Of the temperature of living bodies—Temperature of plants—Power
of plants to resist cold and endure heat—Power of generating
heat—Temperature of animals—Warm-blooded and cold-blooded
animals—Temperature of the higher animals—Temperature of the different
parts of the animal body—Temperature of the human body—Power of
maintaining that temperature at a fixed point whether in intense
cold or intense heat—Experiments which prove that this power is a
vital power—Evidence that the power of generating heat is connected
with the function of respiration—Analogy between respiration and
combustion—Phenomena connected with the functions of the animal body,
which prove that its power of generating heat is proportionate to
the extent of its respiration—Theory of the production of animal
heat—Influence of the nervous system in maintaining and regulating the
process—Means by which cold is generated, and the temperature of the
body kept at its own natural standard during exposure to an elevated
temperature.
482. Closely connected with the function of respiration, is the power
which all living beings possess of resisting within a certain range
the influence of external temperature. The plant is warmer than the
surrounding air in winter, and colder in summer. A thermometer placed
at the bottom of a hole bored into the centre of a living tree,
precaution being taken to keep off as much as possible all external
influence either of heat or cold, does not rise and fall according to
the changes of external temperature; but rises when the external air
is cold, and falls when it is warm. Thus, in a cold day in spring, the
wind being north, at six o’clock in the evening, the temperature of
the external air being 47°, that of a tree was 55°. On another cold
day in the same month, there being snow and hail, and the wind in the
north-east, at six o’clock in the evening, the external temperature
being 39°, that of the tree was 45°. On the contrary, in one
experiment, when the temperature of the air was 57½°, that of the tree
was only 55°; and when the temperature of the air was 62°, that of the
tree was 56°.
483. These experiments afford an explanation of circumstances familiar
to common observation. Every one has noticed that the snow which
falls on grass and trees melts rapidly, while that on the adjoining
gravel walks often remains a long time unthawed. Moist dead sticks are
constantly found frozen hard in the same garden with tender growing
twigs, which are not in the least degree affected by the frost. Every
winter in our own climate tender herbaceous plants resist degrees of
cold which freeze large bodies of water.
484. But the colder, and the warmer the climate, the more strikingly
does the plant exemplify the power with which it is endowed of
resisting external temperature. In the northern parts of America the
temperature is often 50° below zero; yet, though exposed to this
intense degree of cold, the spruce fir, the birch, the juniper, &c.
preserve their vitality uninjured. From numerous experiments which
have been performed expressly with a view to ascertain this point, it
is found that a plant which has been once frozen is invariably dead
when thawed. It is also proved by direct experiment, that if the sap
be removed from its proper vessels, it freezes at 32°, the ordinary
freezing point. In the northern parts of America, then, the plant must
preserve in its living vessels its sap from freezing, when exposed to
a temperature of 50° below zero; which sap out of these vessels would
congeal at the ordinary freezing point; that is, the plant of this
climate is endowed with the power of resisting a degree of cold ranging
from the ordinary freezing point to 50° below zero; a property which
can be referred only to a vital power, by the operation of which the
plant generates within itself a degree of heat sufficient to counteract
the external cold.
485. The opposite faculty of resisting the influence of external heat
is exemplified by the trees and shrubs of tropical climates, often
surrounded by a temperature of 104°, which they resist just as the
plant of the northern clime resists the intense degrees of cold to
which it is exposed.
486. That the plant is endowed with the power of generating heat is
demonstrated by the phenomena which attend the performance of some of
its vital processes, such as those of germination and flowering. During
the germination of barley, the thermometer was observed to rise in the
course of one night to 102°. The bulb of a thermometer applied to the
surface of the spadix of an arum maculatum, indicated a temperature
7° higher than that of the external air; but in an arum cordifolium,
at the Isle of France, a thermometer placed in the centre of five
spadixes stood at 111°; and in the centre of twelve at 121°, though the
temperature of the external air was only 66°.
487. Animals indicate in a still more striking degree the power of
generating heat. The lower the animal in the scale of organization,
indeed, the nearer it approaches to the plant in the comparative
feebleness of this function. The heat of worms, insects, crustacea,
mollusca, fishes, and amphibia, is commonly only two or three degrees
above that of the medium in which they are immersed. Absolutely
colder than the higher animals, they are at the same time incapable
of resisting any considerable changes in the temperature of the
surrounding medium, whether from heat to cold or from cold to heat.
The higher animals, on the contrary, maintain their heat steadily at a
fixed point, or very nearly at a fixed point, however the temperature
of the surrounding medium may change. Hence animals are divided
into two great classes, the cold-blooded and the warm-blooded. The
temperature of the cold-blooded is lower than that of the warm-blooded,
and it varies with the heat of the surrounding medium; the temperature
of the warm-blooded is higher than that of the cold-blooded, and
it remains nearly at the same fixed point, however the heat of the
surrounding medium may change.
488. The temperature natural to the higher animals differs somewhat
according to their class. The temperature of the bird is the highest,
and is pretty uniformly about 103° or 104°; that of the mammiferous
quadruped is 100 or 101°; that of the human species is 97° or 98°.
489. The temperature of the animal body is not precisely the same in
every part of it. The ball of the thermometer introduced within the
rectum of the dog stood at 100½; within the substance of the liver at
100¾; within the right ventricle of the heart at 101°, and within the
cavity of the stomach at 101°. In the brain of the lamb it stood at
104°; in the rectum at 105°; in the right ventricle of the heart, and
in the substance of the liver and of the lungs, at 106°; and in the
left ventricle of the heart at 107°.
