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Large Intestine

1) The primary function of the large intestine is the reabsorption of water and inorganic salts. The only secretion of any importance is mucus, which acts as a lubricant during the transport of the intestinal contents.
2) Extends from ileocecal junction to anus
3) Consists of cecum, colon, rectum, anal canal
4) Movements sluggish (18-24 hours)

Unique features:

a. Teniae coli – three bands of longitudinal smooth muscle in its muscularis
b. Haustration – pocket like sacs caused by the tone of the teniae coli
c. Epiploic appendages – fat-filled pouches of visceral peritoneum

Mucosa

The surface of the mucosa is relatively smooth as there are no plicae circulares or intestinal villi. Crypts of Lieberkühn are present and usually longer and straighter than those of the small intestine. Goblet cells account for more of the epithelial cells than in the small intestine

Lamina propria

There is only little lamina propria squeezed between the glands

Submucosa

Considerable amounts of fat may be found in the submucosa

Muscularis externa

The appearance of the muscularis externa is different from that of the small intestine. The inner circular layer of muscle forms the usual sheath around the large intestine, but the outer longitudinal muscle layer forms three flattened strands, the taenia coli. Only a thin layer of longitudinal muscle surrounds the inner circular muscle layer between the taenia coli

Adventitia

The adventitia forms small pouches (appendices epiploicae) filled with fatty tissue along the large intestine.

Vermiform appendix

1) Is a small blind-ending diverticulum from the cecum. The most important features of the appendix is the thickening of its wall, which is mainly due to large accumulations of lymphoid tissue in the lamina propria and submucosa.
2) Intestinal villi are usually absent, and crypts do not occur as frequently as in the colon.
3)  There is often fatty tissue in the submucosa.
4)  The muscularis externa is thinner than in the remainder of the large intestine
5)The outer, longitudinal smooth muscle layer of the muscularis externa does not aggregate into taenia coli.

Anal canal

a. The mucosa has a characteristic surface relief of 5-10 longitudinal folds, the anal columns.
b. Small mucosal folds between the anal columns (anal valves) form the pectinate line.
c. Crypts disappear below the pectinate line and the epithelium changes from the tall, columnar type seen in other parts of the large intestine to a stratified squamous epithelium.
d. The muscularis externa gradually becomes thicker and forms the involuntary internal anal sphincter.

Histological slides are available below

Large Intestine

Appendix

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