This condition is the protrusion of the intestine outside the body through the anus or through an artificial anus. The commonest form is the ordinary Prolapsus ani. This only occurs when the sphincter is greatly relaxed by catarrh or by violent pressing at stool, but when it has frequently happened the sphincter atrophies and the prolapse occurs very readily.

There are, strictly speaking, two forms of prolapsus ani. In the one little more than the mucous membrane is protruded, and is seen to be continuous with the skin at the anus. In the other form there is really an invagination as well as a protrusion. The lower part of the rectum is so fixed that the whole wall cannot be protruded, but only its mucous membrane as in the form just considered. But the upper, more movable part may be invaginated into the lower part and then protruded. This mostly occurs as a consequence of violent peristaltic contraction of the rectum in cases of severe diarrhoea with tenesmus.

In both forms the exposed mucous'membrane becomes inflamed and is liable to bleeding. The inflammation sometimes causes through time adhesion and fixation of the bowel in its abnormal position.