Baron Freiherr Justus von Liebig, one of the most illustrious
chemists of his age, was born on May 12, 1803, at Darmstadt,
Germany, the son of a drysalter. It was in his father's business
that his interest in chemistry first awoke, and at fifteen he
became an apothecary's assistant. Subsequently, he went to
Erlangen, where he took his doctorate in 1822; and afterwards, in
Paris, was admitted to the laboratory of Gay-Lussac as a private
pupil. In 1824 he was appointed a teacher of chemistry in the
University of Giessen in his native state. Here he lived for
twenty-eight years a quiet life of incessant industry, while his
fame spread throughout Europe. In 1845 he was raised to the
hereditary rank of baron, and seven years later was appointed by
the Bavarian government to the professorship of chemistry in the
University of Munich. Here he died on April 18, 1873. The treatise
on "Animal Chemistry, or Organic Chemistry in its Relations to
Physiology and Pathology," published in 1842, sums up the results
of Liebig's investigations into the immediate products of animal
life. He was the first to demonstrate that the only source of
animal heat is that produced by the oxidation of the tissues.
_I.--Chemical Needs of Life_
Animals, unlike plants, require highly organised atoms for nutriment;
they can subsist only upon parts of an organism. All parts of the animal
body are produced from the fluid circulating within its organism. A
destruction of the animal body is constantly proceeding, every motion is
the result of a transformation of its structure; every thought, every
sensation is accompanied by a change in the composition of the substance
of the brain. Food is applied either in the increase of the mass of a
structure (nutrition) or in the replacement of a structure wasted
(reproduction).
Equally important is the continual absorption of oxygen from the
atmosphere. All vital activity results from the mutual action of the
oxygen of the atmosphere and the elements of food. According to
Lavoisier, an adult man takes into his system every year 827 lb. of
oxygen, and yet he does not increase in weight. What, then, becomes of
this oxygen?--for no part of it is again expired as oxygen. The carbon
and hydrogen of certain parts of the body have entered into combination
with the oxygen introduced through the lungs and through the skin, and
have been given out in the form of carbonic acid and the vapour of
water.
Now, an adult inspires 32-1/2 oz. of oxygen daily; this will convert the
carbon of 24 lb. of blood (80 per cent. water) into carbonic acid. He
must, therefore, take as much nutriment as will supply the daily loss.
And, in fact, it is found that he does so; for the average amount of
carbon in the daily food of an adult man is 14 oz., which requires 37
oz. of oxygen for its conversion into carbonic acid. The amount of food
necessary for the support of the animal body must be in direct ratio to
the quantity of oxygen taken into the system. A bird deprived of food
dies on the third day; while a serpent, which inspires a mere trace of
oxygen, can live without food for three months. The number of
respirations is less in a state of rest than in exercise, and the amount
of food necessary in both conditions must vary also.
The capacity of the chest being a constant quantity, we inspire the same
volume of air whether at the pole or at the equator; but the weight of
air, and consequently of oxygen, varies with the temperature. Thus, an
adult man takes into the system daily 46,000 cubic inches of oxygen,
which, if the temperature be 77° F., weighs 32-1/2 oz., but when the
temperature sinks to freezing-point will weigh 35 oz. It is obvious,
also, that in an equal number of respirations we consume more oxygen at
the level of the sea than on a mountain. The quantity of oxygen inspired
and carbonic acid expired must, therefore, vary with the height of the
barometer. In our climate the difference between summer and winter in
the carbon expired, and therefore necessary for food, is as much as
one-eighth.
_II.--The Cause of Animal Heat_
Now, the mutual action between the elements of food and the oxygen of
the air is the source of animal heat.
This heat is wholly due to the combustion of the carbon and hydrogen in
the food consumed. Animal heat exists only in those parts of the body
through which arterial blood (and with it oxygen in solution)
circulates; hair, wool, or feathers, do not possess an elevated
temperature.
