The Probert Encyclopaedia of Medicine

RECTUM

The rectum is the lowest portion of the large intestine, terminating at the anus. The rectum is S- shaped, about 22 cm long, and is formed of muscle layers similar to those elsewhere in the intestinal tract. The lower end of the rectum is funnel-shaped and terminates in the anal canal which is lined with a special form of skin. The rectum is 'slung' in the bottom of the pelvis between the levator ani muscles. Below this level and around the anal canal is a strong sphincter of voluntary muscle - the external sphincter. The internal sphincter is the thickened lower end of the involuntary rectal muscle. Stretching or weakness of the levator ani muscle, or nervous conditions causing paralysis of this or the external sphincter allow the rectum to drop. The lining mucous membrane is then 'prolapsed'. On either side of the anal canal below the levator ani muscle is a space containing fat - the ischio-rectal fossa. This is sometimes the seat of infection and abscess formation.
The anal canal has arteries and veins from the systemic circulation coming from the perineum and buttocks. The arteries join with the branches of the inferior mesenteric artery while the veins join with the tributaries of the portal vein which drain the rectum. If the portal veins are blocked or compressed (as in extreme constipation or in more serious disorders which affect the liver, such as cirrhosis) the haemorrhoidal veins become dilated or 'varicose'. These varicosities around and inside the anal canal immediately underneath the lining membrane are called ' piles' or haemorrhoids. Sometimes during strain at stool the veins are temporarily distended and one of them bursts, producing either brisk haemorrhage or a perianal haematoma (a form of external ' pile'). The rectum is normally empty except immediately before defaecation. The passage of the faecal mass from the colon into the rectum produces the desire to evacuate. If the rectum is insensitive, no such desire occurs, and extreme constipation arises.
Research Rectum