Serosa or Adventitia
|Transverse section of ureter.|
The muscular coat (muscular layer, muscular fibers, muscularis propria, muscularis externa) is a region of muscle in many organs in the vertebrate body, adjacent to the submucosa membrane. It is responsible for gut movement such as peristalsis.
It usually has two distinct layers of smooth muscle:
- inner and "circular"
- outer and "longitudinal"
However, there are some exceptions to this pattern.
- In the stomach and colon, there are three layers to the muscularis externa.
- In the upper esophagus, part of the externa is skeletal muscle, rather than smooth muscle.
- in the pyloric stomach, it forms the pyloric sphincter
- in the anal canal, it forms the anal sphincter
Additional images 
- muscular+layer at eMedicine Dictionary
- Histology at nhmccd.edu
- Bioweb at UWLAX Muscularis externa of the colon
- Bioweb at UWLAX Smooth muscle layers of the gut
- Histology at OU 52_05 - "Duodenum"
- Organology at UC Davis Digestive/mammal/system1/system7 - "Mammal, whole system (LM, Low)"
- UIUC Histology Subject 23
- BU Histology Learning System: 11601ooa - "Muscle Tissue: smooth muscle, muscularis externa"
- BU Histology Learning System: 10802loa - "Digestive System: Alimentary Canal - esophagus "
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