ANIMAL HEAT.
545. The true sources of animal heat, or calorification, are still
imperfectly known. No hypothesis has, as yet, received the concurrent
assent of physiologists. We see certain phenomena, but the ultimate
causes are hidden from our view. Its regular production, to a certain
degree, is essential both to animal and vegetable life.
546. There is a tendency between bodies of different temperature to an
equilibrium of heat. Thus, if we touch or approach a hot body, the
heat, or caloric passes from that body to our organs of feeling, and
gives the sensation of heat. On the contrary, when we touch a cold
body, the heat passes from the hand to that body, and causes a
sensation of cold.
547. The greater number of animals appear cold when we touch them;
and, indeed, the temperature of their bodies is not much above that of
the atmosphere, and changes with it. In man, and other animals that
approach him in their organization, it is otherwise. They have the
faculty of producing a sufficient quantity of caloric to maintain
their temperatures nearly at the same degree, under all atmospheric
changes, and keep themselves warm.
548. Those animals whose proper heat is not very perceivable, are
called _cold_-blooded; as most species of fishes, toads, snakes,
turtles, and reptiles generally. Those animals which produce
sufficient heat independently of the atmosphere surrounding them, are
called _warm_-blooded; as man, birds, quadrupeds, &c.
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545-570. _What is said respecting animal heat?_ 545. Are the true
sources of animal heat known? What do we see? 546. What is the tendency
between bodies of different temperatures? Give an explanation. 547.
What is said of the temperature of animals? 548. What is meant by
cold-blooded animals? By warm-blooded animals?
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549. The temperature of man is about 98 deg., (Fahrenheit's thermometer,)
and that of some other animals is higher; the temperature of birds,
for example, is about 110 deg. It is obvious, that in most parts of the
globe, the heat of the atmosphere is, even in summer, less than that
of the human body. In our latitude, the mercury rarely attains 98 deg.,
and sometimes it descends to several degrees below zero.
550. Captain Parry, with his ship's company, in his voyage of
discovery to the arctic regions, wintered in a climate where the
mercury was at 40 deg., and sometimes at 55 deg. below zero. Captain Back
found it 70 deg. below zero. These were 72 deg. and 102 deg. below the
freezing point, or about 200 deg. below that of their own bodies, and
still they were able to resist this low temperature, and escape being
"frost-bitten."
551. Captain Lyon, who accompanied Captain Parry in his second voyage
to the northern regions, found the temperature of an arctic fox to be
106 deg., while that of the atmosphere was 32 deg. below zero; making a
difference between the temperature of the fox and that of the
atmosphere, of 138 deg. Captain Scoresby found the temperature of a
whale, in the Arctic Ocean, to be 104 deg., or nearly as high as that of
other animals of the same kind in the region of the equator, while the
temperature of the ice was as low as 32 deg., and the water was nearly as
cold. These facts show what a strong counteracting energy there is in
animals against the effects of cold.
552. On the other hand, it has been ascertained by numerous and
well-conducted experiments, that the human body can be exposed, even
for a length of time, to a very high temperature, without essentially
elevating that of the body. Chantrey, the sculptor, often entered the
furnace, heated for drying his moulds, when the temperature indicated
by the thermometer was 330 deg. Chaubert, the Fire-King, is said to have
entered ovens when heated to 600 deg. In 1774, Sir Charles Blagden
entered a room in which the mercury rose to 260 deg. He remained eight
minutes without suffering.
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549. What is the temperature of the human body? Of birds? How does the
heat of the atmosphere in summer, in our latitude, compare with that
of the human system? 550. What is related of Captain Parry? Of Captain
Back? 551. Of Captain Lyon? Of Captain Scoresby? What do these facts
show? 552. What has been ascertained on the other hand?
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553. In order to render it certain that there was no fallacy, says Sir
Charles Blagden, "in the degree of heat shown by the thermometer, but
that the air breathed was capable of producing all the well-known
effects of such a heat on inanimate matter, I put some eggs and
beefsteak upon a tin frame placed near the thermometer, and farther
distant from the cockle than from the wall of the room. In about
twenty minutes the eggs were taken out, roasted quite hard; and in
forty-seven minutes, the steak was not only dressed, but almost dry."
554. If a thermometer be placed under the tongue of a healthy person,
in all climates and seasons the temperature will be found nearly the
same. Sir Charles Blagden, "while in the heated room, breathed on a
thermometer, and the mercury sank several degrees; and when he expired
forcibly, the air felt cool as it passed through the nostrils, though
it was scorching hot when it entered them in inspiration."
_Observation._ Did not the human body possess within itself the power
of generating and removing heat, so as to maintain nearly an equality
of temperature, the most fatal consequences would ensue. In northern
latitudes, especially, in severe weather of winter, the blood would be
converted into a solid mass, and on the other hand, the fatty
secretion, when subjected to equatorial heat, would become fluid, and
life would be extinguished.
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What is related of Chantrey? Of Chaubert? Of Sir Charles Blagden? 553.