As animal heat depends upon respired oxygen, it will vary according to
the respiratory apparatus of the animal. Thus the temperature of a child
is 102° F., while that of an adult is 99-1/2° F. That of birds is higher
than that of quadrupeds or that of fishes or amphibia, whose proper
temperature is 3° F higher than the medium in which they live. All
animals, strictly speaking, are warm-blooded; but in those only which
possess lungs is their temperature quite independent of the surrounding
medium. The temperature of the human body is the same in the torrid as
in the frigid zone; but the colder the surrounding medium the greater
the quantity of fuel necessary to maintain its heat.
The human body may be aptly compared to the furnace of a laboratory
destined to effect certain operations. It signifies nothing what
intermediate forms the food, or fuel, of the furnace may assume; it is
finally converted into carbonic acid and water. But in order to sustain
a fixed temperature in the furnace we must vary the quantity of fuel
according to the external temperature.
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; and everyone knows that the animals of prey in the Arctic
regions far exceed in voracity those in 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 the equivalent for a certain amount of food.
The more warmly we are clothed the less food we require. If in hunting
or fishing we were exposed to the same degree of cold as the Samoyedes
we could with ease consume ten pounds of flesh, and perhaps half a dozen
tallow candles into the bargain. The macaroni of the Italian, and the
train oil of the Greenlander and the Russian, are fitted to administer
to their comfort in the climate in which they have been born.
The whole process of respiration appears most clearly developed in the
case of a man exposed to starvation. Currie mentions the case of an
individual who was unable to swallow, and whose body lost 100 lb. in one
month. The more fat an animal contains the longer will it be able to
exist without food, for the fat will be consumed before the oxygen of
the air acts upon the other parts of the body.
There are various causes by which force or motion may be produced. But
in the animal body we recognise as the ultimate cause of all force only
one cause, the chemical action which the elements of the food and the
oxygen of the air mutually exercise on each other. The only known
ultimate cause of vital force, either in animals or in plants, is a
chemical process. If this be prevented, the phenomena of life do not
manifest themselves, or they cease to be recognisable by our senses. If
the chemical action be impeded, the vital phenomena must take new forms.
The heat evolved by the combustion of carbon in the body is sufficient
to account for all the phenomena of animal heat. The 14 oz. of carbon
which in an adult are daily converted into carbonic acid disengage a
quantity of heat which would convert 24 lb. of water, at the temperature
of the body, into vapour. And if we assume that the quantity of water
vaporised through the skin and lungs amounts to 3 lb., then we have
still a large quantity of heat to sustain the temperature of the body.
_III.--The Chemistry of Blood-Making_
Physiologists conceive that the various organs in the body have
originally been formed from blood. If this be admitted, it is obvious
that those substances alone can be considered nutritious that are
capable of being transformed into blood.
When blood is allowed to stand, it coagulates and separates into a
watery fluid called serum, and into the clot, which consists principally
of fibrine. These two bodies contain, in all, seven elements, among
which sulphur, phosphorus, and nitrogen are found; they contain also the
earth of bones. The serum holds in solution common salt and other salts
of potash and soda, of which the acids are carbonic, phosphoric, and
sulphuric acids. Serum, when heated, coagulates into a white mass called
albumen. This substance, along with the fibrine and a red colouring
matter in which iron is a constituent, constitute the globules of blood.
Analysis has shown that fibrine and albumen are perfectly identical in
chemical composition. They may be mutually converted into each other. In
the process of nutrition both may be converted into muscular fibre, and
muscular fibre is capable of being reconverted into blood.
All parts of the animal body which form parts of organs contain
nitrogen. The principal ingredients of blood contain 17 per cent. of
nitrogen, and there is no part of an active organ that contains less
than 17 per cent. of this element.
The nutritive process is simplest in the case of the carnivora, for
their nutriment is chemically identical in composition with their own
tissues. The digestive apparatus of graminivorous animals is less
simple, and their food contains very little nitrogen. From what
constituents of vegetables is their blood produced?
Chemical researches have shown that all such parts of vegetables as can
afford nutriment to animals contain certain constituents which are rich
in nitrogen; and experience proves that animals require for their
nutrition less of these parts of plants in proportion as they abound in
the nitrogenised constituents. These important products are specially
abundant in the seeds of the different kinds of grain, and of peas,
beans, and lentils. They exist, however, in all plants, without
exception, and in every part of plants in larger or smaller quantity.
