to a considerable height above the surface of the earth, it is
evident, that it must press strongly on the surfaces of bodies. It is
thought to extend nearly fifty miles above the surface of the earth,
and must therefore press heavily on this surface. This may be evinced
by different experiments, performed by means of the air pump.
Another property of the air, by which it is distinguished from most
other fluids, is its elasticity. It may be compressed into a less
space than it naturally occupies, and when the compressing force is
removed, it expands to its former bulk, by its spring or elasticity.
Indeed it is always compressed into less space than it would
naturally occupy, by the weight of the superincumbent air.
The trachea, or windpipe, commences at the further end of the mouth,
between the root of the tongue, and the passage into the stomach: its
upper part is termed the larynx; it forms the projection in the fore
part of the neck, which is more prominent in the male than the
female: its opening is called the glottis, and is covered with a
small valve, or lid, called the epiglottis, which is open while we
breathe, but shuts when we swallow any thing, to prevent its getting
into the lungs: sometimes, however, particularly when we attempt to
speak at the time we swallow, a small portion of our food or drink
gets into the larynx, and excites violent coughing until it is thrown
back again.
The windpipe is composed of cartilaginous rings, covered with
membrane, which keep it open: after having run downwards for the
space of a few inches, it divides into two great branches, each of
which is subdivided into a vast number of ramifications, ultimately
terminating in little vesicles, which, when distended with air, make
up the greatest part of the bulk of the lungs.
The cavity in which the lungs are contained is called the thorax, or
chest: and is bounded by the ribs, and backbone or spine, and
separated from the abdomen by a muscular membrane, called the
diaphragm. The thorax, by the action of the diaphragm and intercostal
muscles, is alternately enlarged and diminished. Suppose then the
thorax to be in its least state; if it become larger, a vacuum will
be formed, into which the external air will descend by its weight,
filling and distending the vesicles of the lungs.
The thorax, thus dilated, is brought back to its former magnitude,
principally by the relaxation of the muscles, which distended it, and
the natural elasticity of the parts, aided by the contraction of the
abdominal muscles; the thorax being thus diminished, a quantity of
air is expelled from the lungs. The muscles which distend the thorax
beginning again to act, the air reenters; and this alternate dilatation
and contraction, is called respiration. The entrance of the air into
the lungs, is termed inspiration, and its expulsion, expiration.
To form a more accurate idea of the manner in which respiration is
performed, let us suppose this room to be filled with water. On
enlarging the thorax, in the manner before mentioned, the water by
its weight would rush in, and fill the newly formed void; and, upon
the diminution of the capacity of the thorax, a part of this water
would be expelled. Just in the same manner the air will alternately
enter and be expelled from the lungs by this alternate dilatation and
contraction of the thorax.
Respiration is a function of such consequence, that death follows if
it is suspended for a few minutes only. By means of this function the
blood is elaborated, and rendered fit to nourish the body; by means
of it the system is, most probably, supplied with irritability; by
means of it the nervous energy is, most likely, conveyed into the
body, to be expended in sensation, and muscular motion. It appears,
likewise, that in this way, animals are supplied with that heat which
preserves their temperatures nearly the same, whatever may be the
temperatures of surrounding bodies.
If any number of inanimate bodies, possessed of different degrees of
heat, be placed near each other, the heat will begin to pass from the
hotter bodies to the colder, till there be an equilibrium of
temperature. But this is by no means the case with respect to
animated matter; for whatever be the degree of heat peculiar to
individual animals, they preserve it, nearly unchanged, in every
temperature, provided the temperature be not altogether incompatible
with life or health. Thus, we find, from experiments that have been
made, that the human body is not only capable of supporting, in
certain circumstances, without any material change in its
temperature, a degree of heat considerably above that at which water
boils; but it likewise maintains its usual temperature, whilst the
surrounding medium is several degrees below frost.
