My dear Sir,
The facts detailed in my last letter will satisfy you as to the
manner in which the increase of mass in an animal, that is, its
growth, is accomplished; we have still to consider a most important
question, namely, the function performed in the animal system by
substances destitute of nitrogen; such as sugar, starch, gum,
The most extensive class of animals, the graminivora, cannot live
without these substances; their food must contain a certain amount
of one or more of them, and if these compounds are not supplied,
death quickly ensues.
This important inquiry extends also to the constituents of the food
of carnivorous animals in the earliest periods of life; for this
food also contains substances, which are not necessary for their
support in the adult state. The nutrition of the young of carnivora
is obviously accomplished by means similar to those by which the
graminivora are nourished; their development is dependent on the
supply of a fluid, which the body of the mother secretes in the
shape of milk.
Milk contains only one nitrogenised constituent, known under the
name of caseine; besides this, its chief ingredients are butter
(fat), and sugar of milk. The blood of the young animal, its
muscular fibre, cellular tissue, nervous matter, and bones, must
have derived their origin from the nitrogenised constituent of
milk—the caseine; for butter and sugar of milk contain no nitrogen.
Now, the analysis of caseine has led to the result, which, after the
details I have given, can hardly excite your surprise, that this
substance also is identical in composition with the chief
constituents of blood, fibrine and albumen. Nay more—a comparison
of its properties with those of vegetable caseine has shown—that
these two substances are identical in all their properties;
insomuch, that certain plants, such as peas, beans, and lentils, are
capable of producing the same substance which is formed from the
blood of the mother, and employed in yielding the blood of the young
The young animal, therefore, receives in the form of caseine,—which
is distinguished from fibrine and albumen by its great solubility,
and by not coagulating when heated,—the chief constituent of the
mother's blood. To convert caseine into blood no foreign substance
is required, and in the conversion of the mother's blood into
caseine, no elements of the constituents of the blood have been
separated. When chemically examined, caseine is found to contain a
much larger proportion of the earth of bones than blood does, and
that in a very soluble form, capable of reaching every part of the
body. Thus, even in the earliest period of its life, the development
of the organs, in which vitality resides, is, in the carnivorous
animal, dependent on the supply of a substance, identical in organic
composition with the chief constituents of its blood.
What, then, is the use of the butter and the sugar of milk? How does
it happen that these substances are indispensable to life?
Butter and sugar of milk contain no fixed bases, no soda nor potash.
Sugar of milk has a composition closely allied to that of the other
kinds of sugar, of starch, and of gum; all of them contain carbon
and the elements of water, the latter precisely in the proportion to
There is added, therefore, by means of these compounds, to the
nitrogenised constituents of food, a certain amount of carbon; or,
as in the case of butter, of carbon and hydrogen; that is, an excess
of elements, which cannot possibly be employed in the production of
blood, because the nitrogenised substances contained in the food
already contain exactly the amount of carbon which is required for
the production of fibrine and albumen.
In an adult carnivorous animal, which neither gains nor loses
weight, perceptibly, from day to day, its nourishment, the waste of
organised tissue, and its consumption of oxygen, stand to each other
in a well-defined and fixed relation.
The carbon of the carbonic acid given off, with that of the urine;
the nitrogen of the urine, and the hydrogen given off as ammonia and
water; these elements, taken together, must be exactly equal in
weight to the carbon, nitrogen, and hydrogen of the metamorphosed
tissues, and since these last are exactly replaced by the food, to
the carbon, nitrogen, and hydrogen of the food. Were this not the
case, the weight of the animal could not possibly remain unchanged.
But, in the young of the carnivora, the weight does not remain
unchanged; on the contrary, it increases from day to day by an
This fact presupposes, that the assimilative process in the young
animal is more energetic, more intense, than the process of
transformation in the existing tissues. If both processes were
equally active, the weight of the body could not increase; and were
the waste by transformation greater, the weight of the body would
Now, the circulation in the young animal is not weaker, but, on the
contrary, more rapid; the respirations are more frequent; and, for
equal bulks, the consumption of oxygen must be greater rather than
smaller in the young than in the adult animal. But, since the
metamorphosis of organised parts goes on more slowly, there would
ensue a deficiency of those substances, the carbon and hydrogen of
which are adapted for combination with oxygen; because, in the
carnivora, nature has destined the new compounds, produced by the
metamorphosis of organised parts, to furnish the necessary
resistance to the action of the oxygen, and to produce animal heat.
What is wanting for these purposes an Infinite Wisdom has supplied
to the young in its natural food.
