The large intestine continues from the small intestine. It is divided into the caecum, colon and rectum. The colon, in turn, consists of the ascending, transverse, descending and sigmoid colons. The main function of the large intestine is elimination of feaces and absorption of water and vitamins. Its length is approximately 160cm. in live persons this length is somewhat greater due to considerable elasticity of tissues. The length of the caecum makes up about 4.66% of the whole large intestine. The ascending colon constitutes approximately 16.17%, the transverse colon – 34.55%; the descending colon is 13.72% and the sigmoid colon – 29.59%. The average diameter of the large intestine is 5-8cm, and it tends to decrease in the direction of the rectum. An empty large intestine of an adult weights approximately 370g.
The caecum continues from the ileum. It is shaped like a pouch, which hands downward. Occasionally the caecum can be cone-shaped. Its length is 4-8cm. Its posterior surface adjoins the iliac and major psoas muscles. The front surface adjoins the anterior abdominal wall. The caecum does not a mesentery. From the caecum extends the vermiform appendix, which pertains to the immune system. The appendix usually begins on the posteromedial surface of the caecum. Its length varies between 2 and 20 cm (on average 8cm). Its diameter is 0.5 -1.0cm. The appendix may begin on any other side of the caecum, and may extend in different directions. Usually it is situated within the right iliac fossa; its free end is turned down and medially, reaching the pelvic brim and sometimes entering the lesser pelvis. It can also be situated behind the caecum (retrocecal position) or may lie retroperitoneally. Normally the appendix has a mesentery, which connects it to the wall of the caecum and the end part of the ileum.
The ascending colon is 18-20cm long. Its position may vary. Its posterior wall occupies the right lateral section of the posterior abdominal wall. The ascending colon extends upward, passing first in front of the quadrate muscle of the abdomen, and then over the right kidney, which is situated retroperitoneally. Near the inferior/visceral surface of the liver the ascending colon turns forward and to the left, forming the right colic (hepatic) flexure.
The transverse colon usually passes across the abdomen in a downward arch pattern. It begins in the right subcostal region, at a level of the tenth costal cartilage, and ends in the left subcostal region. Its length is on average 50cm. (ranging between 40 and 50cm). within the left subcostal region, at a level of the ninth costal cartilage, the transverse colon forms the left (splenic) flexure, which is shaped like a sharp angle. The transverse colon is covered from all sides by the peritoneum and has a mesentery, which fixes it to the posterior wall of the peritoneal cavity. The anterior surface of the transverse colon is fixed to the greater curvature of the stomach and the upper part of the duodenum by the gastrocolic ligament, which is formed by the upper part of the greater omentum.
The top of the right colic flexure adjoins to the liver, and the left colic flexure – to the stomach and spleen. Below the transverse colon lie loops of the small intestine; behind it lies the duodenum and pancreas. When the stomach is empty the front surface of the transverse colon adjoins the anterior abdominal wall, and when it is full, the colon is pushed back.
The descending colon begins at the left colic flexure and continues into the sigmoid colon at the level of the iliac crest. The average length of the descending part is 25cm. the descending colon projects within the left lumbar region. The front surface of the descending colon adjoins the anterior abdominal wall. To its right lie the loops of the jejunum, and to its left – the left abdominal wall. To its right lie the loops of the jejunum, and to its left – the left abdominal wall. The descending colon is covered by the peritoneum from the front and sides (mesoperitoneal position).
The sigmoid colon begins at the level of the left iliac crest and ends at the level of the promontory, where it continues into the rectum. Its average length is 40-45cm (ranging between 12 and 84cm). the sigmoid colon forms 1-2 curves, which adjoin the front surface of the left iliac bone and partially descend into the pelvis. It lies intraperitoneally and has a mesentery the mesentery allows this colon considerable mobility.
The rectum is the terminal part of the large intestine. Its average length is 15cm, and the diameter varies between 2.5 and 7.5cm. The rectum consists of the ampoule and the anal canal lies inside the perineum. Behind the ampoule are the sacrum and coccyx. In front of it, in males, lie the prostate, urinary bladder, seminal vesicles and ampoules of the right and left deferent ducts; in females there are the uterus and vagina. The anal canal ends with the anus.
The rectum forms flexures in the sagittal plane. The sacral flexure is protrudes backward, and corresponds to the curvature of the sacrum. The perineal flexure protrudes forward, and is situated inside the perineum (in front of the coccyx0. Flexures of the rectum within the frontal plane are inconstant. The upper part of the rectum is covered with the peritoneum from all sides, the middle part-from only the three sides, and the lower part does not have a serosa lining.
In the region of the anal canal a thickening of muscularis layer forms the internal (involuntary) anal sphincter. Directly beneath the skin of this region is the voluntary external anal sphincter, which is one of the muscles of the perineum. Both sphincters close the anal orifice, opening it during defection.
An external characteristic of the large intestine is the presence of three muscle bands 3-6mm wide, called taenia coli. The free, mesocolic and omental taeniae coli begin near the base of the appendix and stretch up to the rectum. These bands form as a result of an unequal distribution of fascicles of the longitudinal muscle layer along the large intestine, which is thickened within these ribbons.
On the walls of the large intestine there are characteristic fingerlike, foliate or sack-like omental appendices, which are covered with by visceral peritoneum. Their average length is 3-4cm, and their density increases in the direction of the rectum. These appendages may serve for amortization during peristalsis and act as fat depots. Because the taeniae colons form bulging protrusions called haustra.
The walls of large are formed by the mucosa, submucosa, muscular layer and serosa (adventitia).