Give Sir Charles's own statement. 554. What is said of the temperature
of the human tongue? Mention the experiment by Sir Charles Blagden.
What would be the effect if the human system did not maintain an
equality of temperature?
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555. To enable man, and other warm-blooded animals, to maintain this
equilibrium of temperature under such extremes of heat and cold,
naturally suggests two inquiries: 1st. By what organs is animal heat
generated? 2d. By what means is its uniformity maintained?
556. The ancients had no well-arranged theory on the subject of animal
heat. They believed that the chief object of respiration was to cool
the blood, and that the heart was the great furnace where all the heat
was generated. At a later period, Mayow, from his discoveries
respecting respiration, asserted that the object of respiration was to
produce heat, and denied that the blood was cooled in the lungs.
557. When it was discovered that, both in combustion and respiration,
carbonic acid was produced and oxygen absorbed, it led Dr. Black to
conclude that breathing was a kind of combustion by which all the heat
of the body was produced. This theory was objected to, because, if all
the heat was generated in the lungs, like those parts of a stove in
contact with the fuel, they would be at a higher temperature than
those parts at a distance, which was known not to exist.
558. The next theory, and one which received the sanction of the
scientific men of Europe, was proposed by Dr. Crawford. He agreed with
Dr. Black that heat not only was generated in the lungs, but that the
arterial blood had a greater capacity for heat than the venous, and
that this increase of capacity takes place in the lungs. At the moment
heat is generated, a portion of it, under the name of latent heat, is
absorbed and conveyed to the different parts of the body Wherever
arterial blood is converted into venous, this latent heat is given
out. But, unfortunately for this theory, Dr. Davy proved the capacity
of both, for heat, to be nearly the same.
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555. What inquiries are naturally suggested? 556. What was the theory
of the ancients? What did Mayow assert at a later period? 557. What
was the theory of Dr. Black? The objection? 558. What was the theory
of Dr Crawford?
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559. No one can doubt that respiration and animal heat are closely
connected. Those animals whose respiratory apparatus is the most
extended, have the highest temperature. An example is seen in birds,
whose organs of respiration extend over a large part of the body, and
their temperature is 12 deg. above man; while the respiratory apparatus of
cold-blooded animals, as some kinds of fish, is imperfect, and only a
small quantity of blood is subjected, at any time, to the effects of
respiration.
560. To understand the process by which heat is generated in the human
system and in animals, it will be necessary to state: 1st. That the
apparent heat of a body, as perceived by the touch, or as indicated by
a thermometer, is not the measurement of heat contained in the body,
or its capacity for heat.
_Illustration._ If we mix one pound of water, at the temperature of
60 deg., with another pound at 91 deg., the resulting temperature will be
exactly the medium, or 75 1/2 deg. But, if we mix a pound of water at 60
deg. with a pound of quicksilver at 91 deg., the resulting temperature
will be only 61 deg., because the capacity of water for heat is so much
greater than that of quicksilver, that the heat which raised the
quicksilver 31 deg. will raise the water only 1 deg.
561. 2d. When the density and the arrangement of the atoms of a body
are changed, its capacity to hold heat in a latent state is altered.
If it will retain more, heat will be absorbed from contiguous and
surrounding substances; but, if its capacity for caloric is lessened,
heat will be set free and given out to surrounding bodies.
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The objection? 559. In what do all the physiologists of the present
day concur? How is it proved that respiration and animal heat are
closely connected? 560. What is said of the apparent heat of bodies?
How is this illustrated? 561. What is the effect when the density and
the arrangement of the atoms of a body are changed?
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_Illustrations._ 1st. Ice and salt, (Chl. of Sodium,) when mixed, are
converted into a fluid. In this state they will hold more heat than
when solid. The heat necessary to produce this change is drawn from
the surrounding medium, which is made proportionally colder by the
loss of caloric imparted to the ice and salt. It is by this chemical
process that "ice-cream" is made.
2d. On the other hand, mix water and sulphuric acid, (oil of vitriol,)
of the temperature of 60 deg., and the mixture will become quite warm,
and will freely impart its heat to surrounding and contiguous objects.
562. The same principle is exhibited, when oxygen unites with an
inflammable body, as in the burning of wood, coal, oil, &c. In
combustion, the oxygen of the atmosphere unites with carbon and
hydrogen, and carbonic acid and water are produced. This process,
according to all the known laws of caloric, is attended with heat. The
quantity of heat disengaged in combustion is always in proportion to
the amount of carbon and hydrogen consumed; thus a piece of wood
weighing one pound, in burning slowly, would give out the same
quantity of heat as a pound of shavings of the same wood, in burning
rapidly. Upon these principles, the production of animal heat may be
understood.
563. The food contains carbon and hydrogen. These exist in the chyle.
The old and waste atoms of the body likewise contain the same
elements. In the lungs the oxygen and nitrogen of the inspired air are
separated. It is now supposed that the oxygen enters the capillary
vessels of the lungs, and mingles with the blood, with which it is
carried to the heart and thence to the nutrient capillary vessels of
every part of the system.
