Anyone, who has ever used a test tube in a high
school laboratory, or even used a frying pan, knows, that the more heat
applied, the faster a chemical reaction can be expected to take place. Yet the
body, inall of the chemical processes carried on in digestion, does its work at
the ordinary temperature of the human body, 98°F-less than the temperature of
bath water.
It is able to do so at a good pace in large part because of
enzymes-catalytic substances that promote biochemical reactions without
themselves being used up in the process. When food has been chewed and
salivated in the mouth, it is swallowed and enters the esophagus. This division
of the alimentary canal has no digestive function. It carries food downward
from the mouth a distance of nine or ten inches to the stomach.
In the
esophagus, food is moved by peristaltic action, which is achieved as two sets
of muscles work together. One set is circular and squeezes inward when it
contracts; the other is longitudinal and pushes food toward the stomach. It is
because of peristaltic action that you can swallow food in any-body position.
