The sigmoid colon (or pelvic colon) is the part of the large intestine that is closest to the

rectum The rectum is the final straight portion of the large intestine in humans and some other mammals, and the Gastrointestinal tract, gut in others. The adult human rectum is about long, and begins at the rectosigmoid junction (the end of the s ...

and

anus The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, d ...

. It forms a loop that averages about in length. The loop is typically shaped like a Greek letter sigma (ς) or Latin letter S (thus ''

sigma Sigma (; uppercase Σ, lowercase σ, lowercase in word-final position ς; grc-gre, σίγμα) is the eighteenth letter of the Greek alphabet. In the system of Greek numerals, it has a value of 200. In general mathematics, uppercase Σ is used as ...

'' + ''

-oid In linguistics, a suffix is an affix which is placed after the stem of a word. Common examples are case endings, which indicate the grammatical case of nouns, adjectives, and verb endings, which form the conjugation of verbs. Suffixes can carry ...

''). This part of the colon normally lies within the

pelvis The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis, or pelvic skeleton). The ...

, but due to its freedom of movement it is liable to be displaced into the

abdominal cavity The abdominal cavity is a large body cavity in humans and many other animals that contains many organs. It is a part of the abdominopelvic cavity. It is located below the thoracic cavity, and above the pelvic cavity. Its dome-shaped roof is the th ...

Structure

The sigmoid colon begins at the superior aperture of the lesser pelvis, where it is continuous with the iliac colon, and passes transversely across the front of the

sacrum The sacrum (plural: ''sacra'' or ''sacrums''), in human anatomy, is a large, triangular bone at the base of the spine that forms by the fusing of the sacral vertebrae (S1S5) between ages 18 and 30. The sacrum situates at the upper, back part ...

to the right side of the pelvis. It then curves on itself and turns toward the left to reach the middle line at the level of the third piece of the

, where it bends downward and ends in the

. Its function is to expel solid and gaseous waste from the gastrointestinal tract. The curving path it takes toward the anus allows it to store gas in the superior arched portion, enabling the colon to expel gas without excreting faeces simultaneously.

Coverings

The sigmoid colon is completely surrounded by peritoneum (and thus is not retroperitoneal), which forms a

mesentery The mesentery is an organ that attaches the intestines to the posterior abdominal wall in humans and is formed by the double fold of peritoneum. It helps in storing fat and allowing blood vessels, lymphatics, and nerves to supply the intestines ...

( sigmoid mesocolon), which diminishes in length from the center toward the ends of the loop, where it disappears, so that the loop is fixed at its junctions with the iliac colon and rectum, but enjoys a considerable range of movement in its central portion.

Nerve supply

Pelvic splanchnic nerves Pelvic splanchnic nerves or nervi erigentes are splanchnic nerves that arise from sacral spinal nerves S2, S3, S4 to provide parasympathetic innervation to the organs of the pelvic cavity. Structure The pelvic splanchnic nerves arise from t ...

are the primary source for

parasympathetic The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of t ...

innervation. Lumbar splanchnic nerves provide sympathetic innervation via the inferior mesenteric ganglion.

Relations

Behind the sigmoid colon are the external iliac vessels, ovary, obturator nerve, the left Piriformis, and left sacral plexus of nerves. In front, it is separated from the bladder in the male, and the uterus in the female, by some coils of the

small intestine The small intestine or small bowel is an organ in the gastrointestinal tract where most of the absorption of nutrients from food takes place. It lies between the stomach and large intestine, and receives bile and pancreatic juice through the p ...

Clinical significance

Diverticulosis Diverticulosis is the condition of having multiple pouches (diverticulum, diverticula) in the colon (anatomy), colon that are not inflamed. These are outpockets of the colonic mucosa and submucosa through weaknesses of muscle layers in the col ...

often occurs in the sigmoid colon in association with increased intraluminal pressure and focal weakness in the colonic wall. It is a common cause of

hematochezia Haematochezia is the passage of fresh blood through the anus path, usually in or with stools (contrast with melena). The term is from Greek αἷμα ("blood") and χέζειν ("to defaecate"). Hematochezia is commonly associated with lower gastro ...

. Volvulus occurs when a portion of the bowel twists around its mesentery, which can lead to obstruction and infarction. Volvulus in the elderly commonly occurs in the sigmoid colon, whereas in infants and children it is more likely to occur in the midgut. This may correct itself spontaneously or the rotation may continue until the blood supply of the gut is cut off completely.

Additional images

Image:Dickdarm-Schema.svg, 1:

Ascending colon ''Ascending'' is a science fiction novel by the Canadian writer James Alan Gardner, published in 2001 by HarperCollins Publishers under its various imprints.HarperCollins, Avon, HarperCollins Canada, SFBC/Avon; paperback edition 2001, Eos Books. ...

2: Transverse colon
3:

Descending colon In the anatomy of humans and homologous primates, the descending colon is the part of the colon extending from the left colic flexure to the level of the iliac crest (whereupon it transitions into the sigmoid colon). The function of the descendin ...

4: Sigmoid colon
5:

Rectum The rectum is the final straight portion of the large intestine in humans and some other mammals, and the Gastrointestinal tract, gut in others. The adult human rectum is about long, and begins at the rectosigmoid junction (the end of the s ...

Image:Gray1076.png, Iliac colon, sigmoid or pelvic colon, and rectum seen from the front, after removal of pubic bones and bladder Image:WeiblichesBeckenMedian.gif, Sagittal section of the lower part of a female trunk, right segment

References

External links

* - "The large intestine." * {{Authority control Digestive system Large intestine