constant supply of certain matters, animal food, and of oxygen, in
the shape of atmospheric air. During every moment of life, oxygen is
absorbed from the atmosphere in the organs of respiration, and the
act of breathing cannot cease while life continues.
The observations of physiologists have demonstrated that the body of
an adult man supplied abundantly with food, neither increases nor
diminishes in weight during twenty-four hours, and yet the quantity
of oxygen absorbed into his system, in that period, is very
considerable. According to the experiments of Lavoisier, an adult
man takes into his system from the atmosphere, in one year, no less
than 746 pounds weight of oxygen; the calculations of Menzies make
the quantity amount even to 837 pounds; but we find his weight at
the end of the year either exactly the same or different one way or
the other by at most a few pounds. What, it may be asked, has become
of the enormous amount of oxygen thus introduced into the human
system in the course of one year? We can answer this question
satisfactorily. No part of the oxygen remains in the body, but is
given out again, combined with carbon and hydrogen. The carbon and
hydrogen of certain parts of the animal body combine with the oxygen
introduced through the lungs and skin, and pass off in the forms of
carbonic acid and vapour of water. At every expiration and every
moment of life, a certain amount of its elements are separated from
the animal organism, having entered into combination with the oxygen
of the atmosphere.
In order to obtain a basis for the approximate calculation, we may
assume, with Lavoisier and Seguin, that an adult man absorbs into
his system 32 1/2 ounces of oxygen daily,--that is, 46,037 cubic
inches = 15,661 grains, French weight; and further, that the weight
of the whole mass of his blood is 24 pounds, of which 80 per cent.
is water. Now, from the known composition of the blood, we know that
in order to convert its whole amount of carbon and hydrogen into
carbonic acid and water, 64.102 grains of oxygen are required. This
quantity will be taken into the system in four days and five hours.
Whether the oxygen enters into combination directly with the
elements of the blood, or with the carbon and hydrogen of other
parts of the body, it follows inevitably--the weight of the body
remaining unchanged and in a normal condition--that as much of these
elements as will suffice to supply 24 pounds of blood, must be taken
into the system in four days and five hours; and this necessary
amount is furnished by the food.
We have not, however, remained satisfied with mere approximation: we
have determined accurately, in certain cases, the quantity of carbon
taken daily in the food, and of that which passes out of the body in
the faeces and urine combined--that is, uncombined with oxygen; and
from these investigations it appears that an adult man taking
moderate exercise consumes 13.9 ounces of carbon, which pass off
through the skin and lungs as carbonic acid gas. [1]
It requires 37 ounces of oxygen to convert 13 9/10 of carbon into
carbonic acid. Again; according to the analysis of Boussingault,
(Annales de Chim. et de Phys., lxx. i. p.136), a horse consumes 79
1/10 ounces of carbon in twenty-four hours, a milch cow 70 3/4
ounces; so that the horse requires 13 pounds 3 1/2 ounces, and the
cow 11 pounds 10 3/4 ounces of oxygen. [2]
As no part of the oxygen taken into the system of an animal is given
off in any other form than combined with carbon or hydrogen, and as
in a normal condition, or state of health, the carbon and hydrogen
so given off are replaced by those elements in the food, it is
evident that the amount of nourishment required by an animal for its
support must be in a direct ratio with the quantity of oxygen taken
in to its system. Two animals which in equal times take up by means
of the lungs and skin unequal quantities of oxygen, consume an
amount of food unequal in the same ratio. The consumption of oxygen
in a given time may be expressed by the number of respirations; it
is, therefore, obvious that in the same animal the quantity of
nourishment required must vary with the force and number of
respirations. A child breathes quicker than an adult, and,
consequently, requires food more frequently and proportionably in
larger quantity, and bears hunger less easily. A bird deprived of
food dies on the third day, while a serpent, confined under a bell,
respires so slowly that the quantity of carbonic acid generated in
an hour can scarcely be observed, and it will live three months, or
longer, without food. The number of respirations is fewer in a state
of rest than during labour or exercise: the quantity of food
necessary in both cases must be in the same ratio. An excess of
food, a want of a due amount of respired oxygen, or of exercise, as
also great exercise (which obliges us to take an increased supply of
food), together with weak organs of digestion, are incompatible with
health.
