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Other Terms: Large bowel, Intestinum crassum, Gros intestin, Großer Darm, Intestino grande
The large intestine extends from the end of the ileum to the anus. It is about 1.5 meters long and is one-fifth of the whole extent of the digestive tract. It is largest at its commencement at the caecum and gradually diminishes as far as the rectum. Here, there is a dilatation of considerable size just above the anal canal. It differs from the small intestine in its greater caliber, its more fixed position, its sacculated form, and in possessing fatty appendages on its external coat. It also differs in that its longitudinal muscular fibers do not form a continuous layer around the gut but instead are arranged in three longitudinal bands called the taeniae coli. In its course the large intestine arches around the convolutions of the small intestine. It commences in the right iliac region in a dilated portion called the caecum. It ascends through the right lumbar and hypochondriac regions to the undersurface of the liver. It bends at the right colic flexure to the left and passes transversely across the abdomen on the confines of the epigastric and umbilical regions to the left hypochondriac region. It then bends again at the left colic flexure and descends through the left lumbar and iliac regions to the pelvis. Here, it bends forming an S-shape region called the sigmoid colon. From here it continues along the posterior wall of the pelvis to the anus. The large intestine is divided into the caecum, colon, rectum, and anal canal. The colon is further divided into the ascending, transverse, descending, and sigmoid colons.
The wall of the large intestine has four layers: tunica mucosa, tela submucosa, tunica muscularis, and tunica serosa. The tunica mucosa in the caecum and colon is pale, smooth, and destitute of villi. It is raised into numerous semiliunar folds which correspond to the intervals between the haustra. In the rectum, it is thicker, darker, more vascular, and connected loosely to the muscular coat. The tunica mucosa consists of a muscular layer, a quantity of restiform tissue in which the vessels ramify, a basement membrane and columnar epithelium. It resembles the epithelium found in the small intestine. The tela submucosa connects the tunica mucosa and tunica muscularis layers. It is the vascularization layer. The tunica muscularis consists of an external longitudinal and internal circular layer. The longitudinal fibers do not form a continuous layer over the entire surface of the large intestine. In the caecum and colon, they are collected into longitudinal bands known as the taeniae coli. In the sigmoid colon, the longitudinal fibers become more scattered. Around the rectum, they spread out and form a layer. The tunica serosa is derived from the peritoneum. It invests the different portions of the large intestine to a variable extent. The caecum is completely covered by tunica serosa. The ascending and descending parts of the colon are usually covered only in front and at the sides. A variable part of the posterior surface is uncovered. The transverse colon is almost completely invested. The sigmoid colon is entirely surrounded. The rectum is covered above on its anterior surface and sides.
The nerves are derived from the sympathetic plexuses around the branches of the superior and inferior mesenteric arteries. They are distributed in a similar way to those found in the small intestine.
The arteries supplying the colon are derived from the colic and sigmoid branches of the mesenteric arteries. They give off large branches, which ramify between and supply the muscular coats. After dividing into small vessels in the tela submucosa, they pass to the tunica mucosa. The rectum is supplied by the superior rectal artery which branches off the inferior mesenteric. The anal canal is supplied by the middle rectal artery from the internal iliac artery and the inferior rectal artery from the internal pudendal artery.
Veins of the rectum commence in a plexus of vessels which surround the anal canal. Within the vessels forming this plexus are smaller saccular dilatations just within the margin of the anus. From the plexus, about six vessels are given off. These ascend between the tunica muscularis and tunica mucosa for about 12.5 centimeters, running parallel to each other. They then pierce the muscular coat. By their union, they form a single trunk, the superior rectal vein. This arrangement is termed the rectal plexus. This plexus communicates with the tributaries of the middle and inferior rectal veins.
The caecum is the commencement of the large intestine. It is the large pouch situated below the colic valve. Its open end is directed upward and communicates directly with the colon. It is situated in the right iliac fossa above the lateral half of the inguinal ligament. It rests on the iliacus and psoas major and usually lies in contact with the anterior abdominal wall. It is usually entirely enveloped by peritoneum. The caecum lies quite free in the abdominal cavity and has a considerable amount of movement. The vermiform appendix is found off the inferior side of the the caecum.
