Page 548 - Atlas of Histology with Functional Correlations

the small intestine, the duodenum, the submucosa contains numerous branched

mucous glands.

Muscularis Externa

The muscularis externa is a thick, smooth muscle layer located inferior to

submucosa. Except for the large intestine, this contains an inner layer of circular
smooth muscle and an outer layer of longitudinal smooth muscle. Situated
between the two smooth muscle layers are connective tissue and another nerve
plexus called the myenteric (Auerbach) nerve plexus. This plexus also
contains postganglionic parasympathetic neurons and controls the motility of

smooth muscles in the muscularis externa.

Serosa

The serosa is the outermost layer of the abdominal portion of the esophagus,

stomach, and small intestine and is continuous with the mesentery and the lining
of the abdominal cavity. The serosa is a serous membrane of simple squamous
epithelium called mesothelium and a thin underlying loose connective tissue

that surrounds the visceral organs. If mesothelium covers the visceral organs, the
organs are intraperitoneal and the outermost layer is called serosa. Serosa also
covers ascending and descending colon only on the anterior and lateral surfaces.
The posterior surfaces of the colon are bound to the posterior abdominal body
wall and are not covered by the mesothelium or suspended by a mesentery.

Adventitia

When the digestive tube is not covered by mesothelium, it lies outside the

peritoneal cavity and is called retroperitoneal. Here, the outermost layer of the
organ adheres to the body wall and consists only of a connective tissue called
adventitia.

The characteristic features of each layer of the digestive tube and their
functions are discussed in detail with illustration of the different parts of the

digestive tract.

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