the sensibility and excitability of this organ, and so intimate is its
connection with every other part of the body, that the whole system is
invigorated by aliment, as soon as it comes in contact with it. 2. By
mastication. This moves a number of muscles and blood-vessels situated
near the brain and heart, and of course imparts impressions to them.
3. By deglutition, which acts upon similar parts, and with the same
effect. 4. By its presence in the stomach, in which it acts by its
quantity and quality. Food, by distending the stomach, stimulates the
contiguous parts of the body. A moderate degree of distention of the
stomach and bowels is essential to a healthy excitement of the system.
Vegetable aliment and drinks, which contain less nourishment than animal
food, serve this purpose in the human body. Hay acts in the same manner
in a horse. Sixteen pounds of this light food in a day are necessary
to keep up such a degree of distension in the stomach and bowels of
this animal, as to impart to him his natural grade of strength and
life. The _quality_ of food, when of a stimulating nature, supplies
the place of its distension from its quantity. A single onion will
support a lounging highlander on the hills of Scotland for four and
twenty hours. A moderate quantity of salted meat, or a few ounces of
sugar, have supplied the place of pounds of less stimulating food. Even
indigestible substances, which remain for days, or perhaps weeks in the
stomach, exert a stimulus there which has an influence upon animal life.
It is in this way the tops of briars, and the twigs of trees, devoid
not only of nourishing matter, but of juices, support the camel in his
journies through the deserts of the eastern countries. Chips of cedar
posts moistened with water have supported horses for two or three weeks,
during a long voyage from Boston to Surinam; and the indigestible cover
of an old Bible preserved the life of a dog, accidentally confined in
a room at Newcastle upon Tyne, for twenty days. 5. Food stimulates the
whole body by means of the process of digestion which goes forward in
the stomach. This animal function is carried on by a process, in which
there is probably an extrication of heat and air. Now both these, it
has been remarked, exert a stimulus in promoting animal life.
Drinks, when they consist of fermented or distilled liquors, stimulate
from their quality; but when they consist of water, either in its simple
state, or impregnated with any sapid substance, they act principally by
distention.
II. The chyle acts upon the lacteals, mesenteric glands, and thoracic
duct, in its passage through them; and it is highly probable, its first
mixture with the blood in the subclavian vein, and its first action on
the heart, are attended with considerable stimulating effects.
III. The blood is a very important internal stimulus. It has been
disputed whether it acts by its quality, or only by distending the
blood-vessels. It appears to act in both ways. I believe with Dr.
Whytt, that the blood stimulates the heart and arteries by a specific
action. But if this be not admitted, its influence in extending the
blood-vessels in every part of the body, and thereby imparting extensive
and uniform impressions to every animal fibre, cannot be denied. In
support of this assertion it has been remarked, that in those persons
who die of hunger, there is no diminution of the quantity of blood in
the large blood-vessels.
IV. A certain _tension_ of the glands, and of other parts of the body,
contributes to support animal life. This is evident in the vigour which
is imparted to the system, by the fulness of the seminal vesicles and
gall bladder, and by the distension of the uterus in pregnancy. This
distension is so great, in some instances, as to prevent sleep for many
days and even weeks before delivery. It serves the valuable purpose of
rendering the female system less liable to death during its continuance,
than at any other time. By increasing the quantity of life in the body,
it often suspends the fatal issue of pulmonary consumption, and ensures
a temporary victory over the plague and other malignant fevers; for
death, from those diseases, seldom takes place, until the stimulus, from
the distension of the uterus, is removed by parturition.
