rials for growth and
repair of tissues. In order to be used by the body for any purpose,
nutrients must first go through a series of complicated changes known as
digestion, which renders them soluble so that they can soak through the
walls of the intestine.

THE DIGESTIVE PROCESS

Digestion begins in the mouth. There the food is crushed and its fibres
separated by the teeth, it is moistened by the saliva, and substances in
the saliva begin a chemical action upon the starch. Chewing should be
sufficient to reduce the food to a soft mass well moistened with saliva.
Slow eating is desirable, but the emphasis should be placed on thorough
chewing. For instance, long intervals between bites are of no special
benefit if mouthfuls of food are washed down by swallows of water.

After it has been swallowed, the food passes into the stomach and
remains there for a variable length of time, while it undergoes further
preparation for absorption. It is moved about by the contraction of the
muscular walls of the stomach, so that it becomes mixed with the stomach
juices and more thoroughly softened. Some digestion of proteids goes on
in the stomach, and a little absorption through the walls.

Little by little the food is discharged from the stomach into the small
intestine, and the most important part of digestion then begins. It is
acted upon chemically by a fluid flowing into the intestine from an
organ called the pancreas; this pancreatic juice acts upon all three
nutrients and is of great importance in the digestive process. The bile
and other juices that flow into the intestine perform important
functions also.

The food masses are moved along by rhythmic contractions of the
intestine, and absorption goes on when the food has been so changed that
it can soak through the intestinal walls into the blood and lymph
vessels. The small intestine is about 20 feet long, and consequently
affords a large surface for absorption, as does also the large
intestine, into which the small intestine opens. The blood and lymph
carry the digested food substances to all parts of the body, and thus
the different tissues are provided with the materials they need for
growth, repair, and energy. Excess of food substances may be stored as
fat or expelled from the body.

As the blood and lymph go through the tissues they take from the tissues
the refuse, or the part that remains after the fuel substances have been
consumed. This refuse from the tiss