The small intestine is an organ in the digestive system that is composed of numerous muscles, glands, and membranes. First, it is connected to the walls of the abdomen by mesentery, a double layer of serous membrane (a smooth membrane with cells that release fluids). Also on the outside wall is serosa ( a loose connective tissue covered in mucus) protecting the small intestine from friction damage. Then, deeper inside the small intestine is the muscularis extrema which is a mix of two different muscles (circle and longitudinal) that help food travel down the intestine without going back up it. Even deeper inside is the submucosa, which is a dense bunch of connective tissues that has a lot of nerves and blood vessels within it. Alongside the submucosa is the Brunner's Gland which is tubular submucosal gland that secretes alkaline secretion. Within this gland is the mucosa, which is a smooth layer of muscle that helps with the contraction and relaxation of the intestine (for digestion). Finally, all the way in the middle of the intestine is the lumen. This is the cavity where food passes through as it is being digested.
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The small intestine contains a number of different types of tissues; epithelial tissue, muscle tissue, and glandular cells to secrete digestive enzymes, as well as a certain amount of nerve tissue (autonomic, not central).
The small intestines wall is manly used in the digestive system to continue the process of breaking down foods.