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Small intestine muscle layers. Light micrograph of a longitudinal section through the muscular wall of the small intestine. The small intestine is the principal site of absorption of digested food. Here, the muscle layers that surround the intestine are shown. Above is the circular muscle layer with the muscle fibres cut in cross section. Below is the longitudinal muscle layer. The long shape of the longitudinally sectioned fibres are seen with their cell nuclei (oval). These layers of smooth, involuntary muscle are responsible for the intestine's continuous peristaltic activity. Peristalsis moves the food along the intestine. Magnification: x200 at 35mm size.
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