V. The exercises of the faculties of the mind have a wonderful influence
in increasing the quantity of human life. They all act by _reflection_
only, after having been previously excited into action by impressions
made upon the body. This view of the _re-action_ of the mind upon the
body accords with the simplicity of other operations in the animal
economy. It is thus the brain repays the heart for the blood it conveys
to it, by re-acting upon its muscular fibres. The influence of the
different faculties of the mind is felt in the pulse, in the stomach,
and in the liver, and is seen in the face, and other external parts of
the body. Those which act most unequivocally in promoting life are the
understanding, the imagination, and the passions. Thinking belongs to
the understanding, and is attended with an obvious influence upon the
degree and duration of life. Intense study has often rendered the body
insensible to the debilitating effects of cold and hunger. Men of great
and active understandings, who blend with their studies temperance and
exercise, are generally long lived. In support of this assertion, a
hundred names might be added to those of Newton and Franklin. Its truth
will be more fully established by attending to the state of human life
in persons of an opposite intellectual character. The cretins, a race
of idiots in Valais, in Switzerland, travellers tell us, are all short
lived. Common language justifies the opinion of the stimulus of the
understanding upon the brain: hence it is common to say of dull men,
that they have scarcely ideas enough to keep themselves awake.
The imagination acts with great force upon the body, whether its
numerous associations produce pleasure or pain. But the passions pour
a constant stream upon the wheels of life. They have been subdivided
into emotions and passions properly so called. The former have for their
objects present, the latter, future good and evil. All the objects of
the passions are accompanied with desire or aversion. To the former
belong chiefly, hope, love, ambition, and avarice; to the latter,
fear, hatred, malice, envy, and the like. Joy, anger, and terror,
belong to the class of emotions. The passions and emotions have been
further divided into stimulating and sedative. Our business at present
is to consider their first effect only upon the body. In the original
constitution of human nature, we were made to be stimulated by such
passions and emotions only as have moral good for their objects. Man
was designed to be always under the influence of hope, love, and joy.
By the loss of his innocence, he has subjected himself to the dominion
of passions and emotions of a malignant nature; but they possess, in
common with such as are good, a stimulus which renders them subservient
to the purpose of promoting animal life. It is true, they are like the
stimulus of a dislocated bone in their operation upon the body, compared
with the action of antagonist muscles stretched over bones, which
gently move in their natural sockets. The effects of the good passions
and emotions, in promoting health and longevity, have been taken notice
of by many writers. They produce a flame, gentle and pleasant, like oil
perfumed with frankincense in the lamp of life. There are instances
likewise of persons who have derived strength and long life from the
influence of the evil passions and emotions that have been mentioned.
Dr. Darwin relates the history of a man, who used to overcome the
fatigue induced by travelling, by thinking of a person whom he hated.
The debility induced by disease is often removed by a sudden change in
the temper. This is so common, that even nurses predict a recovery in
persons as soon as they become peevish and ill-natured, after having
been patient during the worst stage of their sickness. This peevishness
acts as a gentle stimulus upon the system in its languid state, and
thus turns the scale in favour of life and health. The famous Benjamin
Lay, of this state, who lived to be eighty years of age, was of a very
irascible temper. Old Elwes was a prodigy of avarice, and every court in
Europe furnishes instances of men who have attained to extreme old age,
who have lived constantly under the dominion of ambition. In the course
of a long inquiry which I instituted some years ago into the state
of the body and mind in old people, I did not find a single person
above eighty, who had not possessed an active understanding, or active
passions. Those different and opposite faculties of the mind, when in
excess, happily supply the place of each other. Where they unite their
forces, they extinguish the flame of life, before the oil which feeds it
is consumed.
In another place I shall resume the influence of the faculties of the
mind upon human life, as they discover themselves in the different
pursuits of men.
I have only to add here, that I see no occasion to admit, with the
followers of Dr. Brown, that the mind is active in sleep, in preserving
the motions of life. I hope to establish hereafter the opinion of Mr.
Locke, that the mind is always passive in sound sleep. It is true it
acts in dreams; but these depend upon a morbid state of the brain, and
therefore do not belong to the present stage of our subject, for I am
now considering animal life only in the healthy state of the body. I
shall say presently, that dreams are intended to supply the absence of
some natural stimulus, and hence we find they occur in those persons
most commonly, in whom there is a want of healthy action in the system,
induced by the excess or deficiency of customary stimuli.
