Anatomy
Peristalsis (gut movement – churning, mixing, kneading, moving contents on) is caused by smooth muscles, internal layer circular and outer longitudinal. In SI, outer layer (muscularis externa) surrounds gut, whereas in LI, there are 3 separate longitudinal bands (taenia coli).
These are shorter (more so when contracted) than the rest of the circumference of the SI, and therefore there is billowing out/sacculation/pouching/haustra. SI folds are circular, SI not (X-ray aid to identification).
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