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Haustra are small pouches in the large intestine that have a segmented appearance; they are formed from longitudinal muscle fibers. Bulges or sacculations develop because the thin muscles known as the taenie are barely shorter than the colon. When the intestinal wall constricts, these circular furrows are formed, and they aid in the expansion and elongation of the large intestine. Haustral contractions occur about every 25 minutes, which cause the colon contents to move to the next haustrum, a single segment or pouch.
The three taeniae are long narrow muscles in the large intestine. It is the puckering action of the tenia coli that is responsible for circular muscle fibers found at about every inch (2.54 cm) in the large intestine. In between the haustra are semilunar folds, known as the plicae semilunares. Tubes with taeniae and semilunar folds are adaptations, which assist in effective regulation of digesta or items in the alimentary tract undergoing digestion.
Other names for these pouches are haustra coli, haustrations of the colon, and sacculations of the colon. Researchers have examined human fetuses to determine when these sacculations or colon tucks first appear, and they found that they develop in human fetuses at about the 10th or 11th week of development. The taeniae, which are linked to the formation of haustral clefts, also appear at this time.
Because the formation of sacculations increases the surface area of the large intestine, this helps the colon achieve its three primary functions of retaining, transporting, and finally eliminating digested material as fecal matter. Muscles of the colon move the watery waste material forward and slowly absorb excess water. Soluble and insoluble indigestible carbohydrates travel down the large intestine.
As material travels from one haustrum to the next, a majority of the water is removed. Digestive remains are mixed with bacteria and mucus to make feces. Next, this material moves to the ascending colon where more water is removed, and the stools become more solid as they travel along into the descending colon. At this point, intestinal bacteria use some of the fiber to nourish themselves.
Haustral churning is the sequential movement of colon contents from one haustra to the next. These contractions are slow movements, during which one haustrum expands as material fills it, causing the muscles to contract, and the contents are pushed to the next haustrum. In this manner, waste material from digestion is moved through the colon. A diet high in fiber and fruits can help most people avoid problems with their colon and assist haustral movement.