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Give the 1st illustration. The 2d. 562. What changes take place when
oxygen unites with an inflammable body? To what is the quantity of
heat proportionate in combustion? Give an example. 563. How are carbon
and hydrogen supplied to the system? How the oxygen? Where does the
oxygen mingle with the blood?
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564. In the capillary vessels, the oxygen of the arterial blood unites
with the carbon and hydrogen which the refuse materials contain, and
carbonic acid and water are formed. The combustion of carbon and
hydrogen in the capillaries of every part of the system, (the lungs
not excepted,) is attended with a disengagement of heat, and the
carbonic acid and water are returned to the lungs in the dark-colored
blood, and evolved from the system.
565. Sir Benjamin Brodie and some others have maintained, that the
heat of the system is generated exclusively by the influence of the
brain and nerves. This theory is discarded by most physiologists; yet
it is true that the nervous system exercises a great influence over
the action of the capillary vessels in the process of nutrition,
secretion, and absorption. When these operations are most active, the
change among the particles of matter of which the body is composed, is
then greatest, and the generation of heat is increased in a
corresponding degree.
566. The necessity of pure, red blood in the production of animal
heat, is shown when the vessels that carry blood to a limb are
ligated, or tied; the part immediately becomes colder. The necessity
of nervous influence is seen in the diminished temperature of a
paralytic limb.
567. Our next inquiry is, By what means is the uniformity of
temperature in the body maintained? As there is a constant generation
of heat in the system, there would be an undue accumulation,--so much
so as to cause disagreeable sensations,--if there were no means by
which it could be evolved from the body, or its production lessened.
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564. Where does it unite with the carbon and hydrogen contained in the
body, and how is heat generated? 565. What was the theory of Sir
Benjamin Brodie? Is this theory in general discarded? What is true of
this theory? 566. How is the necessity of pure, red blood and nervous
action shown in the production of animal heat?
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568. It has been ascertained that the principal means by which the
system is kept at a uniform temperature, is the immense evaporation
from the skin and lungs. These membranes, in an ordinary state, are
constantly giving out water, which is converted into vapor, and
carried off by the surrounding air. The quantity of heat abstracted
from the system to effect this, depends on the rapidity of the change
of air, its temperature, and the amount of water it contains in a
state of vapor. The quantity removed is greatest when the air is warm
and dry, and the change, or current, rapid.
_Observations._ 1st. The first discovery of the use of free
evaporation of the perspiration from the skin in reducing the heat of
the body, and the analogy subsisting between this process and that of
the evaporation of water from a rough porous surface, so constantly
resorted to in warm countries, as an efficacious means of reducing the
temperature of the air in rooms, and of wine and other drinks, much
below that of the surrounding atmosphere, was made by Franklin.
2d. In all ages and climes, it has been observed that the increased
temperature of the skin and system in fevers, is abated as soon as
free perspiration is restored. In damp, close weather, as during the
sultry days of August, although the temperature is lower, we feel a
disagreeable sensation of heat, because the saturation of the air with
moisture lessens evaporation, and thus prevents the escape of heat
through the lungs and skin.
3d. It is on the principle of the evaporation of fluids that warm
vinegar and water, applied to the burning, aching head, cools it, and
imparts to it a comfortable feeling. The same results follow if warm
liquids are applied to the skin in the hot stage of fever; and this
evaporation can be increased by constant fanning.
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568. What are the principal means by which a uniform temperature of
the body is maintained? On what does the quantity of heat abstracted
from the system depend? What discovery relative to animal heat is due
to Franklin? What is said of free perspiration in fevers? What
occasions the disagreeable sensation of heat in damp, close weather?
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4th. It is frequently noticed, in very warm weather, that dogs and
other domestic animals are seen with their tongues out of their
mouths, and covered with frothy secretions. This is merely another
mode of reducing animal heat, as the skin of such animals does not
perspire as much as that of man.
569. Under some circumstances, a portion of the heat of the system is
removed by radiation. When cold air comes in contact with the skin and
mucous membrane of the lungs, heat is removed from the body, as from a
stove, to restore an equilibrium of temperature. The removal of heat
from the body is greatest when we are in a current of cold air, or
when a brisk, cold wind is blowing upon us.
570. As the primary object of the different processes of nutrition is
to supply animal heat, so the action of the different nutritive organs
is modified by the demands of the system for heat. When heat is
rapidly removed from the body, the functional activity of the organs
of nutrition is increased. When the system is warmed by foreign
influence, the activity of the nutritive organs is diminished. This
leads to the natural, and, we may add, instinctive change in the
quality and quantity of food at different seasons of the year.
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569. When is heat radiated from the body? When is it greatest? 570.
What is the primary object of the different processes of nutrition?
When is the activity of the nutritive organs increased? When
diminished? To what does this lead?
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