The nitrogenised compounds of vegetables are called vegetable fibrine,
vegetable albumen, and vegetable casein. All other nitrogenised
compounds occurring in plants are either rejected by animals or else
they occur in the food in such very small proportion that they cannot
possibly contribute to the increase of mass in the animal body.
The chemical analysis of these three substances has led to the
interesting result that they contain the same organic elements, united
in the same proportion by weight; and--which is more remarkable--that
they are identical in composition with the chief constituents of
blood--animal fibrine and animal albumen. By identity, be it remarked,
is not here meant merely similarity, but that even in regard to the
presence and relative amounts of sulphur, phosphorus, and phosphate of
lime no difference can be observed.
How beautifully simple then, by the aid of these discoveries, appears
the process of nutrition in animals, the formation of their organs, in
which vitality chiefly resides. Those vegetable constituents which are
used by animals to form blood contain the essential ingredients of blood
ready formed. In point of fact, vegetables produce in their organism the
blood of all animals; for the carnivora, in consuming the blood and
flesh of the graminivora, consume, strictly speaking, the vegetable
principles which have served for the nourishment of the latter. In this
sense we may say the animal organism gives to blood only its form; and,
further, that it is incapable of forming blood out of other compounds
which do not contain the chief ingredients of that fluid.
Animal and vegetable life are, therefore, closely related, for the first
substance capable of affording nutriment to animals is the last product
of the creative energy of vegetables. The seemingly miraculous in the
nutritive power of vegetables disappears in a great degree, for the
production of the constituents of blood cannot appear more surprising
than the occurrence of the principal ingredient of butter in palm-oil
and of horse-fat and train-oil in certain of the oily seeds.
_IV.--Food the Fuel of Life_
We have still to account for the use in food of substances which are
destitute of nitrogen but are known to be necessary to animal life. Such
substances are starch, sugar, gum, and pectine. In all of these we find
a great excess of carbon, with oxygen and hydrogen in the same
proportion as water. They therefore add an excess of carbon to the
nitrogenised constituents of food, and they cannot possibly be employed
in the production of blood, because the nitrogenised compounds contained
in the food already contain exactly the amount of carbon which is
required for the production of fibrine and albumen. Now, it can be shown
that very little of the excess of this carbon is ever expelled in the
form either of solid or liquid compounds; it must be expelled,
therefore, in the gaseous state. In short, these compounds are solely
expended in the production of animal heat, being converted by the oxygen
of the air into carbonic acid and water. The food of carnivorous animals
does not contain non-nitrogenised matters, so that the carbon and
hydrogen necessary for the production of animal heat are furnished in
them from the waste of their tissues.
The transformed matters of the organs are obviously unfit for the
further nourishment of the body--that is, for the increase or
reproduction of the mass. They pass through the absorbent and lymphatic
vessels into the veins, and their accumulation in these would soon put a
stop to the nutritive process were it not that the blood has to pass
through a filtering apparatus, as it were, before reaching the heart.
The venous blood, before returning to the heart, is made to pass through
the liver and the kidneys, which separate from it all substances
incapable of contributing to nutrition. The new compounds containing the
nitrogen of the transformed organs, being utterly incapable of further
application in the system, are expelled from the body. Those which
contain the carbon of the transformed tissues are collected in the
gall-bladder as bile, a compound of soda which, being mixed with water,
passes through the duodenum and mixes with chyme. All the soda of the
bile, and ninety-nine-hundredths of the carbonaceous matter which it
contains, retain the capacity of re-absorption by the absorbents of the
small and large intestines--a capacity which has been proved by direct
experiment.
The globules of the blood, which in themselves can be shown to take no
share in the nutritive process, serve to transport the oxygen which they
give up in their passage through the capillary vessels. Here the current
of oxygen meets with the carbonaceous substances of the transformed
tissues, and converts their carbon into carbonic acid, their hydrogen
into water. Every portion of these substances which escapes this process
of oxidation is sent back into the circulation in the form of bile,
which by degrees completely disappears.