490. The temperature natural to the human body is 98°. When the human
body is surrounded by an atmosphere at the temperature of 30°, it
must have its heat rapidly extracted by the cold medium; yet the
temperature of the body, however long it remain exposed to such a
degree of cold, does not sink, but keeps steadily at its own standard.
But animals which inhabit the polar regions are often exposed to a cold
40° below zero. The temperature of Melville Island is so low during
five months of the year that mercury congeals, and the temperature is
sometimes 46° below zero; yet the musk oxen, the rein deer, the white
hares, the polar foxes, and the white bears which abound in it maintain
their temperature steadily at their own natural standard.
491. The power which the higher animal possesses of resisting heat
is still more remarkable than its power of resisting cold. On taking
rabbits and guinea-pigs from the temperature of 50°, and introducing
them very rapidly to the temperature of 90°; it was found that the
animals acquired only two or three degrees of heat. How different
the result when the cold-blooded animal is subjected to the same
experiment! The temperature of the surrounding air being 45°, a
thermometer introduced into the stomach of a frog rose to 49°. The
frog being then put into an atmosphere made warm by heated water, and
allowed to stay there twenty minutes, the thermometer on being now
introduced into the stomach rose to 64°.
492. But the human body may be actually placed in a temperature of 60°
above that of boiling water, not only without sustaining the slightest
injury, but without having its own temperature raised excepting by two
or three degrees. The attention of physiologists was first directed
to this curious fact by some remarkable circumstances related by the
servants of a baker at Rochefoucault, who were in the habit of going
into the heated ovens in order to prepare them for the reception of
the loaves. In performing this service, the young women were sometimes
exposed to a temperature as high as 278°. It was stated that they could
endure this intense heat for twelve minutes, without any material
inconvenience, provided they were careful not to touch the surface
of the oven. Subsequently Drs. Fordyce, Blagden, and others, with a
view to ascertain the exact facts, entered a chamber, heated to a
temperature much above that of boiling water, and some of the phenomena
observed during these experiments are highly curious.
493. In the first room entered by these experimentalists, the highest
thermometer varied from 132° to 130°; the lowest stood at 119°. Dr.
Fordyce having undressed in an adjoining cold chamber, went into the
heat of 119°; in half a minute the water poured down in streams over
his whole body, so as to keep that part of the floor where he stood
constantly wet. Having remained here fifteen minutes, he went into the
heat of 130°; at this time the heat of his body was 100°, and his pulse
beat 126 times in a minute. While Dr. Fordyce stood in this situation a
Florence flask was brought in by his order, filled with water heated
to 100°, and a dry cloth with which he wiped the surface of the flask
quite dry; but it immediately became wet again, and streams of water
poured down its sides, which continued till the heat of the water
within had risen to 122°, when Dr. Fordyce went out of the room, after
having remained fifteen minutes in a heat of 130°: just before he left
the room his pulse made 129 beats in a minute; but the heat under his
tongue and in his hand did not exceed 100°.
494. In a subsequent experiment the chamber was entered when the
thermometer stood above 211°. The air heated to this degree, says
Dr. Blagden, felt unpleasantly hot; but was very bearable. Our most
uneasy feeling was a sense of scorching in the face and legs; our legs
particularly suffered very much, by being exposed more fully than any
other part to the body of the stove, heated red hot by the fire within.
Our respiration was not at all affected; it became neither quick nor
laborious; the only difference was a want of that refreshing sensation
which accompanies a full inspiration of cool air. But the most striking
effects proceeded from our power of preserving our natural temperature.
Being now in a situation in which our bodies bore a very different
relation to the surrounding atmosphere from that to which we had been
accustomed, every moment presented a new phenomenon. Whenever we
breathed on a thermometer, the quicksilver sank several degrees. Every
expiration, particularly if made with any degree of violence, gave a
very pleasant impression of coolness to our nostrils, scorched before
by the hot air rushing against them whenever we inspired. In the same
manner our now cold breath agreeably cooled our fingers whenever it
reached them. Upon touching my side, it felt cold like a corpse; and
yet the actual heat of my body, tried under my tongue, and by applying
closely the thermometer to my skin, was 98°, about a degree higher than
its ordinary temperature. When the heat of the air began to approach
the highest degree which this apparatus was capable of producing, our
bodies in the room prevented it from rising any higher; and when it
had been previously raised above that point, invariably sunk it. Every
experiment furnished proofs of this. Mr. Banks and Dr. Solander each
found that his single body was sufficient to sink the quicksilver very
fast, when the room was brought nearly to its maximum of heat.
495. In a third series of experiments the temperature of the chamber
was raised to the 260th degree. At this time, continues Dr. Blagden,
I went into the room, with the addition to my common clothes of a
pair of thick worsted stockings drawn over my shoes, and reaching
some way above my knees. I also put on a pair of gloves, and held a
cloth constantly between my face and the stove (necessary precautions
against the scorching of the red-hot iron). I remained eight minutes in
this situation, frequently walking about to all the different parts of
the room, but standing still most of the time in the coolest spot near
the lowest thermometer. The air felt very hot, but by no means so as to
give pain. I had no doubt of being able to bear a much greater heat;
and all who went into the room were of the same opinion. I sweated, but
not very profusely. For seven minutes my breathing remained perfectly
good; but after that time, I began to feel an oppression in my lungs,
attended with a sense of anxiety; which gradually increasing for the
space of a minute, I thought it most prudent to end the experiment.
