HOW WE DIGEST.
~1.~ Did you ever see a Venus's fly-trap? This curious plant grows in
North Carolina. It is called a fly-trap because it has on each of its
leaves something like a steel-trap, by means of which it catches flies.
You can see one of these traps in the picture. When a fly touches the
leaf, the trap shuts up at once, and the poor fly is caught and cannot
get away. The harder it tries to escape, the more tightly the trap
closes upon it, until after a time it is crushed to death.
[Illustration: VENUS'S FLY-TRAP.]
~2.~ But we have yet to learn the most curious thing about this strange
plant, which seems to act so much like an animal. If we open the leaf
after a few days, it will be found that the fly has almost entirely
disappeared. The fly has not escaped, but it has been dissolved by a
fluid formed inside of the trap, and the plant has absorbed a portion of
the fly. In fact, it has really eaten it. The process by which food is
dissolved and changed so that it can be absorbed and may nourish the
body, is called _digestion_ (di-ges´-tion).
~3.~ The Venus's fly-trap has a very simple way of digesting its food.
Its remarkable little trap serves it as a mouth to catch and hold its
food, and as a stomach to digest it. The arrangement by which our food
is digested is much less simple than this. Let us study the different
parts by which this wonderful work is done.
[Illustration: THE DIGESTIVE TUBE.]
~4. The Digestive Tube.~--The most important part of the work of
digesting our food is done in a long tube within the body, called the
_digestive tube_ or _canal_.
~5.~ This tube is twenty-five or thirty feet long in a full-grown man;
but it is so coiled up and folded away that it occupies but little
space. It begins at the mouth, and ends at the lower part of the trunk.
The greater part of it is coiled up in the abdomen.
~6. The Mouth.~--The space between the upper and the lower jaw is called
the _mouth_. The lips form the front part and the cheeks the sides. At
the back part are three openings. One, the upper, leads into the nose.
There are two lower openings. One of these leads into the stomach, and
the other leads to the lungs. The back part of the mouth joins the two
tubes which lead from the mouth to the lungs and the stomach, and is
called the _throat_. The mouth contains the _tongue_ and the _teeth_.
[Illustration: THE TEETH.]
~7. The Teeth.~--The first teeth, those which come when we are small
children, are called _temporary_ or _milk teeth_. We lose these teeth
as the jaws get larger and the second or _permanent_ teeth take their
place. There are twenty teeth in the first set, and thirty-two in the
second. Very old persons sometimes have a third set of teeth.
[Illustration: SALIVARY GLANDS.]
~8. The Salivary~ (sal´-i-vary)~ Glands.~--There are three pairs of
_salivary glands_. They form a fluid called the _saliva_ (sa-li´-va). It
is this fluid which moistens the mouth at all times. When we eat or
taste something which we like, the salivary glands make so much saliva
that we sometimes say the mouth waters. One pair of the salivary glands
is at the back part of the lower jaw, in front of the ears. The other
two pairs of glands are placed at the under side of the mouth. The
saliva produced by the salivary glands is sent into the mouth through
little tubes called _ducts_.
~9. The Gullet.~--At the back part of the throat begins a narrow tube,
which passes down to the stomach. This tube is about nine inches long.
It is called the _gullet_, _food-pipe_, or _oesophagus_
(e-soph´-a-gus).
~10. The Stomach.~--At the lower end of the oesophagus the digestive
tube becomes enlarged, and has a shape somewhat like a pear. This is the
_stomach_. In a full-grown person the stomach is sufficiently large to
hold about three pints. At each end of the stomach is a narrow opening
so arranged that it can be opened or tightly closed, as may be
necessary. The upper opening allows the food to pass into the stomach,
the lower one allows it to pass out into the intestines. This opening is
called the _pylorus_ (py-lo´-rus), or gate-keeper, because it closes so
as to keep the food in the stomach until it is ready to pass out.
~11.~ In the membrane which lines the stomach there are many little
pocket-like glands, in which a fluid called the _gastric juice_ is
formed. This fluid is one of the most important of all the fluids formed
in the digestive canal.
[Illustration: GASTRIC GLAND.]
~12. The Intestine~(in-tes´-tine).--At the lower end of the stomach
the digestive canal becomes narrow again. This narrow portion, called
the _intestine_, is about twenty-five feet long in a grown person. The
last few feet of the intestine is larger than the rest, and is called
the _colon_. This long tube is coiled up and snugly packed away in the
cavity of the abdomen. In the membrane lining the intestines are to be
found little glands, which make a fluid called _intestinal juice_.
~13. The Liver.~--Close up under the ribs, on the right side of the
body, is a large chocolate-colored organ, called the _liver_. The liver
is about half as large as the head, and is shaped so as to fit snugly
into its corner of the abdomen. The chief business of the liver is to
make a fluid called _bile_, which is very necessary for the digestion of
our food.
~14.~ The bile is a bitter fluid of a golden-brown color. It is carried
to the intestine by means of a little tube or duct, which enters the
small intestine a few inches below the stomach. When the bile is made
faster than it is needed for immediate use, it is stored up in a little
pear-shaped sac called the _gall-bladder_, which hangs from the under
side of the liver.
~15.~ The liver is a very wonderful organ, and does many useful things
besides making bile. It aids in various ways in digesting the food, and
helps to keep the blood pure by removing from it harmful substances
which are formed within the body.
~16. The Pancreas~(pan´-cre-as).--The _pancreas_ is another large and
very important gland which is found close to the stomach, lying just
behind it in the abdominal cavity. The pancreas forms a fluid called the
_pancreatic juice_, which enters the small intestine at nearly the same
place as the bile.
~17. The Spleen.~--Close to the pancreas, at the left side of the body,
is a dark, roundish organ about the size of the fist, called the
_spleen_. It is not known that the spleen has much to do in the work of
digestion, but it is so closely connected with the digestive organs that
we need to know about it.
~18.~ Please note that there are five important organs of digestion. The
mouth, the stomach, the intestines, the pancreas, and the liver.
~19.~ Also observe that there are five digestive fluids, saliva,
gastric juice, bile, pancreatic juice, and intestinal juice.
SUMMARY.
1. The process of dissolving and changing the food so that it may be
absorbed and may nourish the body is digestion.
2. The work of digestion is chiefly done in the digestive tube or canal,
which is about thirty feet in length.
3. The mouth contains the teeth, and has three pairs of salivary glands
connected with it, which make saliva.
4. The gullet leads from the mouth to the stomach.
5. The stomach is pear-shaped, and holds about three pints.
6. It has an upper and a lower opening, each of which is guarded by a
muscle, which keeps its contents from escaping.
7. The lower opening of the stomach is called the pylorus.
8. The stomach forms the gastric juice.
9. The intestines are about twenty-five feet long. They form the
intestinal juice.
10. The liver lies under the ribs of the right side. It is about half as
large as the head. It makes bile.
11. When not needed for immediate use, the bile is stored up in a sac
called the gall-bladder.
12. The pancreas is a gland which lies just back of the stomach. It
makes pancreatic juice.
13. The spleen is found near the pancreas.
14. There are five important digestive organs--the mouth, the stomach,
the intestines, the liver, and the pancreas.
15. There are five digestive fluids--saliva, gastric juice, intestinal
juice, bile, and pancreatic juice.