Second Article.
In the last paper on this subject a few instances were quoted, showing
the great extremes of temperature which human beings and the lower
animals are capable of enduring without injury, and in many cases without
inconvenience. We propose in the present article to notice briefly the
means by which it is believed living creatures are enabled to exhibit
this power; and although physiologists are not unanimous in their
opinions on the subject, yet the views we shall endeavour to explain
are those which are held by the majority of scientific men, and which
are best supported by experiment, by analogy, and by the authority of
illustrious names.
For the purpose of making the subject clear to those who may not be
acquainted with the principles of physiology, “the science of life,” as
it has been happily termed, it may be useful to explain the _rationale_
of an operation continually being performed by all of us, and yet very
little thought of or understood--we mean the process of breathing. It is
found that the natural heat of animals depends on the perfection of the
apparatus by which respiration is performed; those animals which have a
complicated respiratory organization having a high degree of bodily heat,
while those which have more simple and less delicately formed organs have
a temperature very little raised above the medium in which they live.[3]
It is necessary, therefore, to have a clear idea of the process of
respiration before we can understand the connection between it and animal
heat.
The object of respiration is to purify the blood and render it fit for
the various offices it performs in the animal economy. When the blood
leaves the heart to be distributed through the body, it is of a very
bright red colour, but as it proceeds in its course it gradually loses
this and assumes a purple hue; and when, having completed its circulation
through the body, it is returned to the heart again by the veins, it has
entirely lost its former bright colour, and is then very dark, and, from
the impurities it has acquired in its course, unfit for the purposes of
life. To restore its former qualities it is necessary that it should
be brought into contact with the atmosphere, and this takes place in
the lungs. By the action of the muscles of the chest and abdomen, the
interior of the chest is increased in size, an empty space is formed into
which the air instantly descends by the mouth or nostrils, constituting
what is termed the act of inspiration. At the same moment that the
muscles of the chest increase its size and make room for the air to
descend, at that very moment the heart contracts, or in popular language
pulsates or beats, the effect of which is to force the dark-coloured
(venous) blood from the portion of the heart in which it was contained,
into the lungs. The lungs are composed almost entirely of an innumerable
number of vesicles, or minute air-bladders, into which the air descends,
as we have stated. These vesicles are covered with a network of extremely
fine blood-vessels. When the heart pulsates, it fills these vessels
with the dark-coloured blood; and as the air is capable of passing
through both the coats of the vesicle and of the blood-vessels, it of
course comes into direct contact with the blood, and a chemical change
immediately takes place.
This chemical change is necessary for life: the air is changed in its
qualities, and the blood is also changed in its qualities. The air is
changed by having one of its constituent elements (oxygen) abstracted
from it: and the blood is changed by its being impregnated with this
gas, and relieved of another kind called carbonic acid. If from any
circumstance this process is interfered with, the individual dies of
suffocation. A person may be suffocated for want of air, or for want of
pure air. In the former case his death is caused in this manner:--The
wind-pipe being closed, either by pressure, as in the case of criminals
who die by hanging, or by something entering and obstructing it, it
happens that although the muscles of the chest enlarge its internal
area, as before mentioned, the air cannot descend into it. This does
not, however, interfere with the action of the heart, which forces the
dark blood into the minute blood-vessels of the chest, as usual; the
blood passes onward unchanged; it receives no oxygen, nor is its bright
red colour restored. In this state it reaches the chamber of the heart,
from whence it is to be distributed to the head and body; a portion of
it is forced up the vessels which convey it to the brain, and the moment
it reaches this organ, it produces violent convulsions, insensibility,
and in a few moments death. A similar result takes place from breathing
foul air. In this case, although air may descend into the air-vessels
of the lungs, yet, as the grand element, the oxygen, is not present, no
change is produced in the blood; it pursues the same course as that just
pointed out, unchanged in its quality, and the same fatal result is the
inevitable consequence.
The atmosphere in a state of purity is composed of two gases mixed
together; the one termed _oxygen_, the other _nitrogen_. After escaping
from the lungs, the air is found to have undergone a remarkable change;
the oxygen has disappeared, and its place is supplied with an equal
volume of another gas called _carbonic acid_; while at the same time
the air is altogether altered in many of its more important qualities;
it is no longer fit for the purposes of life, nor will a light burn in
it. A person shut up in a confined place without a supply of fresh air,
very soon expires: and a candle placed under a glass vessel filled with
air that has been breathed, immediately goes out. In short, respiration
and combustion are similar processes, and the same result is produced by
both, namely, carbonic acid gas.
This gas is formed by the mixture of oxygen with carbon (charcoal). It is
absorbed very readily by water, and is perhaps best known in the form of
soda water; the aërated liquid sold under that name being nothing more
than water strongly impregnated with carbonic acid gas. It is formed by a
variety of processes--by breathing, by combustion, by fermentation, and
otherwise. In every case, however, its formation is attended with heat.
And now, having thus briefly introduced the subject, we may mention, that
on this fact is founded the theory which attempts to explain the means by
which the animal temperature is produced and maintained. It is founded on
the fact, that whenever oxygen enters into combination with carbon, and
forms carbonic acid gas, heat is always produced.
The most usual manner in which this is effected is by combustion; the
substance which burns, such as wood, or tallow, or coal gas, for example,
consists principally of carbon, and on being ignited, the oxygen of the
atmosphere is made to combine with it, and carbonic acid is the result.
