a. Mouth. In the mouth, food is ground into small particles by the teeth. Food is
moistened and softened by saliva. Saliva is secreted by the salivary glands. This
action of the teeth and saliva prepares the food so it can be easily swallowed. An
enzyme in the saliva (ptyalin) starts the chemical breakdown of starchy foods.
b. Pharynx. The pharynx is a muscular tube forming a passage from the mouth
to the esophagus. When swallowing occurs, the tongue pushes the food to the back of
the mouth. The food passes through the pharynx to the esophagus. The pharynx
connects with both the mouth and nose. Air passes from the nose, through the
pharynx, to the larynx, and to the trachea (windpipe). When food is being swallowed, a
lid-like structure (the epiglottis) closes off the larynx so food will not enter.
c. Esophagus. The esophagus is also a muscular tube. It extends from the
pharynx through the thorax to the upper end of the stomach. The chewed food is
passed through the esophagus by a wavelike motion of its muscular walls. This motion,
called peristalsis, is also found in the intestines.
d. Stomach. The stomach is a muscular, bag-like organ. It is located in the
upper left part of the abdomen. At its upper end, the stomach connects with the
esophagus. At its lower end, the stomach connects with the small intestine. The
stomach secretes several enzymes and an acid which aid in the chemical breakdown of
carbohydrates, proteins, and fats. The churning action of the stomach's wall, the action
of the enzymes, and the action of the acid reduce the food to a semifluid mass called
chyme. The stomach does not empty this mass all at once. At intervals, it squirts
chyme into the small intestine. By the time chyme leaves the stomach, the food is
about half digested.
e. Small Intestine. The small intestine is a muscular tube about 21 feet in
length and 1 inch in diameter. It has many folds and curves and occupies a large part
of the abdomen. Here, the final phase of digestion is completed. In this phase,
enzymes secreted from the pancreas, liver, and walls of the intestine itself mix with the
food as it enters the small intestine and act upon the food as it passes through the
intestine by peristaltic action. When digestion is completed, the digested end products
are absorbed through the wall of the small intestine and into the bloodstream. Certain
ingredients of food are not digested. They cannot be broken down into an absorbable
form by the digestive process. This undigested food passes through the small intestine
into the large intestine.
f. Large Intestine. The large intestine, continuous with the small intestine, is
about 5 feet in length and 2 inches in diameter. It is called the large intestine because
its diameter is approximately twice that of the small intestine. The main function of the
large intestine is the recovery of water. Undigested food, bacteria in the intestine and
secretions from the digestive tract enter the large intestine in a semifluid form. During
the passage of this material through the large intestine, water is absorbed through the
intestinal wall. The remaining solid material is packed or squeezed into a form called
feces. The feces then pass through the rectum.