But the quantity of oxygen received by an animal through the lungs
not only depends upon the number of respirations, but also upon the
temperature of the respired air. The size of the thorax of an animal
is unchangeable; we may therefore regard the volume of air which
enters at every inspiration as uniform. But its weight, and
consequently the amount of oxygen it contains, is not constant. Air
is expanded by heat, and contracted by cold--an equal volume of hot
and cold air contains, therefore, an unequal amount of oxygen. In
summer atmospheric air contains water in the form of vapour, it is
nearly deprived of it in winter; the volume of oxygen in the same
volume of air is smaller in summer than in winter. In summer and
winter, at the pole and at the equator, we inspire an equal volume
of air; the cold air is warmed during respiration and acquires the
temperature of the body. In order, therefore, to introduce into the
lungs a given amount of oxygen, less expenditure of force is
necessary in winter than in summer, and for the same expenditure of
force more oxygen is inspired in winter. It is also obvious that in
an equal number of respirations we consume more oxygen at the level
of the sea than on a mountain.
The oxygen taken into the system is given out again in the same
form, both in summer and winter: we expire more carbon at a low than
at a high temperature, and require more or less carbon in our food
in the same proportion; and, consequently, more is respired in
Sweden than in Sicily, and in our own country and eighth more in
winter than in summer. Even if an equal weight of food is consumed
in hot and cold climates, Infinite Wisdom has ordained that very
unequal proportions of carbon shall be taken in it. The food
prepared for the inhabitants of southern climes does not contain in
a fresh state more than 12 per cent. of carbon, while the blubber
and train oil which feed the inhabitants of Polar regions contain 66
to 80 per cent. of that element.
From the same cause it is comparatively easy to be temperate in warm
climates, or to bear hunger for a long time under the equator; but
cold and hunger united very soon produce exhaustion.
The oxygen of the atmosphere received into the blood in the lungs,
and circulated throughout every part of the animal body, acting upon
the elements of the food, is the source of animal heat.
[Footnote 1: This account is deduced from observations made upon the
average daily consumption of about 30 soldiers in barracks. The food
of these men, consisting of meat, bread, potatoes, lentils, peas,
beans, butter, salt, pepper, &c., was accurately weighed during a
month, and each article subjected to ultimate analysis. Of the
quantity of food, beer, and spirits, taken by the men when out of
barracks, we have a close approximation from the report of the
sergeant; and from the weight and analysis of the faeces and urine,
it appears that the carbon which passes off through these channels
may be considered equivalent to the amount taken in that portion of
the food, and of sour-crout, which was not included in the
estimate.]
[Footnote 2: 17.5 ounces = 0.5 kilogramme.]
LETTER VII
My dear Sir,
The source of animal heat, its laws, and the influence it exerts
upon the functions of the animal body, constitute a curious and
highly interesting subject, to which I would now direct your
attention.
All living creatures, whose existence depends upon the absorption of
oxygen, possess within themselves a source of heat, independent of
surrounding objects.
This general truth applies to all animals, and extends to the seed
of plants in the act of germination, to flower-buds when developing,
and fruits during their maturation.
In the animal body, heat is produced only in those parts to which
arterial blood, and with it the oxygen absorbed in respiration, is
conveyed. Hair, wool, and feathers, receive no arterial blood, and,
therefore, in them no heat is developed. The combination of a
combustible substance with oxygen is, under all circumstances, the
only source of animal heat. In whatever way carbon may combine with
oxygen, the act of combination is accompanied by the disengagement
of heat. It is indifferent whether this combination takes place
rapidly or slowly, at a high or at a low temperature: the amount of
heat liberated is a constant quantity.
