The large intestine, also known as the large bowel, is the last part of the gastrointestinal tract and of the digestive system in tetrapods. Water is absorbed here and the remaining waste material is stored in the rectum as feces before being removed by defecation. The colon is the longest portion of the large intestine, and the terms are often used interchangeably but most sources define the large intestine as the combination of the cecum, colon, rectum, and anal canal. Some other sources exclude the anal canal.
The cecum or caecum is a pouch within the peritoneum that is considered to be the beginning of the large intestine. It is typically located on the right side of the body. The word cecum stems from the Latin caecus meaning blind.
The levator ani is a broad, thin muscle group, situated on either side of the pelvis. It is formed from three muscle components: the pubococcygeus, the iliococcygeus, and the puborectalis.
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 35–40 centimetres (14–16 in) in length. The loop is typically shaped like a Greek letter sigma (ς) or Latin letter S. This part of the colon normally lies within the pelvis, but due to its freedom of movement it is liable to be displaced into the abdominal cavity.
The anal canal is the part that connects the rectum to the anus, located below the level of the pelvic diaphragm. It is located within the anal triangle of the perineum, between the right and left ischioanal fossa. As the final functional segment of the bowel, it functions to regulate release of excrement by two muscular sphincter complexes. The anus is the aperture at the terminal portion of the anal canal.
The external anal sphincter is an oval tube skeletal muscle fibers. Distally, it is adherent to the skin surrounding the margin of the anus. The sphincter exhibits a resting state of tonical contraction.
The internal anal sphincter, IAS, is a ring of smooth muscle that surrounds about 2.5–4.0 cm of the anal canal. It is about 5 mm thick, and is formed by an aggregation of the smooth (involuntary) circular muscle fibers of the rectum. it terminates distally about 6 mm from the anal orifice.
The inferior rectal artery is an artery that supplies blood to the lower third of the anal canal below the pectinate line.
The pelvic cavity is a body cavity that is bounded by the bones of the pelvis. Its oblique roof is the pelvic inlet. Its lower boundary is the pelvic floor.
The rectal venous plexus is the venous plexus surrounding the rectum. It consists of an internal and an external rectal plexus. It is drained by the superior, middle, and inferior rectal veins. It forms a portosystemic (portocaval) anastomosis. This allows rectally administered medications to bypassing first pass metabolism.
The cloacal membrane is the membrane that covers the embryonic cloaca during the development of the urinary and reproductive organs.
The Inferior rectal nerves usually branch from the pudendal nerve but occasionally arises directly from the sacral plexus; they cross the ischiorectal fossa along with the inferior rectal artery and veins, toward the anal canal and the lower end of the rectum, and is distributed to the Sphincter ani externus and to the integument (skin) around the anus.
The transverse folds of rectum are semi-lunar transverse folds of the rectal wall that protrude into the rectum, not the anal canal as that lies below the rectum. Their use seems to be to support the weight of fecal matter, and prevent its urging toward the anus, which would produce a strong urge to defecate. Although the term rectum means straight, these transverse folds overlap each other during the empty state of the intestine to such an extent that, as Houston remarked, they require considerable maneuvering to conduct an instrument along the canal, as often occurs in sigmoidoscopy and colonoscopy.
The pelvic fasciae are the fascia of the pelvis and can be divided into:
An anal sinus is a furrow formed between any two adjacent anal columns of the anal canal. An anal sinus is limited inferiorly by an anal valve. The anal glands open into the anal sinuses.
An anal valve is a small horizontal fold uniting the inferior ends of any two adjacent anal columns of the anal canal. An anal valve forms the inferior boundary of an anal sinus. The anal valves demarcate the level of the pectinate line.
Defecography is a type of medical radiological imaging in which the mechanics of a patient's defecation are visualized in real time using a fluoroscope. The anatomy and function of the anorectum and pelvic floor can be dynamically studied at various stages during defecation.
The rectum is the final straight portion of the large intestine in humans and some other mammals, and the gut in others. The adult human rectum is about 12 centimetres (4.7 in) long, and begins at the rectosigmoid junction at the level of the third sacral vertebra or the sacral promontory depending upon what definition is used. Its diameter is similar to that of the sigmoid colon at its commencement, but it is dilated near its termination, forming the rectal ampulla. It terminates at the level of the anorectal ring or the dentate line, again depending upon which definition is used. In humans, the rectum is followed by the anal canal which is about 4 centimetres (1.6 in) long, before the gastrointestinal tract terminates at the anal verge. The word rectum comes from the Latin rectumintestinum, meaning straight intestine.
In humans, the anus is the external opening of the rectum, located inside the intergluteal cleft and separated from the genitals by the perineum. Two sphincters control the exit of feces from the body during an act of defecation, which is the primary function of the anus. These are the internal anal sphincter and the external anal sphincter, which are circular muscles that normally maintain constriction of the orifice and which relaxes as required by normal physiological functioning. The inner sphincter is involuntary and the outer is voluntary. It is located behind the perineum which is located behind the vulva or scrotum.
The rectococcygeal muscles are two bands of smooth muscle tissue arising from the 2nd and 3rd coccygeal vertebrae, and passing downward and forward to blend with the rectal longitudinal smooth muscle fibers on the posterior wall of the anal canal.
