carbonic, and perhaps azotic parts of
the blood and fluids, and even of the solids, which will speedily
destroy the excitability, and even the organization.
Many of the vegetable narcotics, though they will destroy life when
given in considerable doses, yet when exhibited in less quantities
become very powerful remedies, particularly in cases where the
excitability is accumulated, in consequence of which violent spasms
and inordinate actions take place, which are very quickly calmed by
opium, camphor, musk, asafoetida, ether, &c. medicines that occasion
a speedy exhaustion of the excitability. In diseases of exhaustion,
however, these remedies are improper. The indication here is to
accumulate the irritability, by the introduction of oxygen, and by
the diminution of the action of the stimulants which support life. In
this idea too I dissent from Dr. Brown, who taught that diseases of
exhaustion are to be cured by stimulants, a little less powerful than
those which produced the disease. This subject will however be more
fully discussed hereafter.
This doctrine of animal life, which I have been attempting to
illustrate, and render familiar, exhibits a new view of the manner in
which it is constantly supported. It discovers to us the true means
of promoting health and longevity, by proportioning the number and
force of stimuli to the age, climate, situation, habits, and
temperament, of the human body. It leads us to a knowledge of the
causes of diseases: these we shall find consist either in an
excessive or preternatural excitement in the whole or part of the
human body, accompanied generally with irregular motions, and induced
by natural or artificial stimuli, or in a diminished excitement or
debility in the whole, or in part. It likewise teaches us that the
natural and only efficacious cure of these diseases depends on the
abstraction of stimuli, from the whole, or from a part of the body,
when the excitement is in excess: and in the increase of their number
and force when the contrary takes place.
The light which the discoveries of Galvani, and others who have
followed his steps, begin to throw on physiology, promises, when
aided by the principles of chemistry, and the knowledge of the laws
of life, to produce all the advantages that would result from a
perfect knowledge of the animal functions.
From what has been said, it does not seem improbable that muscular
contraction may depend upon the combination of oxygen with hydrogen
and azote, in consequence of a sort of explosion or discharge
produced by nervous electricity. According to this hypothesis, animal
motion, at least that of animals analogous to man, would be produced
by a beautiful pneumatic structure. This hypothesis, though not
perhaps at this moment capable of strict demonstration, seems
extremely probable, it being countenanced by every observation and
experiment yet made on the subject. It accounts likewise for the
perpetual necessity of inhaling oxygen, and enables us to trace the
changes which this substance undergoes, from the moment it is
received into the system, till the moment it is expelled. By the
lungs it is imparted to the blood; by the blood to the muscular
fibres; in these, during their contraction, it combines with the
hydrogen, and perhaps carbon and azote, to form water and various
salts, which are taken up by the absorbents, and afterwards exhaled
or excreted. We know the necessity of oxygen to muscular motion, and
likewise that this motion languishes when there is a deficiency of
the principle, as in sea scurvy. Thus a boundless region of discovery
seems to be opening to our view: the science of philosophy, which
began with remote objects, now promises to unfold to us the more
difficult and more interesting knowledge of ourselves. Should this
kind of knowledge ever become a part of general education, then the
causes of many diseases being known, and the manner in which the
external powers, with which we are surrounded, act upon us, a great
improvement not only in health, but in morality must be the
consequence.
With respect to its influence on the science of medicine, we may
observe that, from the time of Hippocrates till almost the present
day, medicine has not deserved the name of a science but, as he
called it, of a conjectural art. At present however, by the
application of the laws of life, and of the new chemistry, there is
beginning to appear in physiology and pathology, something like the
simplicity and certainty of truth. In proportion as the laws of
animal nature come to be ascertained, the study of them will excite
more general attention, and will ultimately prove the most popular,
as well as the most curious and interesting branch of philosophy.
This must be productive of beneficial consequences to society, since
these truths, once impressed upon the mind by conviction, will
operate as moral motives, by which the sum of disease and human
misery cannot fail to be greatly diminished.
LECTURE XI.
OF THE NATURE AND CAUSES OF DISEASES.
In the two last lectures I have attempted to investigate the laws of
life. I now proceed to the most important part of our course, and for
which all the preceding lectures were intended to prepare us; I mean
the application of the laws of life to explain the nature and causes
of diseases, and the methods of curing them, which must always be
imperfect, and conjectural, unless the nature of the diseases
themselves be well understood.
We have already seen that life is constantly supported by the action
of the external powers which surround us, and that if the action of
these powers be properly regulated, and at the same time no other
powers be suffered to act on the body, we shall enjoy perfect health,
but if, on the contrary, the exciting powers which support life, act
either too feebly or too powerfully, then the functions will not be
performed with precision and vigour, but irregularly; the mind and
body will become deranged, and death will often take place many years
before the natural period at which that event might be expected.
As health is the greatest blessing which man can enjoy, it is natural
to think, that in the early ages of society, when men began to lose
sight of the dictates of nature, and feel the torture of disease,
they would regard with gratitude those who had contributed towards
their relief, and that they would place their physicians among their
heroes and their gods. In the early ages, however, diseases would be
very few, for it would not be till civilisation had made considerable
progress, that such unnatural modes of life as conduce to their
production, would take place.
As the first professors of physic knew nothing of the animal economy,
and little of the theory of diseases, it is evident that whatever
they did, must have been in consequence of mere random trials. Indeed
it is impossible that this or any other art could originate in any
other manner. Accordingly history informs us that the ancient nations
used to expose their sick in temples, and by the sides of highways,
that they might receive the advice of every one that passed.
It would take up too much time to pursue the history of medicine from
this rude origin, through all its changes and revolutions, till the
present time: let it therefore suffice to say, that after various
theories had been invented and overturned, and after one age had
destroyed the labours of another, though different branches of the
healing art, and particularly anatomy, had been enriched with
valuable discoveries, still a rational theory was wanting; there was
nothing to guide the practitioner in his way, and we may truly say
that till the laws of life, which I have been endeavouring to
illustrate, were investigated by Dr. Brown, medicine could boast of
no theory which had a title to be called philosophical.
The theories of Stahl, Boerhaave, and Cullen, have passed away, and
are almost forgotten, but this, which is founded on nature, and on
fact, will, like the Newtonian philosophy, last for ever. It has
already influenced the practice of medicine, and is taught in almost
all the schools of Europe and America. In this country it seems to
have had less attention paid to it than it deserved, because its
influence was counteracted by the arrogance and profligacy of its
author, as if the grossness of a man's manner affected the
conclusiveness of his arguments; but this influence did not extend
beyond Britain, while the light of his theory illuminated the
opposite hemisphere. And when the manner in which he was persecuted
is recollected, the liberal mind will allow something to the deep
consciousness of neglected merit.
A circumstance much in favour of this doctrine is, that those who
understand its principles thoroughly, are guided by it in their
practice with a certainty and success before unknown. I say those who
understand its principles, for these were not perfectly understood
even by the author himself. He first saw with his mind's eye the
grand outline of the system, from which, for want of proper
reflection, he often drew wrong deductions, and which he often
applied improperly. But whatever errors Brown may have committed in
the application of his system, and however short his doctrines may
fall of a perfect system of medicine, we may venture to predict that
the grand outlines will remain unshaken.