By the time the food has gone a third of the way down the small
intestine, a good share of the starches in it have been turned into
sugar and absorbed by the blood vessels in its wall; and the meats,
milk, eggs, and similar foods have been digested in the same way.
There still remains the bulk of the fats to be disposed of. These fats
are attacked by the pancreatic juice and the bile, and made ready for
digestion. Like other foods, they are then eaten by the cells of the
intestinal wall; but instead of going directly into the blood vessels,
as the sugars and other food substances do, they are passed on into
another set of little tubes or vessels, called the _lymphatics_. In
these they are carried through the _lymph glands_ of the abdomen into
the great _lymph duct_, which finally pours them into one of the great
veins not far from the heart. Tiny, branching lymphatic tubes are found
all over the body, picking up what the cells leave of the fluid which
has seeped out of the arteries for their use and returning it to the
veins through the great lymph duct.
All these different food substances, in the process of digestion, do not
simply soak through the lining cells of the food tube, as through a
blotting paper or straining cloth, but are actually eaten by the cells
and very much changed in the process, and are then passed through the
other side of the cells, either into the blood vessels of the wall of
the intestine or into the lymph vessels, practically ready for use by
the living tissues of the body. It is in the cells then that our food is
turned into blood, and it is there that what we have eaten becomes
really a part of us. It may even be said that we are living upon the
leavings of the little cell citizens that line our food tube; but they
are wonderfully decent, devoted little comrades of the rest of our body
cells, and generous in the amount of food they pass on to the blood
vessels.
As the food-pulp is squeezed on from one coil to another through the
intestine, it naturally has more and more of its nourishing matter
sucked out of it; until, by the time it reaches the last loop of the
twenty feet of the small intestine, it has lost over two-thirds of its
food value.
The Final Stage--the Journey through the Large Intestine. From the
small intestine what remains of the food-pulp is poured into the last
section of the food tube, which enlarges to from two to three inches in
diameter. It is known as the large intestine, or large bowel. This
section is only about five feet long. The first three-fourths of it is
called the _colon_; the last or lowest quarter, the rectum, the
discharge-pipe of the food tube. The principal use of the colon is to
suck out the remaining traces of nourishing matter from the food and the
water in which it is dissolved, thus gradually drying the food-pulp down
to a solid or pasty form, in which condition it collects in a large "S"
shaped loop of the bowel just above the rectum, until discharged.
Public-domain text, read in full here on John Shaqi.
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