The long continuous tube that is the digestive tract is about
9 meters in length. It opens to the outside at both ends, through
the mouth at one end and through the anus at the other. Although
there are variations in each region, the basic structure of
the wall is the same throughout the entire length of the tube.
The wall of the digestive tract has four layers or tunics:
- Mucosa
- Submucosa
- Muscular layer
- Serous layer or serosa
The mucosa, or mucous membrane layer, is the innermost tunic
of the wall. It lines the lumen of the digestive tract. The
mucosa consists of epithelium, an underlying loose connective
tissue layer called lamina
propria, and a thin layer of smooth muscle called
the muscularis mucosa. In certain regions, the mucosa develops
folds that increase the surface area. Certain cells in the
mucosa secrete mucus, digestive enzymes, and hormones. Ducts
from other glands pass through the mucosa to the lumen. In
the mouth and anus, where thickness for protection against
abrasion is needed, the epithelium is stratified squamous
tissue. The stomach and intestines have a thin simple columnar
epithelial layer for secretion and absorption.
The submucosa is a thick layer of loose connective tissue
that surrounds the mucosa. This layer also contains blood
vessels, lymphatic vessels, and nerves. Glands may be embedded
in this layer.
The smooth muscle responsible for movements of the digestive
tract is arranged in two layers, an inner circular layer and
an outer longitudinal layer. The myenteric plexus is between
the two muscle layers.
Above the diaphragm,
the outermost layer of the digestive tract is a connective
tissue called adventitia. Below the diaphragm, it is called
serosa.
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