It usually has two layers of
smooth muscle: • inner and "circular" • outer and "longitudinal" However, there are some exceptions to this pattern. • In the
stomach, there are three layers to the muscular layer. Stomach contains an additional oblique muscle layer just interior to circular muscle layer. • In the upper
esophagus, part of the externa is
skeletal muscle, rather than smooth muscle. • In the
vas deferens of the spermatic cord, there are three layers: inner longitudinal, middle circular, and outer longitudinal. • In the
ureter, the smooth muscle orientation is opposite that of the GI tract. There is an inner longitudinal and an outer circular layer. The inner layer of the muscularis externa forms a
sphincter at two locations of the
gastrointestinal tract: • in the
pylorus of the
stomach, it forms the
pyloric sphincter. • in the
anal canal, it forms the
internal anal sphincter. In the
colon, the fibres of the external longitudinal smooth muscle layer are collected into three longitudinal bands, the
teniae coli. The thickest muscularis layer is found in the
stomach (triple layered) and thus maximum peristalsis occurs in the stomach. Thinnest muscularis layer in the
alimentary canal is found in the
rectum, where minimum peristalsis occurs. ==Function==