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The Digestive System / The Large Intestine
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The large intestine serves as a reservoir for the liquids emptied into it, through the ileocecal valve, from the small intestine. It has a much larger diameter than the small intestine. The large intestine, or colon, may be divided into the cecum, ascending colon, transverse colon, descending colon, and sigmoid colon. The primary function of the colon is to absorb water and electrolytes (substances, such as salts, that in solution take on an electrical charge) from the ileal contents and to store fecal material until it can be evacuated by defecation.
 The cecum, the first part of the large intestine, is a sac with a closed end. It occupies the right iliac fossa, the hollow of the inner side of the ilium (the upper part of the hipbone). The cecum, also spelled caecum, is a pouch or large tubelike structure in the lower abdominal cavity that receives undigested food material from the small intestine and is considered the first region of the large intestine. It is separated from the ileum (the final portion of the small intestine) by the ileocecal valve (also called Bauhin's valve), which limits the rate of food passage into the cecum and may help prevent material from returning to the small intestine.
In humans, the cecum's main functions are to absorb fluids and salts that remain after completion of intestinal digestion and absorption and to mix its contents with a lubricating substance, mucus. The cecum's internal wall is composed of a thick mucous membrane through which water and salts are absorbed. Beneath this lining is a deep layer of muscle tissue that produces churning and kneading motions.
The structure and function of the cecum varies in other animals. Vertebrates such as rabbits and horses, which live on a diet composed only of plant life, have a larger cecum that is an important organ of absorption and contains bacteria that help digest cellulose. Animals that eat only meat have a reduced or absent cecum. In cats and dogs, muscle contractions of the cecum are much more vigorous and are reversible. Materials already passed to the next region of the large intestine can be brought back to the cecum for mixing with new food substances.
Although the ileocecal valve isn’t an organ or gland, it can cause a myriad of symptoms and correct diagnosis of ileocecal valve syndrome is often missed by the medical profession. Knowing the function, symptoms, and problems, relating to this valve might save someone much invasive and expensive testing.
The ileocecal valve is at the very end of the small intestine (ileum) and connects it to the first part of the large intestine. If you drew a line from your umbilicus (your "belly button" to your right anterior superior iliac spine (the most prominent part of your pelvis in the front of your body), the valve would be located just below the midpoint of that line.
The ileocecal valve has two main functions. The first is to prevent the backflow of fecal contents from the colon to the small intestine. The second is to prevent the contents of the ileum from passing into the cecum prematurely.
Gastrin is a hormone produced when food is in the stomach. When the chyme with gastrin approaches the valve, the gastrin causes it to relax. Also following a meal, the gastroileal reflex will open the valve to let the chyme through. At other times, the valve remains shut. Irritation and/or distention of the cecum will keep the valve tightly constricted. The symptoms of ileocecal valve syndrome can be effectively treated by finding an experienced chiropractor in your area.
Natural health care practitioners commonly find that the ileocecal valve can get "stuck" open or closed. When this occurs, toxic waste products can back up into the small intestine causing various syndromes such as sudden back pain, migraine headaches, PMS, and allergies. When the valve is returned to a normal closed or open position, these syndromes resolve, many times quickly.
The ascending colon extends up from the cecum at the level of the ileocecal valve to the bend in the colon called the hepatic flexure, which is located beneath and behind the right lobe of the liver; behind, it is in contact with the rear abdominal wall and the right kidney. The ascending colon is covered by peritoneum except on its posterior surface.
The transverse colon is variable in position, depending largely on the distention of the stomach, but usually is located in the subcostal plane; that is, at the level of the 10th rib. On the left side of the abdomen it ascends to the bend called the splenic flexure, which may make an indentation in the spleen. The transverse colon is bound to the diaphragm opposite the 11th rib by a fold of peritoneum.
The descending colon passes down and in front of the left kidney and the left side of the posterior abdominal wall to the iliac crest, the upper border of the hipbone. The descending colon is more likely than the ascending colon to be surrounded by peritoneum.
The sigmoid colon is commonly divided into iliac and pelvic parts. The iliac colon stretches from the crest of the ilium, or upper border of the hipbone, to the inner border of the psoas muscle, which lies in the left iliac fossa. Like the descending colon, the iliac colon is usually covered by peritoneum. The pelvic colon lies in the true pelvis (lower part of the pelvis) and forms one or two loops, reaching across to the right side of the pelvis and then bending back and, at the midline, turning sharply downward to the point where it becomes the rectum.