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 mid-day, and remains stationary for
some hours. From the diminution of the sensibility and contractility
of the system to the action of impressions, it lessens in the evening,
and becomes again languid at bed-time. These facts will admit of an
extensive application hereafter in our lectures upon the practice of
physic.
LECTURE II.
GENTLEMEN,
The stimuli which have been enumerated, when they act collectively, and
within certain bounds, produce a healthy waking state. But they do not
always act collectively, nor in the determined and regular manner that
has been described. There is, in many states of the system, a deficiency
of some stimuli, and, in some of its states, an apparent absence of
them all. To account for the continuance of animal life under such
circumstances, two things must be premised, before we proceed to take
notice of the diminution or absence of the stimuli which support it.
1. The healthy actions of the body in the waking state consist in a
proper degree of what has been called excitability and excitement. The
former is the medium on which stimuli act in producing the latter. In
an exact proportion, and a due relation of both, diffused uniformly
throughout every part of the body, consists good health. Disease is
the reverse of this. It depends _in part_ upon a disproportion between
excitement and excitability, and in a partial distribution of each of
them. In thus distinguishing the different states of excitement and
excitability in health and sickness, you see I dissent from Dr. Brown,
who supposes them to be (though disproportioned to each other) equably
diffused in the morbid, as well as the healthy state of the body.
2. It is a law of the system, that the absence of one natural stimulus
is generally supplied by the increased action of others. This is more
certainly the case where a natural stimulus is abstracted _suddenly_;
for the excitability is thereby so instantly formed and accumulated,
as to furnish a highly sensible and moveable surface for the remaining
stimuli to act upon. Many proofs might be adduced in support of this
proposition. The reduction of the excitement of the blood-vessels, by
means of cold, prepares the way for a full meal, or a warm bed, to
excite in them the morbid actions which take place in a pleurisy or a
rheumatism. A horse in a cold stable eats more than in a warm one, and
thus counteracts the debility which would otherwise be induced upon his
system, by the abstraction of the stimulus of warm air.
These two propositions being admitted, I proceed next to inquire into
the different degrees and states of animal life. The first departure
from its ordinary and perfect state which strikes us, is in
I. Sleep. This is either natural or artificial. Natural sleep is induced
by a diminution of the excitement and excitability of the system, by
the continued application of the stimuli which act upon the body in
its waking state. When these stimuli act in a determined degree, that
is, when the same number of stimuli act with the same force, and for
the same time, upon the system, sleep will be brought on at the same
hour every night. But when they act with uncommon force, or for an
unusual time, it is brought on at an earlier hour. Thus a long walk
or ride, by persons accustomed to a sedentary life, unusual exercise
of the understanding, the action of strong passions or emotions, and
the continual application of unusual sounds seldom fail of inducing
premature sleep. It is recorded of pope Ganganelli, that he slept more
soundly, and longer than usual, the night after he was raised to the
papal chair. The effects of unusual sounds in bringing on premature
sleep, is further demonstrated by that constant inclination to retire
to bed at an early hour, which country people discover the first and
second days they spend in a city, exposed from morning till night to the
noise of hammers, files, and looms, or of drays, carts, waggons, and
coaches, rattling over pavements of stone. Sleep is further hastened
by the absence of light, the cessation of sounds and labour, and the
recumbent posture of the body on a soft bed.
Artificial sleep may be induced at any time by certain stimulating
substances, particularly by opium. They act by carrying the system
beyond the healthy grade of excitement, to a degree of indirect
debility, which Dr. Brown has happily called the sleeping point. The
same point may be induced in the system at any time by the artificial
abstraction of the usual stimuli of life. For example, let a person shut
himself up at mid-day in a dark room, remote from noise of all kinds,
let him lie down upon his back upon a soft bed in a temperate state of
the atmosphere, and let him cease to think upon interesting subjects,
or let him think only upon one subject, and he will soon fall asleep.
Dr. Boerhaave relates an instance of a Dutch physician, who, having
persuaded himself that waking was a violent state, and sleep the only
natural one of the system, contrived, by abstracting every kind of
stimulus in the manner that has been mentioned, to sleep away whole
days and nights, until at length he impaired his understanding, and
finally perished in a public hospital in a state of idiotism.