The carbon of the food, being converted into carbonic acid within
the body, must give out exactly as much heat as if it had been
directly burnt in oxygen gas or in common air; the only difference
is, the production of the heat is diffused over unequal times. In
oxygen gas the combustion of carbon is rapid and the heat intense;
in atmospheric air it burns slower and for a longer time, the
temperature being lower; in the animal body the combination is still
more gradual, and the heat is lower in proportion.
It is obvious that the amount of heat liberated must increase or
diminish with the quantity of oxygen introduced in equal times by
respiration. Those animals, therefore, which respire frequently, and
consequently consume much oxygen, possess a higher temperature than
others, which, with a body of equal size to be heated, take into the
system less oxygen. The temperature of a child (102 deg) is higher
than that of an adult (99 1/2 deg). That of birds (104 deg to 105.4
deg) is higher than that of quadrupeds (98 1/2 deg to 100.4 deg) or
than that of fishes or amphibia, whose proper temperature is from
2.7 to 3.6 deg higher than that of the medium in which they live.
All animals, strictly speaking, are warm-blooded; but in those only
which possess lungs is the temperature of the body quite independent
of the surrounding medium.
The most trustworthy observations prove that in all climates, in the
temperate zones as well as at the equator or the poles, the
temperature of the body in man, and in what are commonly called
warm-blooded animals, is invariably the same; yet how different are
the circumstances under which they live!
The animal body is a heated mass, which bears the same relation to
surrounding objects as any other heated mass. It receives heat when
the surrounding objects are hotter, it loses heat when they are
colder, than itself.
We know that the rapidity of cooling increases with the difference
between the temperature of the heated body and that of the
surrounding medium; that is, the colder the surrounding medium the
shorter the time required for the cooling of the heated body.
How unequal, then, must be the loss of heat in a man at Palermo,
where the external temperature is nearly equal to that of the body,
and in the polar regions, where the external temperature is from 70
deg to 90 deg lower!
Yet, notwithstanding this extremely unequal loss of heat, experience
has shown that the blood of the inhabitant of the arctic circle has
a temperature as high as that of the native of the south, who lives
in so different a medium.
This fact, when its true significance is perceived, proves that the
heat given off to the surrounding medium is restored within the body
with great rapidity. This compensation must consequently take place
more rapidly in winter than in summer, at the pole than at the
equator.
Now, in different climates the quantity of oxygen introduced into
the system by respiration, as has been already shown, varies
according to the temperature of the external air; the quantity of
inspired oxygen increases with the loss of heat by external cooling,
and the quantity of carbon or hydrogen necessary to combine with
this oxygen must be increased in the same ratio.
It is evident that the supply of the heat lost by cooling is
effected by the mutual action of the elements of the food and the
inspired oxygen, which combine together. To make use of a familiar,
but not on that account a less just illustration, the animal body
acts, in this respect, as a furnace, which we supply with fuel. It
signifies nothing what intermediate forms food may assume, what
changes it may undergo in the body; the last change is uniformly the
conversion of its carbon into carbonic acid, and of its hydrogen
into water. The unassimilated nitrogen of the food, along with the
unburned or unoxidised carbon, is expelled in the urine or in the
solid excrements. In order to keep up in the furnace a constant
temperature, we must vary the supply of fuel according to the
external temperature, that is, according to the supply of oxygen.
In the animal body the food is the fuel; with a proper supply of
oxygen we obtain the heat given out during its oxidation or
combustion. In winter, when we take exercise in a cold atmosphere,
and when consequently the amount of inspired oxygen increases, the
necessity for food containing carbon and hydrogen increases in the
same ratio; and by gratifying the appetite thus excited, we obtain
the most efficient protection against the most piercing cold. A
starving man is soon frozen to death. The animals of prey in the
arctic regions, as every one knows, far exceed in voracity those of
the torrid zone.
In cold and temperate climates, the air, which incessantly strives
to consume the body, urges man to laborious efforts in order to
furnish the means of resistance to its action, while, in hot
climates, the necessity of labour to provide food is far less
urgent.