The Ascending Colon
The ascending colon is continuous with the caecum. It passes upward, opposite the colic valve, to the under surface of the right lobe of the liver, on the right side of the gallbladder. Here, it is lodged in a shallow depression, the colic impression. It then bends abruptly forward and to the left, forming the right colic flexure. It is held in contact with the posterior wall of the abdomen by the peritoneum, which covers its anterior surface and sides. Its posterior surface is connected by loose connective tissue with the iliacus, quadratus lumborum, and with the front of the lower and lateral portions of the right kidney. Sometimes the peritoneum completely invests it and forms a distinct but narrow mesocolon.
The Transverse Colon
The transverse colon is the longest and most movable part of the colon. It passes with a downward convexity from the right hypochondriac region across the abdomen into the left hypochondriac region. Here, it curves sharply on itself beneath the lower end of the spleen, forming the left colic flexure. In its course it arches; the concavity of which is directed backward and little upward. Toward its splenic end, there is often and abrupt u-shaped curve which may descend lower than the main curve. It is almost completely invested by peritoneum. It is connected to the inferior border of the pancreas by the transverse mesocolon. It is in relation, by its upper surface, with the liver and gallbladder, the greater curvature of the stomach, and the lower end of the spleen. It is in relation by its undersurface with the small intestine. Its anterior surface is in relation with the anterior layers of the greater omentum. Its posterior surface is in relation from right to left with the descending portion of the duodenum, the head of the pancreas, and some of the convolutions of the jejunum and ileum.
The Descending Colon
The descending colon passes downward through the left hypochondriac and lumbar regions along the lateral border of the left kidney. At the lower end of the kidney, it turns medially toward the lateral border of the psoas, and then descends to the crest of the ileum, where it ends in the iliac colon. The peritoneum covers its anterior surface and sides, while its posterior surface is connect by connective tissue with the lower and lateral part of the left kidney. It is smaller in caliber and more deeply placed than the ascending colon. It is more frequently covered with peritoneum on its posterior surface than the ascending colon. In front of it are some coils of small intestine.
The Sigmoid Colon
The sigmoid colon forms a loop. It normally lies within the pelvis, but it can be displaced into the abdominal cavity. It is continuous with the descending colon. It 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 mid line at the level of the third piece of the sacrum, where it bends downward and ends in the rectum. It is completely surrounded by peritoneum which forms the sigmoid mesocolon. Behind the sigmoid colon are the external iliac vessels, the left piriformis, and left sacral plexus of nerves. In front , it is separate from the bladder in the male and the uterus in the femal by some coils of the small intestine.
The rectum is continuous above with the sigmoid colon. Below, it ends in the anal canal. From its origin at the level of the third sacra vertebra, it passes downward, lying in the sacrococcygeal curve, and extends for about 2.5 centimeters in front of the tip of the coccyx. It then bends sharply backward into the anal canal. The rectum is about twelve centimeters long. At its commencement, its caliber is similar to that of the sigmoid colon, but near its terminations, it is dilated to form the rectal ampulla. The rectum has no sacculations comparable to those of the colon. When the lower part of the rectum is contracted, its mucous membrane is thrown into longitudinal folds. The peritoneum is related to the upper two-thirds of the rectum. At first, it covers its front and sides, but lower down it only covers its front. The upper part of the rectum is in relation, posterior, with the superior rectal vessels, to the left the piriformis, and left sacral plexus of nerves. In its lower part it lies directly superior to the sacrum, coccyx, and the levator ani. Anterior, it is separated above, in the male, from the fundus of the bladder; in the female, from the intestinal surface of the uterus and its appendages, by some convolutions of the small intestine, and frequently by the sigmoid colon. Inferior, it is in relation in the male with the triangular portion of the fundus of the bladder, the seminal vesicle, and ductus deferens. In the female, it is in relation with the posterior wall of the vagina.
The Anal Canal
The anal canal begins at the level of the apex of the prostate. It is directed downward and backward and ends at the anus. It forms an angle with the lower part of the rectum and measures from 2.5 to 4 centimeters in length. It has no peritoneal covering, but is invested by the external anal sphincter. In the male, are the membranous portion and bulb of the urethra and the fascia of the urogenital diaphragm separated from them by connective tissue. In the female it is separated from the lower end of the vagina by a mass of muscular and fibrous tissue, the perineal body. The lumen of the anal canal presents a number of vertical folds known as rectal columns. These columns are separated from one another by rectal sinuses.