The pharynx is a musculomembranous passage that leads from the nose and
mouth to the esophagus. The passage of food from the pharynx into the esophagus is
the second stage of swallowing. When food is being swallowed, the larynx is closed off
from the pharynx to keep food from getting into the respiratory tract.
The esophagus is a musculomembranous passage about 10 inches long, lined
with a mucous membrane. It leads from the pharynx through the chest to the upper end
of the stomach. Its function is to complete the act of swallowing. The involuntary
movement of material down the esophagus is carried out by the process known as
peristalsis, which is the wavelike action produced by contraction of the muscular wall.
This is the method by which food is moved throughout the alimentary canal.
The stomach is an elongated pouch-like structure lying just below the diaphragm, with
most of it to the left of the midline. It has three divisions: the fundus, the enlarged
portion to the left and above the entrance of the esophagus; the body, the central
portion; and the pylorus, the lower portions. Circular sphincter muscles that act as
valves guard the opening of the stomach. (The cardiac sphincter is at the esophageal
opening, and the pyloric sphincter is at the junction of the stomach and the duodenum,
the first portion of the small intestine.) The cardiac sphincter prevents stomach contents
from reentering the esophagus except when vomiting occurs. In the digestive process,
two important functions of the stomach are:
a. It acts as a storehouse for food, receiving fairly large amounts, churning it,
and breaking it down further for mixing with digestive juices. Semiliquid food is released
in small amounts by the pyloric valve into the duodenum, the first part of the small
b. The glands in the stomach lining produce gastric juices (which contain
enzymes) and hydrochloric acid. The enzymes in the gastric juice start the digestion of
protein foods, milk, and fats. Hydrochloric acid aids enzyme action. The mucous
membrane lining the stomach protects the stomach itself from being digested by the
strong acid and powerful enzymes.
a. The small intestine is a tube about 22 feet long. The intestine is attached to
the margin of a thin band of tissue called the mesentery, which is a portion of the
peritoneum, the serous membrane lining the abdominal cavity. The mesentery supports
the intestine, and the vessels that carry blood to and from the intestine lie within this
membrane. The other edge of the mesentery is drawn together like a fan; the gathered