My pulse, counted as soon as I came into the cool air, for the uneasy
feeling rendered me incapable of examining it in the room, beat at
the rate of 144 pulsations in a minute, which is more than double its
ordinary quickness. In the course of this experiment, and others of
the same kind by several of the gentlemen present, some circumstances
occurred to us which had not been remarked before. The heat, as might
have been expected, felt most intense when we were in motion; and on
the same principle, a blast of the heated air from a pair of bellows
was scarcely to be borne: the sensation in both these cases exactly
resembled that felt in our nostrils on inspiration. It was observed
that our breath did not feel cool to our fingers unless held very
near the mouth; at a distance the cooling power of the breath did
not sufficiently compensate the effect of putting the air in motion,
especially when we breathed with force.
496. On going undressed into the room, the impression of the air was
much more disagreeable than before; but in five or six minutes, a
profuse sweat broke out, which instantly relieved me. During all the
experiments of this day, whenever I tried the heat of my body, the
thermometer always came very nearly to the same point (the ordinary
standard), not even one degree of difference, as in our former
experiments.
497. To prove that there was no fallacy in the degree of heat shown
by the thermometer, but that the air which we breathed was capable of
producing all the well-known effects of such heat on inanimate matter,
we put some eggs and a beef steak upon a tin frame, placed near the
standard thermometer, and farther distant from the stove than the wall.
In about thirty minutes the eggs were taken out roasted quite hard. In
about forty-seven minutes the steak was not only dressed, but almost
dry. Another beef steak was rather overdone in thirty-three minutes. In
the evening when the heat was still greater, we blew upon a third steak
with the bellows, which produced a visible change on its surface, and
hastened its dressing; the greatest part of it was pretty well done in
thirteen minutes.
498. The human body, then, may be exposed to a temperature 50° below
zero, without having its own heat appreciably diminished; it may be
exposed to a temperature 60° above that of boiling water, without
having its own heat increased beyond two or three degrees; or, as
appears from experiments subsequently performed expressly to ascertain
this point, from three to five degrees. In the former case, the body
must generate a degree of heat sufficient to compensate the great
quantity of caloric which is every moment abstracted from it by the
intensely-cold surrounding medium. In the latter case it must generate
a degree of cold sufficient to counteract the great quantity of
caloric which is every moment communicated to it by the intensely-hot
surrounding medium.
499. Powers so wonderful and so opposite appeared to the physiologists
of former times to be involved in such profound mystery, that they
did not even attempt to investigate their nature, or trace their mode
of operation; but satisfied themselves with referring them to some
innate quality of the body, and with considering them as essential
attributes of life. And difficulties connected with the subject still
remain, which the present state of knowledge does not permit us wholly
to surmount; but we are able at least to refer these powers to their
proper seat, and to trace some steps of the processes by which they
produce results so wonderful and beautiful.
500. It is certain that whatever be the ultimate physical processes by
which the generation of heat and the production of cold are effected
in the animal body, the phenomena are dependent on the condition of
life. No such phenomena take place excepting in living bodies. This is
illustrated in a striking manner by a series of experiments performed
by Mr. Hunter. A part of the living human body was immersed in water
gradually made warmer and warmer from 100° to 118°; precisely the same
part of the body, dead, was immersed in the same water, and both parts,
the living and the dead, were continued in this heat for some minutes.
The dead part raised the thermometer to 114°; the living part raised it
to no higher than 102¼°. On applying the thermometer to the sides of
the living part, the quicksilver immediately fell from 118° to 104°;
on applying it close to the dead part, the thermometer did not fall
above a single degree; the living part actually produced a cold space
of water around it. Hence in bathing in water, whether colder or warmer
than the heat of the body, the water soon acquires the same temperature
with that of the body; and, consequently, in a large bath the patient
should move from place to place, and in a small one there should be a
constant succession of water of the intended heat.
501. A fresh, that is, a living egg was put into cold water at about
zero, frozen, and then allowed to thaw. By this process its vitality
was destroyed, and consequently its power of resisting cold and heat
lost. This thawed egg was next put into a cold mixture with an egg
newly laid: the time required for freezing the fresh egg was seven
minutes and a half longer than that required for freezing the thawed
egg.
502. A new-laid egg was put into a cold atmosphere fluctuating between
17° and 15°; it took about half an hour to freeze; but when thawed and
put into an atmosphere at 25° (10° warmer), it froze in half the time.
503. A fresh egg and one that had been frozen and thawed were put into
a cold mixture at 15°; the thawed one soon came to 32°, and began
to swell and congeal; the fresh one sunk to 29½, and in twenty-five
minutes after the dead one, it rose to 32°, and began to swell and
freeze.
504. The result of this experiment upon the fresh egg was similar to
that of analogous experiments made upon the frog, eel, snail, &c. where
life allowed the heat to be diminished 2° or 3° below the freezing
point, and then resisted all further decrease; but the powers of life
having been expended by this exertion, the parts then froze like any
other dead animal matter.
505. The heat of the bird is increased somewhat when it is prepared
for incubation. Some eggs were taken from under a sitting hen whose
temperature was 104°, at the time when the chick was about three-parts
formed. A hole was broken in the shell and the bulb of a thermometer
introduced; the quicksilver rose to 99½°; but in some eggs that were
addled it was proved that their heat was not so high by two degrees, so
that the life of the living egg assisted to support its own temperature.