It is evident, therefore, that animals neither receive their heat
from the bodies which surround them, nor suffer, from the influence
of external circumstances, any material alterations in that heat
which is peculiar to their nature. These general facts are confirmed
and elucidated by many accurate and well authenticated observations,
which show, that the degree of heat in the same genus and species of
the more perfect animals, continues uniformly the same, whether they
be surrounded by mountains of snow, in the neighbourhood of the pole,
or exposed to a vertical sun, in the sultry regions of the torrid
zone.
This stability and uniformity of animal heat, under such a disparity
of external circumstances, and so vast a latitude in the temperature
of the ambient air, prove, beyond doubt, that the living body is
furnished with a peculiar mechanism, or power of generating,
supporting, and regulating its own temperature; and that this is so
wisely adapted to the circumstances of its economy, or so dependent
upon them, that, whatever be the temperature of the atmosphere, it
will have very little influence either in diminishing or increasing
that of the animal.
In order that we may see how this effect is produced, we must examine
the chemical properties of the air. Previously to this, however, it
will be necessary to point out briefly how bodies are affected, with
respect to heat, when they change their form.
When a body passes from a state of solidity to that of fluidity, it
absorbs a quantity of heat, which becomes chemically combined with
it, and insensible to the touch or the thermometer; in the same
manner, when it passes from a fluid state to that of vapour or gas,
it combines with a still larger quantity of heat, which remains
latent in it, so long as it continues in the state of gas, but when
it returns to the liquid or solid state, it gives out the heat which
was combined with it, which, being set at liberty, flows into the
surrounding bodies, and augments their temperature.
This is evinced by the conversion of ice into water, and of water
into steam; and by the return of steam into water. It is evinced
likewise by the evaporation of ether, and by numberless other
experiments.
Modern chemistry has shown that the atmosphere is not a homogeneous
fluid, but consists of two elastic fluids, endowed with opposite and
different properties.
If a combustible body, for instance a candle, be confined in a given
quantity of atmospheric air, it will burn only for a certain time;
after it is extinguished, if another combustible body be lighted and
immersed in the same air, it will not burn, but will immediately be
extinguished.
It has been proved by chemical experiments, that in this instance,
the combustible body absorbs that portion of the air which is fitted
for combustion, but produces no change on that which is unfit: so
that, according to this, the air of the atmosphere consists of two
elastic fluids, one of which is capable of supporting combustion, and
the other not; and that they exist in the proportion of one part of
the former to three of the latter nearly.
These two parts may be separated from each other, and experiments
made with them.
Many metals, and particularly manganese, when exposed to the
atmosphere, attract the combustible air from it, without touching the
other; and it may be procured from these metals by the application of
heat, in very great purity.
Because this air is essential to the formation of acids, it has been
called by chemists the acidifying principle, or oxygen gas.
On plunging a combustible body into the remaining air, it is
instantly extinguished; an animal in the same situation is
immediately deprived of life: from this latter circumstance this air
has been called azote, or azotic gas. If we take three parts of azote
and one of oxygen, and mix them together, we shall form an air in
every respect similar to that of the atmosphere.
If I plunge a piece of iron, previously heated, into oxygen gas, it
will burn with great brilliancy, the gas will be diminished in
quantity, and the iron augmented in weight, and this increase of
weight in the metal will be in proportion to the oxygen which has
disappeared: at the same time a great quantity of heat is given out.
This is the heat which was combined with the oxygen in the state of
gas, and which now becomes free, when the oxygen becomes solid and
joins with the iron.
The same phenomena take place when phosphorus is burned in oxygen
gas; the gas becomes diminished, the phosphorus increased, in weight,
and converted into an acid, and a great quantity of heat is given
out. The same is the case when charcoal is burned in this gas. In
short, in every instance of combustion, the oxygen combines with the
combustible body, and at the same time gives out its heat, which
supported it in the form of gas. This is the case of the combustion
of coal in a common fire, as well as in other cases of combustion;
the heat comes from the air, and not from the coal.
When we examine the phenomena of respiration with attention, we shall
find them very analogous to those of combustion. A candle will not
burn in an exhausted receiver: an animal in the same situation ceases
to live.
