EXCREMENTS OF ANIMALS.
[Of what is animal excrement composed?
Explain the composition of the food of animals.
What does hay contain?
To what does Liebig compare the consumption of food by animals, and
why?]
The first organic manure which we shall examine, is animal _excrement_.
This is composed of those matters which have been eaten by the animal as
food, and have been thrown off as solid or liquid manure. In order that
we may know of what they consist, we must refer to the composition of
food and examine the process of digestion.
The food of animals, we have seen to consist of both organic and
inorganic matter. The organic part may be divided into two classes, _i.
e._, that portion which contains nitrogen--such as gluten, albumen,
etc., and that which does not contain nitrogen--such as starch, sugar,
oil, etc.
The inorganic part of food may also be divided into _soluble_ matter and
_insoluble_ matter.
DIGESTION AND ITS PRODUCTS.
[Of what does that part of dung consist which resembles soot?
What else does the dung contain?
In what manner does the digested part of food escape from the body?]
Let us now suppose that we have a full-grown ox, which is not increasing
in any of his parts, but only consumes food to keep up his respiration,
and to supply the natural wastes of his body. To this ox we will feed a
ton of hay which contains organic matter, with and without nitrogen, and
soluble and insoluble inorganic substances. Now let us try to follow it
through its changes in the animal, and observe its destination. Liebig
compares the consumption of food by animals to the imperfect burning of
wood in a stove, where a portion of the fuel is resolved into gases and
ashes (that is, it is completely burned), and another portion, which is
not thoroughly burned, passes off as _soot_. In the animal action in
question, the food undergoes changes which are similar to this burning
of wood. A part of the food is _digested_ and taken up by the blood,
while another portion remains undigested, and passes the bowels as solid
dung--corresponding to soot. This part of the dung then, we see is
merely so much of the food as passes through the system without being
materially changed. Its nature is easily understood. It contains organic
and inorganic matter in nearly the same condition as they existed in the
hay. They have been rendered finer and softer, but their chemical
character is not materially altered. The dung also contains small
quantities of nitrogenous matter, which _leaked out_, as it were, from
the stomach and intestines. The digested food, however, undergoes
further changes which affect its character, and it escapes from the body
in three ways--_i. e._, through the lungs, through the bladder, and
through the bowels. It will be recollected from the first section of
this book, p. 22, that the carbon in the blood of animals, unites with
the oxygen of the air drawn into the lungs, and is thrown off in the
breath as carbonic acid. The hydrogen and oxygen unite to form a part of
the water which constitutes the moisture of the breath.
[Explain the escape of carbon, hydrogen and oxygen.
What becomes of the nitrogenous parts?
How is the _soluble_ ash of the digested food parted with?
The insoluble?
If any portions of the food are not returned in the dung, how are they
disposed of?]
That portion of the organic part of the hay which has been taken up by
the blood of the ox, and which does not contain nitrogen (corresponding
to the _first_ class of proximates, as described in Sect. I), is emitted
through the lungs. It consists, as will be recollected, of carbon,
hydrogen and oxygen, and these assume, in respiration, the form of
carbonic acid and water.
The organic matter of the digested hay, in the blood, which contains
nitrogen (corresponding to the _second_ class of proximates, described
in Sect. I), goes to the _bladder_, where it assumes the form of urea--a
constituent of urine or liquid manure.
We have now disposed of the imperfectly digested food (dung), and of the
_organic_ matter which was taken up by the blood. All that remains to be
examined is the inorganic or mineral matter in the blood, which would
have become _ashes_, if the hay had been burned. The _soluble_ part of
this inorganic matter passes into the bladder, and forms the _inorganic
part of urine_. The _insoluble_ part passes the bowels, in connection
with the dung.
[How is their place supplied?
Is food put out of existence when it is fed to animals?
What does the solid dung contain? Liquid manure? The breath?]
If any of the food taken up by the blood is not returned as above
stated, it goes to form fat, muscle, hair, bones, or some other part of
the animal, and as he is not growing (not increasing in weight) an
equivalent amount of the body of the animal goes to the manure to take
the place of the part retained.[V]
We now have our subject in a form to be readily understood. We learn
that when food is given to animals it is not _put out of existence_, but
is merely _changed in form_; and that in the impurities of the breath,
we have a large portion of those parts of the food which plants obtain
from air and from water; while the solid and liquid excrements contain
all that was taken by the plants from the soil and manures.
The SOLID DUNG contains the undigested parts of the food, the
_insoluble_ parts of the ash, and the nitrogenous
matters which have _escaped_ from the digestive organs.
"LIQUID MANURE" the nitrogenous or _second class_ of proximates of the
digested food, and the _soluble_ parts of the ash.
THE BREATH contains the _first class_ of proximates, those which contain
carbon, hydrogen and oxygen, but _no nitrogen_.[W]
FOOTNOTES:
[V] This account of digestion is not, perhaps, strictly accurate in a
physiological point of view, but it is sufficiently so to give an
elementary understanding of the character of excrements as manures.
[W] The excrements of animals contain more or less of sulphur, and
sometimes small quantities of phosphorus.