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 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
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 faeces simultaneously.
Coverings The sigmoid colon 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.
Nerve supply 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, 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. ==Clinical significance==