When a candle is confined in a given quantity of atmospheric air, it
will burn only for a certain length of time. On examining the air in
which it has been burned, the oxygen is found to be all extracted,
nothing remaining but azotic gas, and a quantity of carbonic acid
gas, produced by the union of the charcoal of the candle with the
oxygen of the atmospheric air.
In the same manner, if an animal be confined in a given quantity of
atmospheric air, it will live only a short time; on examining the air
in which it has ceased to live, it will be found to have lost its
oxygen: what remains being a mixture of azotic and carbonic acid
gases.
When a candle is enclosed in a given quantity of pure oxygen gas, it
will burn four times as long as in the same quantity of atmospheric
air.
In the same manner it has been proved, that an animal will be four
times as long in consuming a given quantity of pure oxygen gas, as in
rendering unfit for respiration the same quantity of atmospheric air.
Here then we observe a striking similarity between combustion and
animal respiration. The ancients seem to have had a more accurate
idea of respiration than most of the philosophers who followed them.
They supposed that the air contained a principle proper for the
support and nourishment of life, which they called pabulum vitae.
This idea, which was unconnected with any hypothesis, was followed by
systems destitute of foundation. Sometimes it was thought that the
air in the lungs incessantly acted as a stimulus or spur to drive on
the circulation; sometimes the lungs were considered in the light of
a pair of bellows, or fan, to cool the body, which was supposed to be
heated by a thousand imaginary causes: and when philosophers were
convinced, by experiments, that the bulk of the air was diminished by
respiration, they explained it by saying, that the air had lost its
spring.
Modern chemistry however enables us to explain the phenomena of
respiration in a satisfactory manner.
In order to see this, we shall proceed to examine the changes
produced by respiration; firstly, on the air, and secondly, on the
blood.
The air which has served for respiration, is found to contain a
mixture of azotic and carbonic acid gas, with a small quantity of
oxygen gas; and a considerable quantity of water is thrown off from
the lungs, in the form of vapour, during respiration.
From a variety of facts, it appears that oxygen gas is decomposed in
the lungs during respiration; a part of it unites, as we shall
afterwards see, with the iron contained in the blood, and converts it
into an oxid; another and greater portion unites with the carbon,
brought by the venous blood from all parts of the body to the lungs,
and thus forms carbonic acid gas; while another portion of the oxygen
unites with the hydrogen, brought in the same manner by the blood,
and forms water. Thus then we are able to account for the different
products of respiration.
Hence we see, that the explanation of animal heat follows as a simple
and beautiful corollary from the theory of combustion; and we may
consider respiration as an operation in which oxygen gas is
continually passing from the gaseous to the concrete state; it will
therefore give out at every instant the heat which it held in
combination, and this heat, being conveyed by the circulation of the
blood to all parts of the body, is a constant source of heat to the
animal.
These facts likewise enable us to explain the reason, why an animal
preserves the same temperature, notwithstanding the various changes
which occur in the temperature of the surrounding atmosphere. In
winter the air is condensed by the cold, the lungs therefore receive
a greater quantity of oxygen in the same bulk, and the heat
extricated will be proportionally increased. In summer, on the
contrary, the air being rarefied by the heat, a less quantity of
oxygen will be received by the lungs during each inspiration, and
consequently the heat which is extricated must be less.
For the same reason, in northern latitudes, the heat extricated by
respiration will be much greater than in the southern. By this simple
and beautiful contrivance, nature has moderated the extremes of
climate, and enabled the human body to bear vicissitudes which would
otherwise destroy it.
Of all the phenomena of the animal body, there is none at first sight
more remarkable, than that which animals possess of resisting the
extremes of temperature.
The heat of the body, as has already been observed, continues at the
same degree, though the temperature of the atmosphere be sometimes
considerably hotter, at other times considerably colder, than the
animal body: so that man is able to live, and to preserve the
temperature of health, on the burning sands of Africa, and on the
frozen plains of Siberia.
