Also known as retroperitoneal, the rectum and the associated anal canal is the most caudal organ in the digestive system. While it's name might mislead one into believing it is straight (from the Latin, rectus), it actually consists of three lateral curves or bends. These are represented internally as three transverse folds called the rectal valves, which are located at the top of above picture. These valves are extremely important as they are used to separate the feces (stool) from flatus (gas). Moving downwards, one encounters the anal canal. It is controlled by two sphincters, the internal anal sphincter and the external anal sphincter. The former set of muscles is primarily involuntary while the later is voluntarally controlled. Essentially these sphincters keep the anus, the opening at the end of the digestive system, from staying open.