Sigmoid colon
Encyclopedia
The sigmoid 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
.
, where it is continuous with the iliac colon
, and passes transversely across the front of the sacrum
to the right side of the pelvis. (The name sigmoid aptly means S
-shaped.)
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
.
(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.
are the primary source for parasympathetic
innervation. Lumbar splanchnic nerves
provide sympathetic
innervation via the inferior mesenteric ganglion
.
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
.
Pelvis
In human anatomy, the pelvis is the lower part of the trunk, between the abdomen and the lower limbs .The pelvis includes several structures:...
, but on account of its freedom of movement it is liable to be displaced into the abdominal cavity
Abdominal cavity
The abdominal cavity is the body cavity of the human body that holds the bulk of the viscera. It is located below the thoracic cavity, and above the pelvic cavity. Its dome-shaped roof is the thoracic diaphragm , and its oblique floor is the pelvic inlet...
.
Path
It begins at the superior aperture of the lesser pelvisLesser pelvis
The lesser pelvis is that part of the space enclosed by the skeleton of the pelvis which is situated between the pelvic inlet and the pelvic floor. Some authors call this cavity the pelvic cavity...
, where it is continuous with the iliac colon
Iliac colon
The iliac colon is situated in the left iliac fossa, and is about 12 to 15 cm. long.It begins at the level of the iliac crest, where it is continuous with the descending colon, and ends in the sigmoid colon at the superior aperture of the lesser pelvis....
, and passes transversely across the front of the sacrum
Sacrum
In vertebrate anatomy the sacrum is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx...
to the right side of the pelvis. (The name sigmoid aptly means S
S
S is the nineteenth letter in the ISO basic Latin alphabet.-History: Semitic Šîn represented a voiceless postalveolar fricative . Greek did not have this sound, so the Greek sigma came to represent...
-shaped.)
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
Sacrum
In vertebrate anatomy the sacrum is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx...
, where it bends downward and ends in the rectum
Rectum
The rectum is the final straight portion of the large intestine in some mammals, and the gut in others, terminating in the anus. The human rectum is about 12 cm long...
.
Coverings
It is completely surrounded by peritoneumPeritoneum
The peritoneum is the serous membrane that forms the lining of the abdominal cavity or the coelom — it covers most of the intra-abdominal organs — in amniotes and some invertebrates...
(and thus is not retroperitoneal), which forms a mesentery
Mesentery
In anatomy, the mesentery is the double layer of peritoneum that suspends the jejunum and ileum from the posterior wall of the abdomen. Its meaning, however, is frequently extended to include double layers of peritoneum connecting various components of the abdominal cavity.-Mesentery :The...
(sigmoid mesocolon
Sigmoid mesocolon
The sigmoid mesocolon is the fold of peritoneum which retains the sigmoid colon in connection with the pelvic wall.Its line of attachment forms a V-shaped curve, the apex of the curve being placed about the point of division of the left common iliac artery....
), 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
Iliac colon
The iliac colon is situated in the left iliac fossa, and is about 12 to 15 cm. long.It begins at the level of the iliac crest, where it is continuous with the descending colon, and ends in the sigmoid colon at the superior aperture of the lesser pelvis....
and rectum, but enjoys a considerable range of movement in its central portion.
Innervation
Pelvic splanchnic nervesPelvic 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 hindgut.-Structure:...
are the primary source for parasympathetic
Parasympathetic nervous system
The parasympathetic nervous system is one of the two main divisions of the autonomic nervous system . The ANS is responsible for regulation of internal organs and glands, which occurs unconsciously...
innervation. Lumbar splanchnic nerves
Lumbar splanchnic nerves
The lumbar splanchnic nerves are splanchnic nerves that arise from the lumbar part of the sympathetic trunk and travel to an adjacent plexus near the aorta...
provide sympathetic
Sympathetic nervous system
The sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...
innervation via the inferior mesenteric ganglion
Inferior mesenteric ganglion
The inferior mesenteric ganglion is a ganglion located near where the inferior mesenteric artery branches from the abdominal aorta....
.
Relations
Behind the sigmoid colon are the external iliac vessels, the left Piriformis, and left sacral plexusSacral plexus
-External links:*...
of nerves.
In front, it is separated from the bladder
Urinary bladder
The urinary bladder is the organ that collects urine excreted by the kidneys before disposal by urination. A hollow muscular, and distensible organ, the bladder sits on the pelvic floor...
in the male
Male
Male refers to the biological sex of an organism, or part of an organism, which produces small mobile gametes, called spermatozoa. Each spermatozoon can fuse with a larger female gamete or ovum, in the process of fertilization...
, and the uterus
Uterus
The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...
in the female
Female
Female is the sex of an organism, or a part of an organism, which produces non-mobile ova .- Defining characteristics :The ova are defined as the larger gametes in a heterogamous reproduction system, while the smaller, usually motile gamete, the spermatozoon, is produced by the male...
, by some coils of the small intestine
Small intestine
The small intestine is the part of the gastrointestinal tract following the stomach and followed by the large intestine, and is where much of the digestion and absorption of food takes place. In invertebrates such as worms, the terms "gastrointestinal tract" and "large intestine" are often used to...
.