The carbon and hydrogen of butter, and the carbon of the sugar of
milk, no part of either of which can yield blood, fibrine, or
albumen, are destined for the support of the respiratory process, at
an age when a greater resistance is opposed to the metamorphosis of
existing organisms; or, in other words, to the production of
compounds, which, in the adult state, are produced in quantity amply
sufficient for the purpose of respiration.
The young animal receives the constituents of its blood in the
caseine of the milk. A metamorphosis of existing organs goes on, for
bile and urine are secreted; the materials of the metamorphosed
parts are given off in the form of urine, of carbonic acid, and of
water; but the butter and sugar of milk also disappear; they cannot
be detected in the faeces.
The butter and sugar of milk are given out in the form of carbonic
acid and water, and their conversion into oxidised products
furnishes the clearest proof that far more oxygen is absorbed than
is required to convert the carbon and hydrogen of the metamorphosed
tissues into carbonic acid and water.
The change and metamorphosis of organised tissues going on in the
vital process in the young animal, consequently yield, in a given
time, much less carbon and hydrogen in the form adapted for the
respiratory process than correspond to the oxygen taken up in the
lungs. The substance of its organised parts would undergo a more
rapid consumption, and would necessarily yield to the action of the
oxygen, were not the deficiency of carbon and hydrogen supplied from
The continued increase of mass, or growth, and the free and
unimpeded development of the organs in the young animal, are
dependent on the presence of foreign substances, which, in the
nutritive process, have no other function than to protect the
newly-formed organs from the action of the oxygen. The elements of
these substances unite with the oxygen; the organs themselves could
not do so without being consumed; that is, growth, or increase of
mass in the body,—the consumption of oxygen remaining the
same,—would be utterly impossible.
The preceding considerations leave no doubt as to the purpose for
which Nature has added to the food of the young of carnivorous
mammalia substances devoid of nitrogen, which their organism cannot
employ for nutrition, strictly so called, that is, for the
production of blood; substances which may be entirely dispensed with
in their nourishment in the adult state. In the young of carnivorous
birds, the want of all motion is an obvious cause of diminished
waste in the organised parts; hence, milk is not provided for them.
The nutritive process in the carnivora thus presents itself under
two distinct forms; one of which we again meet with in the
In graminivorous animals, we observe, that during their whole life,
their existence depends on a supply of substances having a
composition identical with that of sugar of milk, or closely
resembling it. Everything that they consume as food contains a
certain quantity of starch, gum, or sugar, mixed with other matters.
The function performed in the vital process of the graminivora by
these substances is indicated in a very clear and convincing manner,
when we take into consideration the very small relative amount of
the carbon which these animals consume in the nitrogenised
constituents of their food, which bears no proportion whatever to
the oxygen absorbed through the skin and lungs.
A horse, for example, can be kept in perfectly good condition, if he
obtain as food 15 lbs. of hay and 4 1/2 lbs. of oats daily. If we
now calculate the whole amount of nitrogen in these matters, as
ascertained by analysis (1 1/2 per cent. in the hay, 2.2 per cent.
in the oats), in the form of blood, that is, as fibrine and albumen,
with the due proportion of water in blood (80 per cent.), the horse
receives daily no more than 4 1/2 oz. of nitrogen, corresponding to
about 8 lbs. of blood. But along with this nitrogen, that is,
combined with it in the form of fibrine or albumen, the animal
receives only about 14 1/2 oz. of carbon.
Without going further into the calculation, it will readily be
admitted, that the volume of air inspired and expired by a horse,
the quantity of oxygen consumed, and, as a necessary consequence,
the amount of carbonic acid given out by the animal, are much
greater than in the respiratory process in man. But an adult man
consumes daily abut 14 oz. of carbon, and the determination of
Boussingault, according to which a horse expires 79 oz. daily,
cannot be very far from the truth.
In the nitrogenised constituents of his food, therefore, the horse
receives rather less than the fifth part of the carbon which his
organism requires for the support of the respiratory process; and we
see that the wisdom of the Creator has added to his food the
four-fifths which are wanting, in various forms, as starch, sugar,
&c. with which the animal must be supplied, or his organism will be
destroyed by the action of the oxygen.
It is obvious, that in the system of the graminivora, whose food
contains so small a portion, relatively, of the constituents of the
blood, the process of metamorphosis in existing tissues, and
consequently their restoration or reproduction, must go on far less
rapidly than in the carnivora. Were this not the case, a vegetation
a thousand times more luxuriant than the actual one would not
suffice for their nourishment. Sugar, gum, and starch, would no
longer be necessary to support life in these animals, because, in
that case, the products of the waste, or metamorphosis of the
organised tissues, would contain enough carbon to support the