506. These facts sufficiently show the dependence of the faculty of
generating heat and of producing cold on the powers of life. But the
processes by which, under the agency and control of the vital powers,
these different results are effected, are various, and even opposite.
507. The power of generating heat is connected in the closest manner
with the function of respiration, and is directly dependent upon it.
The evidence of this is indubitable. For—
508. i. Respiration is combustion, and, like ordinary combustion, is
attended with the production of heat. In ordinary combustion oxygen
disappears, and a new compound is formed, consisting of oxygen combined
with the combustible matter; that is, an oxidized body is generated. On
burning a piece of iron wire in oxygen, the oxygen disappears, and the
iron increases in weight. The oxygen combines with the iron, forming
a new product, oxide of iron, and the weight of this new substance
is found on examination to be exactly equal to the weight of the
wire originally employed, added to the quantity of oxygen which has
disappeared.
509. It is precisely the same in respiration. In this process oxygen
combines with combustible matter, carbon: the oxygen disappears, and a
new body, carbonic acid, is generated.
510. ii. One phenomenon which invariably accompanies the combination of
oxygen with combustible matter is the extrication of heat. Whenever a
substance passes from a rarer into a denser state; when, for example, a
gas is converted into a liquid or solid, or when a liquid solidifies,
heat is evolved; because, according to the ordinary theory of
combustion, the denser substance has a less capacity for caloric than
the rarer, and consequently in passing from a rare into a dense state,
a quantity of caloric previously combined or latent within it is set
free. The combined or latent caloric contained in a body is termed its
specific caloric; the caloric which is evolved on its change of state
is named free or sensible caloric.
511. The combination of oxygen with carbon, as in the combination
of oxygen with combustible matter in every other instance, must
be attended with the evolution of heat. Though the product of the
combustion, in the present case, be a gaseous body, carbonic acid,
still, according to the ordinary theory of combustion, carbonic
acid has less specific caloric, or less capacity for caloric, than
oxygen; and therefore in combining with carbon, a portion of its
specific caloric becomes free or sensible, that is, heat is evolved.
But whatever theory of combustion be adopted, the fact is certain,
that whenever oxygen combines with carbon to form carbonic acid,
heat is evolved; not only in the rapid union which takes place in
ordinary combustion, but also in the slow combination which occurs in
fermentation, putrefaction, and germination; in the latter of which
processes, as in the malting of barley, the temperature rises as high
as 10°. The union of oxygen with carbon in the lungs during respiration
must therefore necessarily produce heat, just as it does in a charcoal
fire, or in any other natural process in which this combination takes
place.
512. iii. Numerous phenomena connected with the animal body show that
its temperature is in strict proportion to the quantity of oxygen which
is consumed in respiration, and to the quantity of carbonic acid which
is formed by the union of oxygen and carbon during the process.
513. In all animals whose respiratory organs are so constructed, that
the consumption of oxygen and the consequent generation of carbonic
acid is minute in quantity, the production of heat is proportionably
small. It has been shown (337 _et seq._), that in almost the entire
class of the invertebrata, the respiratory apparatus is comparatively
minute and imperfect; accordingly, in these animals the power of
generating heat is at the minimum. In the fish, though the respiratory
apparatus be large, and though all the blood of the body circulate
through it (345 _et seq._), yet only a small quantity of air is brought
into contact with the respiratory organ, merely the air contained in
water. In the reptile, though it possess a true and proper lung, and
respire air, yet only one half of the blood of its body circulates
through the comparatively small, imperfectly divided, and simply
constructed air bag, which constitutes its respiratory organ (354).
Hence, the striking contrast exhibited between the temperature of these
cold-blooded creatures and that of the mammiferous quadruped, whose
lung, comparatively large, and composed of innumerable minute and
closely-set air vesicles (fig. CXXXIV. and CXXXV.), presents to the air
an immense extent of surface (370), and the whole mass of whose blood
incessantly traversing this surface, comes at every point into contact
with the air (399).
514. In the various tribes of warm-blooded animals, the elevation
and uniformity of the temperature is strictly proportionate to the
comparative magnitude of the lungs; to the complexity of their
structure; to the minuteness and number of the air vesicles; and,
consequently, to the quantity of oxygen consumed, and of carbonic acid
generated.
515. In all animals with red blood there is a strict relation between
the temperature of the body and the lightness or depth of the colour
of the blood; invariably the deeper the colour, the higher the
temperature. Thus, the blood of the fish and of the reptile is of a
light, and that of the bird of an intense red colour. It has been shown
(229) that the lightness or deepness of the colour of the blood depends
on the quantity of red particles which it contains, and the chemical
action between the air and the blood is carried on chiefly through the
medium of the red particles.
516. Even in the same animal, the temperature differs at different
times, according to the energy with which the process of respiration
is carried on. When the circulation of the blood is sluggish and the
respiration slow and feeble, the quantity of oxygen consumed is small,
and the temperature low; when, on the contrary, the circulation is
rapid, and the respiration energetic, the quantity of oxygen consumed
is large, and the temperature proportionably high. Whatever diminishes
the quantity of air that flows to the lungs, and the quantity of blood
that circulates through them, diminishes the temperature. Malformation
of the heart, in consequence of which a quantity of blood is sent to
the system without passing through the lungs, as in the individuals
termed Ceruleans: disease of the lungs, by which the access of air
to the air vesicles is obstructed, as in asthma, are morbid states
invariably attended with a diminution of the temperature.