The layers that make up the wall of the colon are similar in some respects to those of the small bowel; there are distinct differences, however. The external aspect of the colon differs markedly from that of the small bowel because of features known as the haustra, taeniae, and appendices epiploicae.
The haustra, bulges or sacculations, are formed by constricting circular furrows of varying depths. The three taeniae are long, narrow bands of longitudinal muscle fibres, about one centimetre in width, that are approximately equally spaced around the circumference of the colon. Between the thick bands of the taeniae there is a thin coating of longitudinal muscle fibres.
The appendices epiploicae are collections of fatty tissue beneath the covering membrane. On the ascending and descending colon they are usually found in two rows, whereas on the transverse colon they form one row.
The mucous membrane of the colon has a characteristic structure. It lacks the villi and the folds known as plicae circulares characteristic of the small intestine. It contains many solitary lymphatic nodules but no Peyer's patches. The surface epithelium is columnar, and there are many goblet cells. Characteristic of the colonic mucosa are deep tubular pits, increasing in depth toward the rectum.
 The arterial blood supply to the large intestine is supplied by branches of the superior and inferior mesenteric arteries (both of which are branches of the abdominal aorta) and the hypogastric branch of the internal iliac (which supplies blood to the pelvic walls and viscera, the genital organs, the buttocks, and the inside of the thighs). The vessels form a continuous row of arches from which vessels arise to enter the large intestine. Venous blood is drained from the colon from branches that form venous arches similar to those of the arteries. These eventually drain into the superior and inferior mesenteric veins, which ultimately join with the splenic vein to form the portal vein.
The innervation of the large intestine is similar to that of the small intestine.
The rectum, which is a continuation of the sigmoid colon, begins in front of the midsacrum (the sacrum is the triangular bone near the base of the spine and between the two hipbones). It ends in a dilated portion called the rectal ampulla, which in front is in contact with the rear surface of the prostate in the male and with the posterior vaginal wall in the female. Posteriorly, the rectal ampulla is in front of the tip of the coccyx (the small bone at the very base of the spine).
At the end of the pelvic colon, the mesocolon, the fold of peritoneum that attaches the colon to the rear wall of the abdomen and pelvis, ceases, and the rectum is then covered by peritoneum only at its sides and in front; lower down, the rectum gradually loses the covering on its sides, until only the front is covered. At about the junction of the middle and lower thirds of the rectum, about 7.5 centimetres from the anus, the anterior peritoneal covering is also folded back onto the bladder and the prostate or the vagina.
Near the termination of the sigmoid colon and the beginning of the rectum, the colonic taeniae spread out to form a wide external longitudinal muscle coat. At the lower end of the rectum muscle fibres of the longitudinal and circular coats tend to intermix. The internal circular muscle coat terminates in the thick rounded internal anal sphincter muscle. The smooth muscle fibres of the external longitudinal muscle coat of the rectum terminate by interweaving with striated muscle fibres of the levator ani, a broad muscle that forms the floor of the pelvis. A second sphincter, the external anal sphincter, is composed of striated muscle and is divided into three parts known as the subcutaneous, superficial, and deep external sphincters. Thus, the internal sphincter is composed of smooth muscle and is innervated by the autonomic nervous system, while the external sphincters are of striated muscle and have somatic (voluntary) innervation provided by nerves called the pudendal nerves.
The mucosal lining of the rectum is similar to that of the sigmoid colon but becomes thicker and better supplied with blood vessels, particularly in the lower rectum. In the rectal ampulla are two to three large crescent-like folds known as rectal valves. These folds, or valves, are caused by an invagination, or infolding, of the circular muscle and submucosa. The columnar epithelium of the rectal mucosa changes to the stratified squamous (scalelike) type in the lower rectum a few centimetres above the pectinate line, which is the junction between squamous mucous membrane of the lower rectum and the skin lining the lower portion of the anal canal.
Arterial blood is supplied by branches from the inferior mesenteric artery and the right and left internal iliac arteries. Venous drainage from the anal canal and rectum is provided by a rich network of veins called the internal and external hemorrhoidal veins.
 Movie: This shows the inner wall of the colon with it's rich supply of blood vessels just as the rest of the digestive tract. Large veins can be seen as dark twisted lines, while the colonoscope is moved back and forth in order to examine the entire wall of the bowel. ( QuickTime 739k)
Movie: This video shows a large tumor, or growth, inside the colon. It is bleeding a bit, and is around about 3/4 of the entire inner wall. It had to be surgically removed. ( QuickTime 1087k)
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