Introduction
The anus is the opening at the end of the digestive tract where stool leaves the body; the rectum is the section of the digestive tract above the anus where stool is held before it passes out of the body through the anus.
The anus is formed partly from the surface layers of the body, including the skin, and partly from the intestine. The rectal lining consists of glistening orange-tan tissue containing mucus glands--much like the rest of the intestinal lining. The lining of the rectum is relatively insensitive to pain, but the nerves from the anus and nearby external skin are very sensitive to pain.
The veins from the rectum and anus drain into the portal vein, which leads to the liver, and then into the general circulation. The lymph vessels of the rectum drain into lymph nodes in the lower abdomen; those of the anus drain into the lymph nodes in the groin.
A muscular ring (anal sphincter) keeps the anus closed. This sphincter is controlled subconsciously by the autonomic nervous system (see Section 6, Chapter 76); however, the lower part of the sphincter can be relaxed or tightened at will.
To diagnose disorders of the anus and rectum, a doctor inspects the skin around the anus for any abnormality. With a gloved finger, the doctor probes the rectum. For women, this is often done along with a manual examination of the vagina.
Next, a doctor looks into the anus and rectum with a 3- to 10-inch rigid viewing tube (anoscope or proctoscope). A longer, flexible tube (sigmoidoscope (see Section 9, Chapter 119)) may then be inserted so that the doctor can observe as much as 2 or more feet of the large intestine. An anoscopy or sigmoidoscopy is generally uncomfortable but not painful; however, if the area in or around the anus proves to be painful because of an abnormal condition, a local, regional, or even general anesthetic may be given before examination proceeds. Sometimes a cleansing enema to rid the lower part of the large intestine of stool is given before sigmoidoscopy. Tissue and stool samples for microscopic examination and cultures may be obtained during sigmoidoscopy. A barium enema x-ray may also be performed.
| 