517. When a warm-blooded animal is placed in an elevated temperature,
its consumption of oxygen is comparatively small; when it is placed
in a cold atmosphere, and the production of a large quantity of heat
is necessary to maintain its temperature at its natural standard, its
consumption of oxygen is proportionably large; accordingly, it is
established by direct experiment that the same animal consumes a much
larger quantity of oxygen in winter than in summer.
518. Due allowance being made for the difference in their bulk, young
animals consume less oxygen than adults; and they have a less power of
generating heat. Different species of young animals differ from each
other in their power of generating heat, and the closest relation is
observable between the difference in their power of consuming oxygen
and that of generating heat. Puppies and kittens require so small
a quantity of oxygen for supporting life, that they may be wholly
deprived of this gas for twenty minutes, without material injury, while
adult animals of the same species perish when deprived of it only for
four minutes. As long as these young creatures retain the power of
sustaining life for so protracted a period without oxygen, they are
wholly incapable of maintaining their own temperature; on free exposure
to air, even in summer, the heat of their body sinks rapidly, and if
this exposure be continued long, they perish of cold. In like manner,
young sparrows and other birds which are naked when hatched, consume
little oxygen, and are incapable of maintaining their temperature; but
can support life when deprived of oxygen much longer than adult birds
of the same species; while young partridges which are able to retain
their own temperature at the period of quitting the shell, die when
deprived of oxygen as rapidly as the adult bird.
519. The state of hybernation illustrates in the same striking manner
the relation between respiration and the generation of heat. One of the
most remarkable phenomena connected with this curious state, is the
reduction, sometimes even the apparent suspension, of respiration; and
in all cases of hybernation, the respiratory function is performed in
a feeble manner, and only at distant intervals. Exactly in proportion
to the diminution of the respiration, is the reduction of the power of
generating heat; so that when the state of hybernation is established,
the temperature of the external parts of the body sinks nearly to that
of the surrounding medium; while the internal parts, the blood, and
the vital organs are only a degree or two higher. In experiments made
to reduce an hybernating animal to a torpid state by cold artificially
produced, De Saissy found that he could not bring on the state of
hybernation by the reduction of temperature alone, without also
constraining the respiration.
520. These and other analogous facts abundantly establish the relation
between the function of respiration and that of calorification, and
lead to the general conclusion that the generation of animal heat is
in the direct ratio of the quantity of air and blood which are brought
into contact, and which act on each other in a given time. Yet an
attempt has recently been made by an ingenious physiologist[3] to
disturb this induction, and to show that the production of animal heat
is not in the direct ratio of the quantity of oxygen inhaled, but in
the inverse ratio of the quantity of blood exposed to this principle.
This position is maintained on the following grounds:—
[3] An Experimental Inquiry into the Laws which regulate the Phenomena
of Organic and Animal Life. By George Calvert Holland, M.D. and more
complete than the expirations; it is a state of continual sighing. In
like manner, in certain diseases, such as asthma, the inspirations
greatly preponderate both in frequency and energy over the
expirations. In such conditions of the system the blood accumulates
in preternatural quantity in all the internal organs; but more
especially in the lungs; and two consequences follow: first, there
is a remarkable diminution in the energy of all the vital actions;
and secondly there is a proportionate diminution in the production of
animal heat.
521. Inspiration favours the flow of blood to the lungs; expiration
retards it: consequently, if from any causes the inspirations
preponderate in number and proportion over the expirations, a greater
quantity of blood than usual will be accumulated in the lungs. There
are conditions of the system in which this preponderance of the
inspirations actually takes place; when the mind is under the influence
of certain emotions, for example, as when it is depressed by anxiety
and fear. In this state the inspirations are more frequent
522. On the contrary, as it is the effect of inspiration to facilitate
the motion of the blood through the lungs, so it is the effect of
expiration to retard it; hence, when the expirations preponderate the
opposite state of the system is induced; all the vital actions are
performed with increased energy; the heart beats with unusual vigor;
the pulse becomes quick and strong; a larger quantity of blood is
determined to the surface of the body, and this excited state of the
system is always attended with an augmentation of the temperature.
523. As in the first state there is a greater and in the second a
smaller quantity of blood than natural contained in the lungs, the
inference deduced by Dr. Holland is, that the production of animal heat
is in the inverse ratio of the quantity of blood exposed to oxygen. But
this inference is neither logical nor sound.
524. If, as a comparison of all the phenomena of respiration exhibited
throughout the entire range of the animal kingdom, shows the production
of animal heat to be in the direct ratio of the quantities of air and
blood which are brought into contact, and which re-act on each other,
every phenomenon of respiration must be in harmony with this law, and,
accordingly, when really understood, it is found to be so.
525. Inspiration, by the dilatation of the thorax, and consequently of
the lungs incident to that action, is favorable to the flow of blood to
the lungs. But it is only a certain degree of dilatation of the lungs
that is favorable to the flow of blood through them (407 _et seq._).
If the dilatation be carried beyond a certain point, the quantity of
blood transmitted through the pulmonary tissue is diminished (406);
if the dilatation be carried farther, the transmission of the blood
may be wholly stopped (417). The quantity of the blood which flows to
the lungs, and the quantity which circulates through them, are not
then identical. So large a quantity may flow to them as to impede or
retard or wholly stop the pulmonary circulation. In proportion to the
accumulation of blood in the lung must necessarily be the distension of
the pulmonary tissue; in that proportion the lung must be approximated
to its condition in the experiment in which it was distended with water
(417), when it did not transmit a single particle of blood. Further,
in proportion to the preternatural distension of the pulmonary tissue
with blood must be the exclusion of air from the air vesicles for the
lungs can contain only a certain quantity of blood and air (418.3), so
that the blood can preponderate only by the exclusion of the air.