In thus anticipating a view of the cause of sleep, I have said nothing
of the effects of diseases of the brain in inducing it. These belong to
another part of our course. The short explanation I have given of its
cause was necessary in order to render the history of animal life, in
that state of the system, more intelligible.
At the usual hour of sleep there is an abstraction of the stimuli of
light, sound, and muscular motion. The stimuli which remain, and act
with an increased force upon the body in sleep, are
1. The heat which is discharged from the body, and confined by means of
bed-clothes. It is most perceptible when exhaled from a bed-fellow. Heat
obtained in this way has sometimes been employed to restore declining
life to the bodies of old people. Witness the damsel who lay for this
purpose in the bosom of the king of Israel. The advantage of this
external heat will appear further, when we consider how impracticable
or imperfect sleep is, when we lie under too light covering in cold
weather.
2. The air which is applied to the lungs during sleep probably acts with
more force than in the waking state. I am disposed to believe that more
air is phlogisticated in sleep than at any other time, for the smell of
a close room in which a person has slept one night, we know, is much
more disagreeable than that of a room, under equal circumstances, in
which half a dozen people have sat for the same number of hours in the
day time. The action of decomposed air on the lungs and heart was spoken
of in a former lecture. An increase in its quantity must necessarily
have a powerful influence upon animal life during the sleeping state.
3. Respiration is performed with a greater extension and contraction
of the muscles of the breast in sleep than in the waking state; and
this cannot fail of increasing the impetus of the blood in its passage
through the heart and blood-vessels. The increase of the fulness and
force of the pulse in sleep, is probably owing in part to the action
of respiration upon it. In another place I hope to elevate the rank of
the blood-vessels in the animal economy, by showing that they are the
fountains of power in the body. They derive this pre-eminence from the
protection and support they afford to every part of the system. They are
the perpetual centineals of health and life; for they never partake in
the repose which is enjoyed by the muscles and nerves. During sleep,
their sensibility seems to be converted into contractility, by which
means their muscular fibres are more easily moved by the blood than in
the waking state. The diminution of sensibility in sleep is proved by
many facts to be mentioned hereafter; and the change of sensibility into
contractility will appear, when we come to consider the state of animal
life in infancy and old age.
4. Aliment in the stomach acts more powerfully in sleep than in the
waking state. This is evident from digestion going on more rapidly
when we are awake than when we sleep. The more slow the digestion, the
greater is the stimulus of the aliment in the stomach. Of this we have
many proofs in daily life. Labourers object to milk as a breakfast,
because it digests too soon; and often call for food in a morning, which
they can feel all day in their stomachs. Sausages, fat pork, and onions
are generally preferred by them for this purpose. A moderate supper is
favourable to easy and sound sleep; and the want of it, in persons who
are accustomed to that meal, is often followed by a restless night. The
absence of its stimulus is probably supplied by a full gall-bladder
(which always attends an empty stomach) in persons who are not in the
habit of eating suppers.
5. The stimulus of the urine, accumulated in the bladder during
sleep, has a perceptible influence upon animal life. It is often so
considerable as to interrupt sleep; and it is one of the causes of our
waking at a regular hour in the morning. It is moreover a frequent cause
of the activity of the understanding and passions in dreams; and hence
we dream more in our morning slumbers, when the bladder is full, than we
do in the beginning or middle of the night.
6. The fæces exert a constant stimulus upon the bowels in sleep. This
is so considerable as to render it less profound when they have been
accumulated for two or three days, or when they have been deposited in
the extremity of the alimentary canal.
