The rectum is a large tube that forms the end of the large intestine and terminates the anus. It is used for the storage of feces until they can be expelled. The rectum is able to stretch to several times it's size when empty, and nerves attached to the muscles surrounding the rectum sense when it is full and contract the muscles to force feces out through the anus. Unlike the rest of the intestine, it is not designed to extract nutrients or water. As such, feces generally leave the anus with much the same consistency as they enter the rectum.
The rectum, courtesy NIH, via Wikipedia.
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