526. In those states of the system, then, in which the preponderance
of the inspirations induces a preternatural accumulation of blood in
the lungs, the production of animal heat is diminished for a two-fold
reason; first, because the distension of the pulmonary tissue with
blood retards the pulmonary circulation, and proportionally lessens
the quantity of blood which is brought into contact with the air;
and, secondly, because the distended blood-vessels compress the air
vesicles, and so diminish the quantity of air which is brought into
contact with the blood.
527. It follows that the diminution of temperature which takes place in
this condition of the system is not because the production of animal
heat is in the inverse ratio of the quantity of blood which is exposed
to oxygen; but because from a two-fold operation there is a diminution
of the quantity of blood and of oxygen which are brought into contact.
528. The reason is equally obvious why there is an increase of
the temperature in those conditions of the system in which the
expirations preponderate over the inspirations. Expiration, it is
true, somewhat retards the circulation of the blood through the lungs,
but the preponderance of this respiratory action does not raise the
temperature by the retardation of the flow of blood through the lungs,
and the consequent diminution of the quantity transmitted in a given
time; for though expiration somewhat retards the circulation of the
blood through the branches of the pulmonary artery, it promotes its
circulation through the branches of the pulmonary veins (fig. CXL. 10).
It is indeed by the action of expiration that the aërated blood is
transmitted from the lungs to the left heart to be sent out renovated
to the system. Expiration has no influence whatever over the aëration
of the blood. Before the action of expiration takes place, the blood
is already aërated. The office of expiration is to remove from the
system the air which has served for respiration, and to transmit to the
system the blood which has been subjected to respiration. Consequently,
in those states of the system in which the expirations preponderate,
the temperature is increased, not because the expiratory actions, by
lessening the quantity of blood in the lungs, diminish the quantity
exposed to oxygen, but because they transmit to the system oxygenated
blood as rapidly as it is formed, that is, blood which either produces
animal heat in the act of its formation, or which generates it as it
flows through the system.
529. These conditions establish the conclusion deduced, as has
been stated, from the comparison of the phenomena of respiration
exhibited throughout the entire range of the animal kingdom. But if
the production of animal heat be really the result of combustion,
if that combustion take place in the lung, and if the lung be thus
the focus whence the heat radiates to every other part of the body,
why is not the heat of this organ and of the parts in its immediate
neighbourhood higher than the temperature of the rest of the body?
Some of the internal organs are indeed a degree or two hotter than the
general mass of the circulating blood (469), and among these the lung
is admitted to rank perhaps the very highest. But how can a quantity of
caloric sufficient to maintain the heat of the body in a temperature
of forty degrees below zero radiate from an organ the temperature of
which is only two or three degrees above that of the body itself? It
is estimated that, in every minute, during the calm respiration of a
healthy man of ordinary stature, 26·6 cubic inches of carbonic acid,
at the temperature of 50° Fahr. are emitted, and that an equal volume
of oxygen is withdrawn from the atmosphere. From these data it is
calculated that, in an interval of twenty-four hours, not less than
eleven ounces of carbon are consumed. Why is the lung, the seat of this
combustion, not only not greatly warmer than any other organ; but why
is it not even consumed by the fire which is thus incessantly burning
within it?
530. It has been shown (468 and 469) that when the carbon of the
blood unites in the lung with the oxygen of the air, the nature of
the blood, in consequence of the abstraction of carbon, undergoes an
essential change, passing from venous into arterial. By an elaborate
series of experiments, conducted with extraordinary care and skill,
it would appear that arterial has a greater capacity for caloric than
venous blood, in the proportion of 114·5 to 100. In consequence of this
difference in the constitution of the two kinds of blood, the heat
generated in the lung by the combustion of carbon, instead of being
evolved or becoming sensible (510. ii.), and so raising the temperature
of the organ, goes to satisfy the increased capacity for caloric of
arterial blood, is spent, not in rendering the fluid sensibly warmer,
but in augmenting its specific caloric (510. ii.). Arterial blood is
not increased in temperature,[4] but with its absolute quantity of
caloric augmented, flows from the lung to the left heart (fig. CXL.
10), and thence to the system (fig. CXL. 6). In the system, in every
organ, at every point of the component tissue of every organ and at
every moment of time, the blood repasses from the arterial to the
venous state: by this transition its capacity for heat is diminished;
the venous cannot retain in it the same quantity of caloric as the
arterial blood, consequently a portion of caloric is extricated; that
which was latent becomes sensible, and caloric being set free the
temperature is raised. In this process the lung is not burnt, it is
only rendered just sensibly warmer than any other part of the body,
though it be the organ by which the whole mass of blood receives its
caloric, because it is only in the capillary part of the systemic
circulation, when the arterial blood again passes into the venous
state, that the caloric acquired is liberated. In this manner, gently,
steadily, uninterruptedly, an abundant, unceasing, and equable current
of heat is distributed to every part and particle of the system.