7. The partial and irregular exercises of the understanding and passions
in dreams have an occasional influence in promoting life. They occur
only where there is a deficiency of other stimuli. Such is the force
with which the mind acts upon the body in dreams, that Dr. Brambilla,
physician to the emperor of Germany, informs us, that he has seen
instances of wounds in soldiers being inflamed, and putting on a
gangrenous appearance in consequence of the commotions excited in their
bodies by irritating dreams[92]. The stimulating passions act through
the medium of the will; and the exercises of this faculty of the mind
sometimes extend so far as to produce actions in the muscles of the
limbs, and occasionally in the whole body, as we see in persons who walk
in their sleep. The stimulus of lust often awakens us with pleasure or
pain, according as we are disposed to respect or disobey the precepts
of our Maker. The angry and revengeful passions often deliver us, in
like manner, from the imaginary guilt of murder. Even the debilitating
passions of grief and fear produce an indirect operation upon the system
that is favourable to life in sleep, for they excite that distressing
disease called the night mare, which prompts us to speak, or halloo, and
by thus invigorating respiration, overcomes the languid circulation of
the blood in the heart and brain. Do not complain then, gentlemen, when
you are bestrode by this midnight hag. She is kindly sent to prevent
your sudden death. Persons who go to bed in good health, and are found
dead the succeeding morning, are said most commonly to die of this
disease.
[92] A fever was excited in Cinna the poet, in consequence of his
dreaming that he saw Cæsar, the night after he was assassinated,
and was invited to accompany him to a dreary place, to which
he pointed, in order to sup with him. Convulsions and other
diseases, I believe, are often excited in the night, by
terrifying or distressing dreams.
_Plutarch's Life of M. Brutus._
I proceed now to inquire into the state of animal life in its different
stages. I pass over for the present its history in generation. It will
be sufficient only to remark in this place, that its first motion is
produced by the stimulus of the male seed upon the female ovum. This
opinion is not originally mine. You will find it in Dr. Haller[93]. The
pungent taste which Mr. John Hunter discovered in the male seed renders
it peculiarly fit for this purpose. No sooner is the female ovum thus
set in motion, and the f[oe]tus formed, than its capacity of life is
supported,
1. By the stimulus of the heat which it derives from its connection with
its mother in the womb.
2. By the stimulus of its own circulating blood.
3. By its constant motion in the womb after the third month of
pregnancy. The absence of this motion for a few days is always a sign
of the indisposition or death of a f[oe]tus. Considering how early a
child is accustomed to it, it is strange that a cradle should ever have
been denied to it after it comes into the world.
[93] "Novum f[oe]tum a seminis masculi _stimulo_ vitam
concepisse."--_Elementa Physiologiæ_, vol. viii. p. 177.
II. In infants there is an absence of many of the stimuli which support
life. Their excretions are in a great measure deficient in acrimony, and
their mental faculties are too weak to exert much influence upon their
bodies. But the absence of stimulus from those causes is amply supplied
1. By the very great excitability of their systems to those of light,
sound, heat, and air. So powerfully do light and sound act upon them,
that the Author of nature has kindly defended their eyes and ears from
an excess of their impressions by imperfect vision and hearing, for
several weeks after birth. The capacity of infants to be acted upon
by moderate degrees of heat is evident from their suffering less from
cold than grown people. This is so much the case, that we read, in Mr.
Umfreville's account of Hudson's Bay, of a child that was found alive
upon the back of its mother after she was frozen to death. I before
hinted at the action of the air upon the bodies of new-born infants in
producing the red colour of their skins. It is highly probable (from
a fact formerly mentioned) that the first impression of the atmosphere
which produces this redness is accompanied with pain, and this we know
is a stimulus of a very active nature. By a kind law of sensation,
impressions, that were originally painful, become pleasurable by
repetition or duration. This is remarkably evident in the impression now
under consideration, and hence we find infants at a certain age discover
signs of an increase of life by their delightful gestures, when they are
carried into the open air. Recollect further, gentlemen, what was said
formerly of excitability predominating over sensibility in infants. We
see it daily, not only in their patience of cold, but in the short time
in which they cease to complain of the injuries they meet with from
falls, cuts, and even severe surgical operations.
2. Animal life is supported in infants by their sucking, or feeding,
nearly every hour in the day and night when they are awake. I explained
formerly the manner in which food stimulated the system. The action
of sucking supplies, by the muscles employed in it, the stimulus of
mastication.