It is obvious that in the system of the graminivora, whose food contains
relatively so small a proportion of the constituents of blood, the
process of metamorphosis in existing tissues, and consequently their
restoration or reproduction, must go on far less rapidly than in the
carnivora. Otherwise, a vegetation a thousand times as luxuriant would
not suffice for their sustenance. Sugar, gum, and starch, which form so
large a proportion of their food, would then be no longer necessary to
support life in these animals, because in that case the products of
waste, or metamorphosis of organised tissues, would contain enough
carbon to support the respiratory process.
When exercise is denied to graminivorous and omnivorous animals this is
tantamount to a deficient supply of oxygen. The carbon of the food, not
meeting with a sufficient supply of oxygen to consume it, passes into
other compounds containing a large excess of carbon--or, in other words,
fat is produced. Fat is thus an abnormal production, resulting from a
disproportion of carbon in the food to that of the oxygen respired by
the lungs or absorbed by the skin. Wild animals in a state of nature do
not contain fat. The production of fat is always a consequence of a
deficient supply of oxygen, for oxygen is absolutely indispensable for
the dissipation of excess of carbon in the food.
_V.--Animal Life-Chemistry_
The substances of which the food of man is composed may be divided into
two classes--into nitrogenised and non-nitrogenised. The former are
capable of conversion into blood, the latter incapable of this
transformation. Out of those substances which are adapted to the
formation of blood are formed all the organised tissues. The other class
of substances in the normal state of health serve to support the process
of respiration. The former may be called the plastic elements of
nutrition; the latter, elements of respiration.
Among the former we may reckon--vegetable fibrine, vegetable albumen,
vegetable casein, animal flesh, animal blood.
Among the elements of respiration in our food are--fat, starch, gum,
cane sugar, grape-sugar, sugar of milk, pectine, bassorine, wine, beer,
spirits.
The nitrogenised constituents of vegetable food have a composition
identical with that of the constituents of the blood.
No nitrogenised compound the composition of which differs from that of
fibrine, albumen, and casein, is capable of supporting the vital process
in animals.
The animal organism undoubtedly possesses the power of forming from the
constituents of its blood the substance of its membranes and cellular
tissue, of the nerves and brain, of the organic part of cartilages and
bones. But the blood must be supplied to it ready in everything but its
form--that is, in its chemical composition. If this is not done, a
period is put to the formation of blood, and, consequently, to life.
The whole life of animals consists of a conflict between chemical forces
and the vital power. In the normal state of the body of an adult these
stand in equilibrium: that is, there is equilibrium between the
manifestations of the causes of waste and the causes of supply. Every
mechanical or chemical agency which disturbs the restoration of this
equilibrium is a cause of disease.
Death is that condition in which chemical or mechanical powers gain the
ascendancy, and all resistance on the part of the vital force ceases.
This resistance never entirely departs from living tissues during life.
Such deficiency in resistance is, in fact, a deficiency in resistance to
the action of the oxygen of the atmosphere.
Disease occurs when the sum of vital force, which tends to neutralise
all causes of disturbance, is weaker than the acting cause of
disturbance.
Should there be formed in the diseased parts, in consequence of the
change of matter, from the elements of the blood or of the tissue, new
products which the neighbouring parts cannot employ for their own vital
functions; should the surrounding parts, moreover, be unable to convey
these products to other parts where they may undergo transformation,
then these new products will suffer, at the place where they have been
formed, a process of decomposition analogous to putrefaction.
In certain cases, medicine removes these diseased conditions by exciting
in the vicinity of the diseased part, or in any convenient situation, an
artificial diseased state (as by blisters), thus diminishing by means of
artificial disturbance the resistance offered to the external causes of
change in these parts by the vital force. The physician succeeds in
putting an end to the original diseased condition when the disturbance
artificially excited (or the diminution of resistance in another part)
exceeds in amount the diseased state to be overcome.
The accelerated change of matter and the elevated temperature in the
diseased part show that the resistance offered by the vital force to the
action of oxygen is feebler than in the healthy state. But this
resistance only ceases entirely when death takes place. By the
artificial diminution of resistance in another part, the resistance in
the diseased organ is not, indeed, directly strengthened; but the
chemical action, the cause of the change of matter, is diminished in the
diseased part, being directed to another part, where the physician has
succeeded in producing a still more feeble resistance to the change of
matter, to the action of oxygen.