Sigmoid colon

Sigmoid colon
Drawing of colon seen from front (sigmoid colon coloured blue)
Front of abdomen, showing surface markings for liver, stomach and large intestine
Latin	Colon sigmoideum
Gray's	subject #249 1182
Artery	Sigmoid arteries
Nerve	Inferior mesenteric ganglia and sacral nerve
Precursor	Hindgut

The sigmoid colon (pelvic colon) is the part of the large intestine that is closest to the rectum and anus. It forms a loop that averages about 40 cm in length, and normally lies within the pelvis, but on account of its freedom of movement it is liable to be displaced into the abdominal cavity.

Path

It 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 to the left side of the pelvis. (The name sigmoid aptly means S-shaped.)

It then curves on itself and turns toward the right to reach the middle line at the level of the third piece of the sacrum, where it bends downward and ends in the rectum.

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 feces simultaneously.

Coverings

It is completely surrounded by peritoneum (and thus is not retroperitoneal), which forms a mesentery (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.

Innervation

Pelvic splanchnic nerves are the primary source for parasympathetic innervation. Lumbar splanchnic nerves provide sympathetic innervation via the inferior mesenteric ganglion.

Relations

Behind the sigmoid colon are the external iliac vessels, 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.

Additional images

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  1: Ascending colon
  2: Transverse colon
  3: Descending colon
  4: Sigmoid colon
  5: Rectum

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  Intestines

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  Sigmoid colon and rectum, showing distribution of branches of inferior mesenteric artery and their anastomoses.

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  Iliac colon, sigmoid or pelvic colon, and rectum seen from the front, after removal of pubic bones and bladder.

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  Sagittal section of the lower part of a female trunk, right segment.

References

This article incorporates text from a public domain edition of Gray's Anatomy.

External links

• Search sigmoid+colon at eMedicine Dictionary
• SUNY Figs 37:06-07 - "The large intestine."
• Superior & Inferior Mesenteric Artery at The Anatomy Lesson by Wesley Norman (Georgetown University)