The alterations of temperature which the human body has been known to
bear, without any fatal or even bad effects, are not less than 400
degrees or 500 degrees of Fahrenheit. The natural heat of the human
body is 96 degrees or 97 degrees. In the West Indies, the heat of the
atmosphere is often 98 degrees or 99 degrees, and sometimes rises
even to 126 degrees, or 30 degrees above the temperature of the human
body, notwithstanding which, a thermometer put in the mouth points to
96 degrees or 97 degrees. The inhabitants of the hot regions of
Surinam support, without inconvenience, the heat of their climate. We
are assured that in Senegal, about the latitude of 17 degrees, the
thermometer in the shade generally stands at 108 degrees, without any
fatal effects to men or animals. The Russians often live in places
heated by stoves to 108 degrees or 109 degrees, and some philosophers
in this country, by way of experiment, remained a considerable time
in a room heated above the boiling point of water.
On the other hand, an equal excess of cold seems to have no greater
effect in altering the degree of heat proper to animal bodies.
Delisle has observed a cold in Siberia 70 degrees below the zero of
Fahrenheit's scale, notwithstanding which animals lived. Gmelin has
seen the inhabitants of Jeniseisk under the 58th degree of northern
latitude, sustaining a degree of cold, which in January became so
severe, that the spirit in the thermometer was 126 degrees below the
freezing point. Professor Pallas in Siberia, and our countrymen at
Hudson's Bay, have experienced a degree of cold almost equal to this.
All these facts tend to prove, that the heat of animals continues
without alteration in very different temperatures. Hence it is
evident that they must be able to produce a greater degree of heat,
when surrounded by a cold medium; and on the contrary, that they must
effect a diminution of the heat, when the surrounding medium is very
hot.
All these circumstances may be accounted for, by the principles we
have laid down; the decomposition of oxygen in the lungs.
There have not been wanting, however, some very eminent
physiologists, who have contended that animal heat is produced
chiefly by the nerves. They have brought forward in proof of this the
well known fact, that when the spinal marrow is injured, the
temperature of the body generally becomes diminished; and that in a
paralytic limb the heat is less than ordinary, though the strength
and velocity of the pulse remain the same. These facts, and others of
a similar nature, have induced them to believe, that the nervous
system is the chief cause and essential organ of heat; and they have
adduced similar arguments, to prove that nutrition is performed by
the nerves, for a limb which is paralytic from an injury of the
nerves, wastes, though the circulation continues. The truth is, that
the nerves exert their influence upon these, and all other functions
of the body, and modify their action. The liver secretes bile, but if
the nerves leading to it be destroyed, the secretion of bile will
cease; but who will say, that the bile is secreted by the nerves? The
nitric acid will dissolve metals, and this solution will go on more
quickly if heat be applied; but surely the nitric acid is the
solvent, the heat being only an aiding cause.
But though the human body has been so wisely constructed, as to bear,
without inconvenience, a considerable variation of temperature; yet
this latitude has its limits, which depend upon the capability of
extricating heat from the atmosphere. There must be a limit below
which the diminution of heat takes place faster than its production.
If this be continued, or increased, the heat of the animal must
diminish, the functions lose their energy, and an insuperable
inclination to sleep is felt, in which if the sufferer indulge, he
will be sure to wake no more.