[4] It is not a perfectly accurate statement that the temperature of
venous and arterial blood is precisely the same. The latest and best
experiments concur in showing that arterial blood, at least in the
heart and the great arterial trunks, is one or two degrees warmer than
venous blood. The weight of evidence from experiment is also in favour
of the opinion, that the different parts of the body are _somewhat_
less warm as they recede from the lungs and heart; but the difference
is so slight that it may be disregarded in the general argument.
531. Such is the celebrated theory of animal heat suggested by Dr.
Crawford, of which it has been justly said, that it affords one of the
most beautiful specimens of the application of physical and chemical
reasoning to the animal economy that has ever been presented to the
world.
532. The main position on which this theory rests—that arterial
possesses a greater capacity for caloric than venous blood—professes
to be founded on experiments which, though of a delicate and complex
nature, are nevertheless uniform and decisive in their results.
In consequence of their extreme interest and importance, these
experiments have been subjected, by different physiologists, to rigid
examination, with a somewhat conflicting result. The greater number
of experimentalists maintain that Crawford’s experiments are correct
in all the essential points, and that the objections which have been
urged against them do not really affect them; while others are of
opinion that, even although it must, upon the whole, be admitted that
the specific heat of arterial is greater than that of venous blood;
yet that the excess is so small as to be inadequate to account for
the effects attributed to it. Dr. Davy’s experiments, which of all
that have been instituted are generally conceived to be the most
unfavourable to the theory of Crawford, do not afford uniform results.
Three experiments out of four indicate a greater capacity in arterial
than in venous blood; in those in which the experimentalist himself
places the most confidence, in the relative proportion of 913 to 903;
while, according to Crawford, the relative proportion is 114·5 to 100.
533. But when this subject is closely considered, the discrepancy
in question turns out to be of no real consequence. There is a
modification of the theory, which removes every difficulty, and
dispenses with the necessity of any regard whatever to the point in
dispute.
534. It has been shown (444 _et seq._), that during the process of
respiration more oxygen disappears than is accounted for by the
carbonic acid that is generated; that this excess of oxygen is absorbed
by the blood; and that in the lung the oxygen merely enters into a
state of loose combination with the blood, the union being intimate
and complete only in the system. The complete chemical combination
of the oxygen with the carbon takes place, then, not in the lungs,
but in the capillary arteries of the system; consequently it is only
while flowing in capillary arteries that carbonic acid is formed; that
is, it is only in these vessels that the arterial combustion takes
place: of course, therefore, it is only in these vessels that heat
is extricated, and only from them that it can be communicated to the
adjacent parts. According to this view, wherever there is a capillary
artery, the combustion of carbon incessantly goes on, and there caloric
is as incessantly set free; but since there is not a point of any
tissue, in which there are not capillary arteries, there is not a point
from which caloric does not radiate. As soon as formed, carbonic acid
passes from the capillary arteries into the capillary veins; by the
veins it is transmitted to the lungs; and by the lungs it is expelled
from the system. The real operations carried on in the lungs, then,
are the transmission of oxygen and the extrication of carbonic acid;
but this organ is not the seat of the essential and ultimate part
of the function; it is merely the portal through which the elements
employed in the process have their entrance and exit. Thus the question
concerning the greater capacity of arterial blood for caloric is of
no importance whatever: the phenomena may be equally accounted for,
whatever be, in this respect, the constitution of the blood.
535. The result of the whole is, the complete establishment of the
fact, that the production of heat in the animal body is a chemical
operation, dependent on the combination of oxygen with carbon in the
capillary arteries of the system; that is, it is the result of the
burning of charcoal at every point of the body.
536. The agent which maintains and regulates this internal fire is the
nervous system. There is, indeed, reason to suppose that the nervous
system, in some mode or other, contributes to the actual production
of animal heat. It is established by direct experiment, that the
quantity of carbonic acid formed in the system is inadequate to the
supply of the caloric expended by it; that in a given time more heat
is abstracted from the body by the surrounding medium, than can be
accounted for by the consumption of the amount of carbonic acid thrown
off by the lungs during the same interval. There is evidence that the
source of this additional heat is the nervous system.
537. The influence exerted by the nervous system over the production
of animal heat, is demonstrated by the fact, established by numerous
observations and experiments, that whatever weakens the nervous power,
proportionally diminishes the capacity of producing heat. For,
1. The destruction of a portion of the spinal cord diminishes the
temperature of an animal without, as far as is ascertained, the
disturbance of any other function.
2. The privation of the heart and blood-vessels of the nervous
influence, as by decapitation, though the passage of the blood through
the lungs and its ordinary change from the venous to the arterial state
be maintained by artificial respiration, greatly diminishes, if it do
not altogether suspend, the generation of animal heat.
3. The abolition of sensibility by the administration of a narcotic
poison, artificial respiration being maintained, as effectually
disturbs the generation of animal heat as decapitation; while the power
of generating heat is restored, in the exact proportion to the return
of the sensibility by the cessation of the action of the poison.
4. The temperature of an organ is found, by direct experiment, to be
diminished by the division of the nerves that supply it with nervous
influence. The nerves that supply the horn were divided on one side
of the body in a young deer; the other horn was left entire. The
temperature of the horn—the nerves of which had been divided—was
found, after some hours, to be considerably diminished, and it
continued diminished for several days; at length its temperature was
restored. On examining the horn about ten days after the operation
had been performed, the divided nerves were found to be connected by
a newly-formed substance; thus apparently accounting for the loss of
temperature in the first instance, and for its subsequent restoration.