Our clothing is merely an equivalent for a certain amount of food.
The more warmly we are clothed the less urgent becomes the appetite
for food, because the loss of heat by cooling, and consequently the
amount of heat to be supplied by the food, is diminished.
If we were to go naked, like certain savage tribes, or if in hunting
or fishing we were exposed to the same degree of cold as the
Samoyedes, we should be able with ease to consume 10 lbs. of flesh,
and perhaps a dozen of tallow candles into the bargain, daily, as
warmly clad travellers have related with astonishment of these
people. We should then also be able to take the same quantity of
brandy or train oil without bad effects, because the carbon and
hydrogen of these substances would only suffice to keep up the
equilibrium between the external temperature and that of our bodies.
According to the preceding expositions, the quantity of food is
regulated by the number of respirations, by the temperature of the
air, and by the amount of heat given off to the surrounding medium.
No isolated fact, apparently opposed to this statement, can affect
the truth of this natural law. Without temporary or permanent injury
to health, the Neapolitan cannot take more carbon and hydrogen in
the shape of food than he expires as carbonic acid and water; and
the Esquimaux cannot expire more carbon and hydrogen than he takes
in the system as food, unless in a state of disease or of
starvation. Let us examine these states a little more closely.
The Englishman in Jamaica perceives with regret the disappearance of
his appetite, previously a source of frequently recurring enjoyment;
and he succeeds, by the use of cayenne pepper, and the most powerful
stimulants, in enabling himself to take as much food as he was
accustomed to eat at home. But the whole of the carbon thus
introduced into the system is not consumed; the temperature of the
air is too high, and the oppressive heat does not allow him to
increase the number of respirations by active exercise, and thus to
proportion the waste to the amount of food taken; disease of some
kind, therefore, ensues.
On the other hand, England sends her sick to southern regions, where
the amount of the oxygen inspired is diminished in a very large
proportion. Those whose diseased digestive organs have in a greater
or less degree lost the power of bringing the food into the state
best adapted for oxidation, and therefore are less able to resist
the oxidising influence of the atmosphere of their native climate,
obtain a great improvement in health. The diseased organs of
digestion have power to place the diminished amount of food in
equilibrium with the inspired oxygen, in the mild climate; whilst in
a colder region the organs of respiration themselves would have been
consumed in furnishing the necessary resistance to the action of the
atmospheric oxygen.
In our climate, hepatic diseases, or those arising from excess of
carbon, prevail in summer; in winter, pulmonary diseases, or those
arising from excess of oxygen, are more frequent.
The cooling of the body, by whatever cause it may be produced,
increases the amount of food necessary. The mere exposure to the
open air, in a carriage or on the deck of a ship, by increasing
radiation and vaporisation, increases the loss of heat, and compels
us to eat more than usual. The same is true of those who are
accustomed to drink large quantities of cold water, which is given
off at the temperature of the body, 98 1/2 deg. It increases the
appetite, and persons of weak constitution find it necessary, by
continued exercise, to supply to the system the oxygen required to
restore the heat abstracted by the cold water. Loud and long
continued speaking, the crying of infants, moist air, all exert a
decided and appreciable influence on the amount of food which is
taken.
We have assumed that carbon and hydrogen especially, by combining
with oxygen, serve to produce animal heat. In fact, observation
proves that the hydrogen of the food plays a no less important part
than the carbon.
The whole process of respiration appears most clearly developed,
when we consider the state of a man, or other animal, totally
deprived of food.
The first effect of starvation is the disappearance of fat, and this
fat cannot be traced either in the urine or in the scanty faeces.
Its carbon and hydrogen have been given off through the skin and
lungs in the form of oxidised products; it is obvious that they have
served to support respiration.
In the case of a starving man, 32 1/2 oz. of oxygen enter the system
daily, and are given out again in combination with a part of his
body. Currie mentions the case of an individual who was unable to
swallow, and whose body lost 100 lbs. in weight during a month; and,
according to Martell (Trans. Linn. Soc., vol. xi. p.411), a fat pig,
overwhelmed in a slip of earth, lived 160 days without food, and was
found to have diminished in weight, in that time, more than 120 lbs.