The mucosa of the large intestine forms numerous semilunar folds. Their height varies between several millimeters to 1-3cm. These folds are formed by both the mucosa and submucosa layers. In the ampoule of the rectum there are 8-10 longitudinal folds called anal columns. Between them are recesses called anal sinuses. Into their walls open the ducts of 5-38multicellular tubuloalveolar mucous glands, main sections of which are located inside the submucosa of the anal canal. The line drawn through the lower margins of the anal through the lower margins fo the anal columns and sinuses is called the anorectal line.
The mucosa of the large intestine is lined with simple columnar epithelium, which consists of columnar epithelioctytes (absorption cells), goblet cells and Endocrinocytes. At the level of the anal canal simple epithelium passes into stratified cuboidal epithelium. Even further, it transforms abruptly into stratified squamous non-keratinizing, and then keratinizing epithelium.
The lamina propria of mucosa is formed by loose connective tissue, inside it lie 7.5-12 million intestinal glands (crypts of Lierberkuhn), which perform both a secretory and absorptive functions. Approximately 4.5% of these glands are found in the caecum. 90% are in the colon; and 5.5% are in the rectum. The distribution of these glands has certain characteristics, for instance their concentration is higher (by 4-12%) at the levels of taeniae coli than between them. The glands situated on top of the semilunar folds and over sphincters tend to be larger. The walls of the glands are lined by simple epithelium, situated on a basement membrane. The epithelioctytes include primarily goblet cells and absorption cells. A constant element of the epithelium is stem cells. In some places there are also Endocrinocytes. The number of Endocrinocytes increases in the direction of the rectum. Among them are EC-cells, which produce serotonin and melatonin. A-cells, which produce glucagon, and D1 cells, which secrete the vasoactive intestinal peptide.
Along the length of the lamina propria of mucosa there are 5.5-6 thousand solitary lymphoid nodules. Also there are lymphoid and mast cells, and sometimes a few eosinophils and neutrophils. The lamina propria contains blood and lymph capillaries and vessels, non-myelinated nerve cells of the intramural nervous plexus and nerve fibers.
The muscularis of the mucosa is formed by two layers of smooth muscles. The internal layer is circular, and the external layer is oblique and longitudinal. Some fascicles of smooth muscle cells (10-30mm long and (0.2- 20mm in diameter) separate from the muscularis and penetrate the lamina propria. Thin muscle fascicles surround intestinal galnds and participate in secretion.
The submucosa is formed by loose fibrous connective tissue. It contains solitary lymphoid nodules, the submucosal (Meissner’s) plexus, blood and lymph vessels, and mucous glands (at the level of the anal canal)
The muscularis of the large intestine increases in thickness in the direction of the rectum. It is formed by a circular (internal) and a longitudinal (external) layer of muscles. Between these layers lies the myenteric (Auerbach’s) plexus, which is formed by the ganglion cells, glial cells (Schwann and satellite cells) and nerve fibers. Gangliocytes dominate in zones, which correspond to he taeniae coli. The internal part of the circular muscle layer is eh zone of formation of peristaltic waves, which are generated by interstitial (Cajal’s) cell, located inside the submucosa on the boarder with the muscle layer.
In some places of the large intestine, especially in transition segments between its different parts, the circular layer of the muscularis has thickenings. In these places its lumen may be narrowed, which serves to regulate the passage of chyme, or feaces through the intestine during digestion. These muscular thickenings are called colonic sphincters. The caeco-ascending sphincter is defined at the upper edge level of the ileocecal valve. The next sphincter (Girsh’s sphincter) is situated at the right colic flexure. There are three sphincters in the transverse colon. Near the beginning of the transverse colon is the right (Kennon’s) sphincter. The middle transverse and the left Kennon’s sphincters are located closer to the left colic flexure. Within the left colic flexure s the Payer’s sphincter. Another sphincter is located in the transition between the descending and sigmoid colons. Within the sigmoid colon there are the superior and inferior sigmoid sphincters. The sigmoido-rectal (O’Bernier’s) sphincter is situated between the sigmoid colon and rectum.
Between the large intestine and the abdominal walls and neighbouring organs there are ligaments, which are formed by folds of the peritoneum. These ligaments form a fixating apparatus, which prevents shifting and ptosis of the colon, and provide passages for blood vessels. The superior ileocecal ligament is a continuation of the mesentery. It attaches to the lower medial surfaces of the ascending colon. Its base is connected with the peritoneum of the right mesenteric sinus. The mesentericopudendal ligament beings on the mesentery of the terminal part of the ileum. It forms a triangular structure, which descends toward the right edge of the inlet into the lesser pelvis. In the female this ligament continues onto the suspensory ligament of the ovary, and in the males in to the deep ring of the inguinal canal, where it passes into the parietal peritoneum. The left diaphragmaticocolic ligament stretches between the costal part of the diaphragm and the left colic flexure. The bottom of this ligament attaches in the splenic angle between the transverse and descending colon connecting them to each other. Usually this ligament is accreted with the greater omentum. Other ligaments may be present but are not constant.
Innervation: colon – branches of vagus nerves and sympathetic fibers form the superior and inferior mesenteric plexuses; rectum – parasympathetic pelvic nerves and sympathetic fibers form the inferior hypogastric plexus.
Blood supply: colon – superior and inferior mesenteric arteries; rectum – rectal arteries form the inferior mesenteric and iliac arteries.
Venous outflow: colon – superior andinferior mesenteric veins; rectum – inferior mesenteric vein, inferior vena cava (through medial and inferior rectal veins)
Lymph outflow: caecum and appendix – ileocolic, prececal and retrocaecal lymph nodes. Ascending, transverse and descending colons – mesentericolic, paracolic, right, middle and left colic lymph nodes. Sigmoid colon – inferior mesenteric (sigmoid) lymph nodes. Rectum – internal iliac (sacral), subaortal and rectal lymph nodes.