My dear Sir,
Having attempted in my last letter to explain to you the simple and
admirable office subserved by the oxygen of the atmosphere in its
combination with carbon in the animal body, I will now proceed to
present you with some remarks upon those materials which sustain its
mechanisms in motion, and keep up their various functions,—namely,
If the increase in mass in an animal body, the development and
reproduction of its organs depend upon the blood, then those
substances only which are capable of being converted into blood can
be properly regarded as nourishment. In order then to ascertain what
parts of our food are nutritious, we must compare the composition of
the blood with the composition of the various articles taken as
Two substances require especial consideration as the chief
ingredients of the blood; one of these separates immediately from
the blood when it is withdrawn from the circulation.
It is well known that in this case blood coagulates, and separates
into a yellowish liquid, the serum of the blood, and a gelatinous
mass, which adheres to a rod or stick in soft, elastic fibres, when
coagulating blood is briskly stirred. This is the fibrine of the
blood, which is identical in all its properties with muscular fibre,
when the latter is purified from all foreign matters.
The second principal ingredient of the blood is contained in the
serum, and gives to this liquid all the properties of the white of
eggs, with which it is indeed identical. When heated, it coagulates
into a white elastic mass, and the coagulating substance is called
Fibrine and albumen, the chief ingredients of blood, contain, in
all, seven chemical elements, among which nitrogen, phosphorus, and
sulphur are found. They contain also the earth of bones. The serum
retains in solution sea salt and other salts of potash and soda, in
which the acids are carbonic, phosphoric, and sulphuric acids. The
globules of the blood contain fibrine and albumen, along with a red
colouring matter, in which iron is a constant element. Besides
these, the blood contains certain fatty bodies in small quantity,
which differ from ordinary fats in several of their properties.
Chemical analysis has led to the remarkable result, that fibrine and
albumen contain the same organic elements united in the same
proportion,—i.e., that they are isomeric, their chemical
composition—the proportion of their ultimate elements—being
identical. But the difference of their external properties shows
that the particles of which they are composed are arranged in a
different order. (See Letter V).
This conclusion has lately been beautifully confirmed by a
distinguished physiologist (Denis), who has succeeded in converting
fibrine into albumen, that is, in giving it the solubility, and
coagulability by heat, which characterise the white of egg.
Fibrine and albumen, besides having the same composition, agree also
in this, that both dissolve in concentrated muriatic acid, yielding
a solution of an intense purple colour. This solution, whether made
with fibrine or albumen, has the very same re-actions with all
substances yet tried.
Both albumen and fibrine, in the process of nutrition, are capable
of being converted into muscular fibre, and muscular fibre is
capable of being reconverted into blood. These facts have long been
established by physiologists, and chemistry has merely proved that
these metamorphoses can be accomplished under the influence of a
certain force, without the aid of a third substance, or of its
elements, and without the addition of any foreign element, or the
separation of any element previously present in these substances.
If we now compare the composition of all organised parts with that
of fibrine and albumen, the following relations present themselves:—
All parts of the animal body which have a decided shape, which form
parts of organs, contain nitrogen. No part of an organ which
possesses motion and life is destitute of nitrogen; all of them
contain likewise carbon and the elements of water; the latter,
however, in no case in the proportion to form water.
The chief ingredients of the blood contain nearly 17 per cent. of
nitrogen, and from numerous analyses it appears that no part of an
organ contains less than 17 per cent. of nitrogen.
The most convincing experiments and observations have proved that
the animal body is absolutely incapable of producing an elementary
body, such as carbon or nitrogen, out of substances which do not
contain it; and it obviously follows, that all kinds of food fit for
the production either of blood, or of cellular tissue, membranes,
skin, hair, muscular fibre, &c., must contain a certain amount of
nitrogen, because that element is essential to the composition of
the above-named organs; because the organs cannot create it from the
other elements presented to them; and, finally, because no nitrogen
is absorbed from the atmosphere in the vital process.
The substance of the brain and nerves contains a large quantity of
albumen, and, in addition to this, two peculiar fatty acids,
distinguished from other fats by containing phosphorus (phosphoric
acid?). One of these contains nitrogen (Fremy).
Finally, water and common fat are those ingredients of the body
which are destitute of nitrogen. Both are amorphous or unorganised,
and only so far take part in the vital process as that their
presence is required for the due performance of the vital functions.
The inorganic constituents of the body are, iron, lime, magnesia,
common salt, and the alkalies.
The nutritive process is seen in its simplest form in carnivorous
animals. This class of animals lives on the blood and flesh of the
graminivora; but this blood and flesh are, in all their properties,
identical with their own. Neither chemical nor physiological
differences can be discovered.
The nutriment of carnivorous animals is derived originally from
blood; in their stomach it becomes dissolved, and capable of
reaching all other parts of the body; in its passage it is again
converted into blood, and from this blood are reproduced all those
parts of their organisation which have undergone change or
With the exception of hoofs, hair, feathers, and the earth of bones,
every part of the food of carnivorous animals is capable of
In a chemical sense, therefore, it may be said that a carnivorous
animal, in supporting the vital process, consumes itself. That which
serves for its nutrition is identical with those parts of its
organisation which are to be renewed.
The process of nutrition in graminivorous animals appears at first
sight altogether different. Their digestive organs are less simple,
and their food consists of vegetables, the great mass of which
contains but little nitrogen.
From what substances, it may be asked, is the blood formed, by means
of which of their organs are developed? This question may be
answered with certainty.