The whole history of hybernating animals, and the well-established
facts of the periodical accumulation, in various animals, of fat,
which, at other periods, entirely disappears, prove that the oxygen,
in the respiratory process, consumes, without exception, all such
substances as are capable of entering into combination with it. It
combines with whatever is presented to it; and the deficiency of
hydrogen is the only reason why carbonic acid is the chief product;
for, at the temperature of the body, the affinity of hydrogen for
oxygen far surpasses that of carbon for the same element.
We know, in fact, that the graminivora expire a volume of carbonic
acid equal to that of the oxygen inspired, while the carnivora, the
only class of animals whose food contains fat, inspire more oxygen
than is equal in volume to the carbonic acid expired. Exact
experiments have shown, that in many cases only half the volume of
oxygen is expired in the form of carbonic acid. These observations
cannot be gainsaid, and are far more convincing than those arbitrary
and artificially produced phenomena, sometimes called experiments;
experiments which, made as too often they are, without regard to the
necessary and natural conditions, possess no value, and may be
entirely dispensed with; especially when, as in the present case,
Nature affords the opportunity for observation, and when we make a
rational use of that opportunity.
In the progress of starvation, however, it is not only the fat which
disappears, but also, by degrees all such of the solids as are
capable of being dissolved. In the wasted bodies of those who have
suffered starvation, the muscles are shrunk and unnaturally soft,
and have lost their contractibility; all those parts of the body
which were capable of entering into the state of motion have served
to protect the remainder of the frame from the destructive influence
of the atmosphere. Towards the end, the particles of the brain begin
to undergo the process of oxidation, and delirium, mania, and death
close the scene; that is to say, all resistance to the oxidising
power of the atmospheric oxygen ceases, and the chemical process of
eremacausis, or decay, commences, in which every part of the body,
the bones excepted, enters into combination with oxygen.
The time which is required to cause death by starvation depends on
the amount of fat in the body, on the degree of exercise, as in
labour or exertion of any kind, on the temperature of the air, and
finally, on the presence or absence of water. Through the skin and
lungs there escapes a certain quantity of water, and as the presence
of water is essential to the continuance of the vital motions, its
dissipation hastens death. Cases have occurred, in which a full
supply of water being accessible to the sufferer, death has not
occurred till after the lapse of twenty days. In one case, life was
sustained in this way for the period of sixty days.
In all chronic diseases death is produced by the same cause, namely,
the chemical action of the atmosphere. When those substances are
wanting, whose function in the organism is to support the process of
respiration, when the diseased organs are incapable of performing
their proper function of producing these substances, when they have
lost the power of transforming the food into that shape in which it
may, by entering into combination with the oxygen of the air,
protect the system from its influence, then, the substance of the
organs themselves, the fat of the body, the substance of the
muscles, the nerves, and the brain, are unavoidably consumed.
The true cause of death in these cases is the respiratory process,
that is, the action of the atmosphere.
A deficiency of food, and a want of power to convert the food into a
part of the organism, are both, equally, a want of resistance; and
this is the negative cause of the cessation of the vital process.
The flame is extinguished, because the oil is consumed; and it is
the oxygen of the air which has consumed it.
In many diseases substances are produced which are incapable of
assimilation. By the mere deprivation of food, these substances are
removed from the body without leaving a trace behind; their elements
have entered into combination with the oxygen of the air.
From the first moment that the function of the lungs or of the skin
is interrupted or disturbed, compounds, rich in carbon, appear in
the urine, which acquires a brown colour. Over the whole surface of
the body oxygen is absorbed, and combines with all the substances
which offer no resistance to it. In those parts of the body where
the access of oxygen is impeded; for example, in the arm-pits, or in
the soles of the feet, peculiar compounds are given out,
recognisable by their appearance, or by their odour. These compounds
contain much carbon.