DIGESTION OF A MOUTHFUL OF BREAD.
~1.~ Let us suppose that we have eaten a mouthful of bread, and can
watch it as it goes through all the different processes of digestion.
~2. Mastication.~--First, we chew or masticate the food with the teeth.
We use the tongue to move the food from one side of the mouth to the
other, and to keep the food between the teeth.
~3. Mouth Digestion.~--While the bread is being chewed, the saliva is
mixed with it and acts upon it. The saliva moistens and softens the food
so that it can be easily swallowed and readily acted upon by the other
digestive juices. You have noticed that if you chew a bit of hard bread
a few minutes it becomes sweet. This is because the saliva changes some
of the starch of the food into sugar.
~4.~ After we have chewed the food, we swallow it, and it passes down
through the oesophagus into the stomach.
~5. Stomach Digestion.~--As soon as the morsel of food enters the
stomach, the gastric juice begins to flow out of the little glands in
which it is formed. This mingles with the food and digests another
portion which the saliva has not acted upon. While this is being done,
the stomach keeps working the food much as a baker kneads dough. This is
done to mix the gastric juice with the food.
~6.~ After an hour or two the stomach squeezes the food so hard that a
little of it, which has been digested by the gastric juice and the
saliva, escapes through the lower opening, the pylorus, of which we have
already learned. As the action of the stomach continues, more of the
digested food escapes, until all that has been properly acted upon has
passed out.
~7. Intestinal Digestion.~--We sometimes eat butter with bread, or take
some other form of fat in our food. This is not acted upon by the saliva
or the gastric juice. When food passes out of the stomach into the small
intestine, a large quantity of bile is at once poured upon it. This bile
has been made beforehand by the liver and stored up in the gall-bladder.
The bile helps to digest fats, which the saliva and the gastric juice
cannot digest.
~8.~ The pancreatic juice does the same kind of work that is done by the
saliva, the gastric juice, and the bile. It also finishes up the work
done by these fluids. It is one of the most important of all the
digestive juices.
~9.~ The intestinal juice digests nearly all the different elements of
the food, so that it is well fitted to complete the wonderful process by
which the food is made ready to enter the blood and to nourish the body.
~10.~ While the food is being acted upon by the bile, the pancreatic
juice, and intestinal juice, it is gradually moved along the intestines.
After all those portions of food which can be digested have been
softened and dissolved, they are ready to be taken into the blood and
distributed through the body.
~11. Absorption.~--If you put a dry sponge into water, it very soon
becomes wet by soaking up the water. Indeed, if you only touch a corner
of the sponge to the water, the whole sponge will soon become wet. We
say that the sponge absorbs the water. It is in a somewhat similar way
that the food is taken up or absorbed by the walls of the stomach and
intestines. When the food is absorbed, the greater part of it is taken
into the blood-vessels, of which we shall learn in a future lesson.
~12. Liver Digestion.~--After the food has been absorbed, the most of it
is carried to the liver, where the process of digestion is completed.
The liver also acts like an inspector to examine the digested food and
remove hurtful substances which may be taken with it, such as alcohol,
mustard, pepper, and other irritating things.
~13. The Thoracic Duct.~--A portion of the food, especially the digested
fats, is absorbed by a portion of the lymphatic vessels called
_lacteals_, which empty into a small vessel called the _thoracic duct_.
This duct passes upward in front of the spine and empties into a vein
near the heart.
SUMMARY.
How a mouthful of food is digested:
1. It is first masticated--that is, it is chewed and moistened with
saliva.
2. Then it is swallowed, passing through the oesophagus to the
stomach.
3. There it is acted upon, and a part of it digested by the gastric
juice.
4. It is then passed into the small intestine, where it is acted upon by
the bile, the pancreatic fluid, and the intestinal juice.
5. The digested food is then absorbed by the walls of the stomach and
intestines.
6. The greater portion of the food is next passed through the liver,
where hurtful substances are removed.
7. A smaller portion is carried through the thoracic duct and emptied
into a vein near the heart.