The rectum is attached to a bone called the sacrum by the peritoneum. It generally follows the curvature of this bone. Anatomically, the rectum can be divided into three portions, known as the upper, middle, and lower thirds. The lower third merges with the anal canal, which opens to the outside as the anus. Two sphincter muscles control the passage of feces through the anus. The internal sphincter muscle is composed of smooth muscle tissue, and is under involuntary control. The external sphincter is made up of skeletal muscle, and is under voluntary control.