RECTUM, DRAWING. The rectum in frontal section. The rectum is composed of the rectal ampulla upper part and of the anal canal lower part, delimited by the pectinated line , anal canal leads to the anus. The rectal ampulla forms curves, at the origin of folds of the internal mucosa of the rectum : the Houston´s valves. It also presents vertical folds, the anal columns. The wall of the rectum includes three layers of tissue : the muscularis, external, composed of muscle fibers insuring the defecat