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 the most ordinary experience proves that
animals require for their support and nutrition less of these parts
of plants in proportion as they abound in the nitrogenised
constituents. Animals cannot be fed on matters destitute of these
These important products of vegetation are especially abundant in
the seeds of the different kinds of grain, and of peas, beans, and
lentils; in the roots and the juices of what are commonly called
vegetables. They exist, however, in all plants, without exception,
and in every part of plants in larger or smaller quantity.
These nitrogenised forms of nutriment in the vegetable kingdom may
be reduced to three substances, which are easily distinguished by
their external characters. Two of them are soluble in water, the
third is insoluble.
When the newly-expressed juices of vegetables are allowed to stand,
a separation takes place in a few minutes. A gelatinous precipitate,
commonly of a green tinge, is deposited, and this, when acted on by
liquids which remove the colouring matter, leaves a grayish white
substance, well known to druggists as the deposite from vegetable
juices. This is one of the nitrogenised compounds which serves for
the nutrition of animals, and has been named vegetable fibrine. The
juice of grapes is especially rich in this constituent, but it is
most abundant in the seeds of wheat, and of the cerealia generally.
It may be obtained from wheat flour by a mechanical operation, and
in a state of tolerable purity; it is then called gluten, but the
glutinous property belongs, not to vegetable fibrine, but to a
foreign substance, present in small quantity, which is not found in
the other cerealia.
The method by which it is obtained sufficiently proves that it is
insoluble in water; although we cannot doubt that it was originally
dissolved in the vegetable juice, from which it afterwards
separated, exactly as fibrine does from blood.
The second nitrogenised compound remains dissolved in the juice
after the separation of the fibrine. It does not separate from the
juice at the ordinary temperature, but is instantly coagulated when
the liquid containing it is heated to the boiling point.
When the clarified juice of nutritious vegetables, such as
cauliflower, asparagus, mangelwurzel, or turnips, is made to boil, a
coagulum is formed, which it is absolutely impossible to distinguish
from the substance which separates as a coagulum, when the serum of
blood, or the white of an egg, diluted with water, are heated to the
boiling point. This is vegetable albumen. It is found in the
greatest abundance in certain seeds, in nuts, almonds, and others,
in which the starch of the gramineae is replaced by oil.
The third nitrogenised constituent of the vegetable food of animals
is vegetable caseine. It is chiefly found in the seeds of peas,
beans, lentils, and similar leguminous seeds. Like vegetable
albumen, it is soluble in water, but differs from it in this, that
its solution is not coagulated by heat. When the solution is heated
or evaporated, a skin forms on its surface, and the addition of an
acid causes a coagulum, just as in animal milk.
These three nitrogenised compounds, vegetable fibrine, albumen, and
caseine, are the true nitrogenised constituents of the food of
graminivorous animals; all other nitrogenised compounds occurring in
plants, are either rejected by animals, as in the case of the
characteristic principles of poisonous and medicinal plants, 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
The chemical analysis of these three substances has led to the very
interesting result that they contain the same organic elements,
united in the same proportion by weight; and, what is still more
remarkable, that they are identical in composition with the chief
constituents of blood, animal fibrine, and albumen. They all three
dissolve in concentrated muriatic acid with the same deep purple
colour, and even in their physical characters, animal fibrine and
albumen are in no respect different from vegetable fibrine and
albumen. It is especially to be noticed, that by the phrase,
identity of composition, we do not here intend mere similarity, but
that even in regard to the presence and relative amount of sulphur,
phosphorus, and phosphate of lime, no difference can be observed.
How beautifully and admirably simple, with the aid of these
discoveries, appears the process of nutrition in animals, the
formation of their organs, in which vitality chiefly resides! Those
vegetable principles, which in animals are used to form blood,
contain the chief constituents of blood, fibrine and albumen, ready
formed, as far as regards their composition. All plants, besides,
contain a certain quantity of iron, which reappears in the colouring
matter of the blood. Vegetable fibrine and animal fibrine, vegetable
albumen and animal albumen, hardly differ, even in form; if these
principles be wanting in the food, the nutrition of the animal is
arrested; and when they are present, the graminivorous animal
obtains in its food the very same principles on the presence of
which the nutrition of the carnivora entirely depends.
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, only the vegetable principles which have
served for the nutrition of the latter. Vegetable fibrine and
albumen take the form in the stomach of the graminivorous animal as
animal fibrine and albumen do in that of the carnivorous animal.
From what has been said, it follows that the development of the
animal organism and its growth are dependent on the reception of
certain principles identical with the chief constituents of blood.
In this sense we may say that the animal organism gives to the blood
only its form; that it is incapable of creating blood out of other
substances which do not already contain the chief constituents of
that fluid. We cannot, indeed, maintain that the animal organism has
no power to form other compounds, for we know that it is capable of
producing an extensive series of compounds, differing in composition
from the chief constituents of blood; but these last, which form the
starting-point of the series, it cannot produce.
The animal organism is a higher kind of vegetable, the development
of which begins with those substances with the production of which
the life of an ordinary vegetable ends. As soon as the latter has
borne seed, it dies, or a period of its life comes to a termination.
In that endless series of compounds, which begins with carbonic
acid, ammonia, and water, the sources of the nutrition of
vegetables, and includes the most complex constituents of the animal
brain, there is no blank, no interruption. The first substance
capable of affording nutriment to animals is the last product of the
creative energy of vegetables.
The substance of cellular tissue and of membranes, of the brain and
nerves, these the vegetable cannot produce.
The seemingly miraculous in the productive agency of vegetables
disappears in a great degree, when we reflect that the production of
the constituents of blood cannot appear more surprising than the
occurrence of the fat of beef and mutton in cocoa beans, of human
fat in olive-oil, of the principal ingredient of butter in palm-oil,
and of horse fat and train-oil in certain oily seeds.