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                                                GI Tract Layers (labels) - illustration
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                                                The walls of the digestive tract have four layers of tissue: mucosa, submucosa, muscularis externa and serosa. (Description from the National Cancer Institute Visuals Online page.)
 Image and description: https://visualsonline.cancer.gov/details.cfm?imageid=1766
 
 Originator: NCI
 
 Source: The Web site of the National Cancer Institute (https://www.cancer.gov)
 
 Creator: Unknown Illustrator
 
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