538. But although these and other analogous facts prove, beyond all
question, the important influence of the nervous system over the
development of animal heat, yet the mode in which that influence
operates is not ascertained. Its action may be either direct or
indirect. The nerves may possess some specific power of generating
heat,—extricating it immediately from the blood by a process analogous
to secretion,—or they may evolve it indirectly by other operations, as
by some of the processes of nutrition. Each hypothesis is maintained
by able physiologists; but the balance of evidence (as will appear
hereafter) is greatly in favour of the opinion that the influence
of the nervous system over this process is altogether indirect. A
beautiful illustration of this is afforded in the following operation,
which is going on, without ceasing, every instant during life.
539. The skin which forms the external covering of the body is composed
essentially of gelatin. No gelatin is contained in the blood; but the
albumen of the blood is capable of being converted into gelatin by the
addition of oxygen. Albumen is received by the capillary artery of the
skin; the blood, of which albumen forms so important a constituent,
contains a quantity of oxygen which it receives at the moment of
inspiration, and which it retains in a state of loose combination
(470 _et seq._). Under the influence probably of the organic nerve,
the capillary artery chemically combines a portion of the free oxygen
with the albumen of the blood, and gelatin is the result. In this
process the albumen gives off carbon; the blood affords oxygen; the
two elements unite; carbonic acid is formed; and, as in every other
instance in which carbonic acid is formed, heat is evolved. In this
manner a fire is kindled, and is kept constantly burning, where it
is most needed to counteract the influence of external cold, at the
external surface of the body.
540. Such are the main points which have been established in relation
to the production and distribution of animal heat. But it has been
shown that the living body is capable of bearing without injury a
temperature by which it is rapidly consumed when deprived of life. By
what means does the vital power enable the body to resist the influence
of such intense degrees of heat?
541. Two circumstances are observable when the body is placed in a
temperature greatly higher than its own. First, it can endure such
a temperature only in the medium of air. Air can easily be borne at
the temperature of 260°; aqueous vapour at the temperature of 130°
few Europeans are capable of enduring longer than twelve minutes; the
peasants of Finland appear to be able to sustain it, for the space of
half an hour, as high as 167°; but the hottest liquid water-bath which
any one seems to have been able to bear for the space of ten minutes,
is the hottest spring at Barêges, the temperature of which is 113°.
But in heated air the quantity of heat in actual contact with the body
is much less than in the other media; because in proportion as the
air is heated it is expanded, and in proportion as it is expanded the
particles are diminished that come into contact with the body.
542. In the second place, the afflux of the colder fluids from the
central parts of the system to the surface may for a time exert some
influence in keeping down the temperature of the body. But above all
this, in the third place, a two-fold provision is made in the body
itself for the reduction of its temperature when exposed to intense
degrees of heat; by the one, the power with which it is endowed
of producing heat is diminished; by the other, cold is positively
generated.
543. It has been shown (517) that in proportion to the elevation
of the temperature to which the body is exposed the blood becomes
less venalized, and in the proportion in which the blood retains its
arterial character the consumption of oxygen is diminished. Venous
blood contains an excess of carbon, arterial blood an excess of oxygen.
Consequently in proportion as the blood retains its arterial character
it affords less carbon for the combination of oxygen, that is less
inflammable matter. At an elevated temperature therefore there must, of
necessity, be a diminished production of heat within the body, since
the blood contains a diminished quantity of combustible material.
544. Moreover, in proportion to the elevation of the temperature to
which the body is exposed, evaporation takes place from the entire
surface of the pulmonary vesicles. No experiments have been performed
which enable the physiologist to ascertain precisely the quantity of
vapour exhaled from the lungs in a given time, when the body is exposed
to a given degree of heat; but both observation and experiment show
that it is very great. The blood pours out upon the whole surface of
the air vesicles a quantity of moisture in the form of water: by the
surrounding air this water is converted into vapour: by the conversion
of a fluid from the state of a liquid into that of vapour caloric is
absorbed: by the absorption of caloric cold is generated, and that to
such a degree that fluids exposed to the influence of evaporation may
be frozen in the intensest heat of summer. The very process by which
art, aided by science, affords to the inhabitants of warm climates the
luxury of ice, is that by which nature generates cold in the human
lungs when the body is exposed to a temperature above its own. Not
only, then, is the lung the instrument by which the body acquires the
power of evolving heat in greater or less quantity in proportion to
the demands of the system, but this very same organ, under a change
of circumstances, produces the directly contrary effect, and actually
generates cold.
545. In the process of producing cold the skin is a powerful auxiliary
to the lungs. More fluid is, indeed, evaporated from the surface of the
skin in the form of perspiration, than from the lungs in the form of
vapour; the cutaneous, like the pulmonary evaporation, increases in the
ratio of the temperature, and both co-operate in abstracting the excess
of caloric.
546. Finally, in proportion to the elevation of the temperature is
the acceleration of the circulation; the pulse is augmented in power,
and doubled or trebled in frequency (495); but in proportion to
the rapidity of the circulation is the increase of the quantity of
evaporable matter which is transmitted to the evaporating surfaces.
547. From the whole it appears that by the combination of carbon and
oxygen provision is made for the production of the greatest quantity
of caloric that can at any time be required for the wants of the
system; that when a decreased evolution of heat is necessary a smaller
quantity of carbon and oxygen is brought into union, and that when,
from exposure to intense degrees of heat, it is requisite for the
maintenance of the temperature of the body at its own standard, that
it should actually generate cold, it accomplishes this object by the
evaporation of water.