This is confirmed by what happened to Sir Joseph Banks and his party
on the heights of Terra del Fuego. Dr. Solander, who had more than
once crossed the mountains which divide Sweden from Norway, well knew
that extreme cold produces an irresistible torpor and sleepiness, he
therefore conjured the company to keep always in motion, whatever
exertion it might require, and however great might be their
inclination to rest. Whoever sits down, says he, will sleep; and
whoever sleeps will wake no more. Thus, at once admonished and
alarmed, they set forward; but, while they were still upon the naked
rocks, the cold was so intense, as to produce the effects which had
been so much dreaded. Dr. Solander himself was the first who found
the inclination against which he had warned others, irresistible; and
insisted on being suffered to lie down. Sir Joseph entreated and
remonstrated in vain; he lay down upon the ground, though it was
covered with snow; and it was with great difficulty that his friend
kept him from sleeping. One of his black servants also began to
linger, having suffered from the cold in the same manner as the
Doctor. Partly by persuasion, and partly by force, they were got
forwards; soon however they both declared that they would go no
further. Sir Joseph had recourse again to entreaty and expostulation,
but these produced no effect: when the black was told, that if he did
not go on, he would shortly be frozen to death; he answered, that he
desired nothing so much as to lie down and die. The Doctor did not so
explicitly renounce his life, but said, he would go on, if they would
first allow him to take some sleep, though he had before told them,
that to sleep was to perish. They both in a few minutes fell into a
profound sleep, and after five minutes Sir Joseph Banks happily
succeeded in waking Dr. Solander, who had almost lost the use of his
limbs; the muscles were so shrunk, that his shoes fell from his feet;
but every attempt to recal the unfortunate black to life proved
unsuccessful.
As the circulation of the blood is the means by which the heat
produced is conveyed to all parts of the body; and as it is a
function of the highest importance, I shall, in the next lecture,
proceed to the consideration of it.
LECTURE III.
CIRCULATION OF THE BLOOD.
Two kinds of motion may be distinguished in the animal economy; the
one voluntary, or under the command of the will, which takes place at
certain intervals, but may be stopped at pleasure. The other kind of
motion is called involuntary, as not depending on the will, but going
on constantly, without interruption, both when we sleep and when we
wake.
Of the first kind is the motion of the limbs, of which I have already
spoken in general terms; the object of which is, to change the
situation of the animal, and carry it where the will directs.
Among the involuntary motions, the most remarkable is the circulation
of the blood, which I shall proceed to consider in this lecture.
There is one motion, however, which claims a middle place between the
voluntary and involuntary; I mean respiration. This action is so far
under the command of the will, that it may be suspended, increased,
or diminished in strength and frequency: but we can only suspend it
for a very short time; and it goes on regularly during sleep, and in
general, even when we are awake, without the intervention of the
will; its continuation being always necessary, as we have already
seen, to support life.
The motion of the fluids in the living body is regulated by very
different laws, from those which govern the motion of ordinary
fluids, that depend upon their gravity and fluidity: these last have
a general centre of gravitation to which they incessantly tend. Their
motion is from above downwards, when not prevented by any obstacles;
and when they meet with obstruction, they either stop till the
obstacle is removed, or escape where they find the least resistance.
When they have reached the lowest situations, they remain at rest,
unless acted upon by some internal impulse, which again puts them in
motion.
But the motion of the fluids in an animal body, is less uniform,
constant, and regular; it takes place upwards as well as downwards,
and overcomes numerous obstacles; it carries the blood from the
interior parts of the body to the surface, and from the surface back
again to the internal parts; it forces it from the left side of the
body to the right, and with such rapidity that not a particle of the
fluid remains an instant in the same place.
The principal organ concerned in the circulation of the blood, is the
heart; which is a hollow muscle, of a conical figure, with two
cavities, called ventricles; this organ is situated in the thorax or
chest; its apex or point is inclined downwards and to the left side,
where it is received in a cavity of the left lobe of the lungs.
At the basis of the heart on each side are situated two cavities,
called auricles, to receive the blood; and these contracting, force
the blood into the ventricles, which are two cavities in the heart,
separated from each other by a strong muscular partition. The cavity
which is situated on the right side of the heart, is called the right
ventricle, and that on the left the left ventricle. From the right
ventricle of the heart issues a large artery, called the pulmonary
artery, which goes to the lungs, and is there divided and subdivided
into a vast number of branches, the extremities of which are too
small to be visible. These ultimate ramifications unite again into
larger branches; these again into branches still larger, and so
continually, till at last they form four tubes, called the pulmonary
veins, which are inserted into the left auricle of the heart,