Every body knows that heat is produced by this process; but there are
many instances in which the same effect may take place without being so
readily understood. Heat and light are so constantly found united, that
we can hardly conceive how so large a substance as the human body can be
kept constantly warm without the aid of fire. It is, however, effected by
a chemical process identically the same as combustion, except that light
is not produced. The lungs may be regarded as the furnace of the body,
from which it derives its supply of heat; the fuel is the carbon in the
blood; and the wind-pipe is a chimney serving a double purpose: first,
to allow of the passage of fresh air for the process, and then to convey
away the vapour which is produced by it: for the breath which issues from
our lungs is just as much deteriorated in quality as that which escapes
from the chimney of a large furnace after passing through the fire.
This, then, is the process by which the animal heat is maintained.
The blood comes to the lungs loaded with carbon; the air descends the
wind-pipe, consisting of one-third oxygen; the carbon of the blood and
the oxygen of the air unite; the blood is purified, and carbonic acid gas
is produced. This is attended with heat; the purified blood is capable
of absorbing all this heat, and does so. In its progress through the
body, as the blood again becomes impure, it gradually parts with the heat
so acquired, and on again being purified, it receives a fresh supply.
Nothing can be more simple and beautiful than this process; it is in
accordance with every great operation in nature, which is always effected
in the most direct and simple manner; and the proofs that this is the
manner in which nature effects her object in this instance, are numerous
and unanswerable.
There are two circumstances which at first sight may appear to interfere
with the explanation above given of this very beautiful phenomenon.
First, the lungs are found to be but very little warmer than any other
part of the body, although, as we have stated, the animal heat is
produced in them: and, secondly, the quantity of carbon produced by
respiration is very small compared with the genial heat produced by its
conversion into carbonic acid. With regard to the heat of the lungs, a
series of experiments instituted for the purpose of ascertaining how they
were kept at so moderate a degree of temperature, led to the discovery
of an extraordinary change which takes place in the vital fluid after
being purified, which satisfactorily explains the circumstance. The pure
blood is found to have a greater capacity for heat than impure blood:
it will absorb more; and in consequence, all the heat produced by its
purification is immediately absorbed by it, and carried away as fast as
it is generated, to be distributed over the body. As the blood becomes
impure in its progress, it gradually loses its power of retaining the
heat it had so imbibed; and the heat therefore is distributed during the
circulation of the blood, and every part receives a due supply. This
change in the power of the vital fluid to absorb heat, according as it
is more or less pure, is a fact that was not established in the time of
Paley, or he would have been able to add another proof of design to his
unequalled argument.
The quantity of fuel (if we may use the expression) required for
generating the heat of the animal frame, is certainly less than we might
anticipate. All animal and vegetable food contains a considerable portion
of carbon, which of course, after being digested, becomes a part of the
vital fluid, and in this way it is supplied for the process. It is well
known also that in cold climates, where a greater quantity of animal fuel
is required, the inhabitants are extremely fond of fat and oily matters,
which contain more carbon than any other kind of food; yet it would
hardly be imagined that so small a quantity as the eighth part of an
ounce of carbon per hour would be sufficient to maintain the heat of the
body at an uniform temperature of 98 degrees. We are assured by the best
chemists, however, that the average quantity of carbonic acid generated
by a person in health in twenty-four hours is about 40,000 cubic inches,
and this contains only about 11½ ounces of pure carbon. Rather less than
half an ounce is therefore used per hour in preserving the body at its
usual temperature.
The limits of this article prevent our noticing other objections which
have been urged against the theory just described, but the facts it
rests upon can only be overturned by opposing facts which have never
yet been produced. It is certain that carbonic acid is produced during
respiration, that its production is always attended with heat, that pure
(arterial) blood is capable of absorbing a greater portion of heat than
impure (venous) blood, and that the temperature of any part of the body
is according to the supply of blood which it receives; an inflamed part,
becoming very hot, and a limb in which the circulation has been stopped
by a bandage becoming cold. These facts taken together sufficiently prove
the truth of the conclusion that has been drawn from them, and which we
have above very briefly illustrated.
It remains to say a few words on the manner in which the body is relieved
of its superabundant heat, and enabled to bear such high degrees of
temperature as mentioned in the former paper. Franklin was the first
who gave a rational explanation of the phenomenon. He observed that the
evaporation of a small quantity of a liquid from the surface of any
substance would reduce the temperature of a very large body. If we place
a little ether on our hand, and allow it to evaporate, we shall soon
become sensible how much cold may be produced in this way. Wine-coolers
are formed on this principle: they are made of porous earth, through
which the water they contain oozes very gradually, and is evaporated by
the heat of the air: this cools the liquid within, and of course the
decanter of wine contained in it. Now, perspiration cools the body in a
similar manner. If any person looks closely at the fleshy part of his
hand, he will observe that the minute ridges which lie nearly parallel
to each other are covered with an innumerable number of small pores,
through which the perspiration may be seen issuing when the hand is
warm. From microscopic observations it is calculated that the skin is
perforated by 1000 of those pores, or holes, in every square inch, and
that the whole surface of the body therefore contains not less than
2,304,000 pores! When the body is heated to a certain degree, the fluid
portions are all directed to the skin, and escape gradually through these
pores in the form of perspiration, and the cooling power thus produced
is capable of immediately removing the superabundant heat. The moment
perspiration broke out on the bodies of the experimenters who ventured
into the heated oven, all sense of pain was removed; and in many fatal
disorders to which man is subject, the first symptom of returning health
is a similar occurrence. We may add, that a common cold is the effect of
the perspiration being suddenly checked, and that the health of the body
depends on the minute pores we have referred to being kept open and in
action.