2.3.5. Large Intestine

The large intestine extends from the ileocecal junction to the anus and is about 1.5m long. On the surface, you can identify bands of longitudinal muscle fibers called taeniae coli, each about 5mm wide. There are three bands and they start at the base of the appendix and extend from the cecum to the rectum. Along the sides of the taeniae, you will find tags of peritoneum filled with fat, called epiploic appendages (or appendices epiploicae). The sacculations, called haustra, are characteristic features of the large intestine, and distinguish it from the rest of the intestinal tract.

Its function is to absorb water from the remaining indigestible food matter, and then to pass useless waste material from the body.

COLON_Image4_eng_namesThe large intestine consists of the following parts:

1. Cecum. about 6cm long and is a blind cul-de-sac which lies in the right iliac fossa. It is the part of the colon below the opening of the ileum into the colon. The cecum lies immediately behind the abdominal wall and greater omentum. There is frequently a peritoneal recess behind the cecum called the retrocecal recess and the appendix is sometimes hiding within this recess and may extend as far superiorly as the liver.

Hanging off the cecum is the vermiform appendix which opens into the cecum about 2cm below the ileocecal opening. The average length of the appendix is about 10cm and may lie in different positions. It has its own mesentery called themesoappendix which carries the appendicular artery.

If the cecum is opened, you can identify the opening of the ileum into the cecum. This opening is surrounded by thickened muscle which forms the iliocolic valve. In this image, you can see the first part of the ascending colon with its semilunar folds.

 2. Ascending colon. The ascending colon is smaller in caliber than the cecum from where it starts. It passes upward, opposite the colic valve, to the under surface of the right lobe of the liver, on the right of the gall-bladder, where it is lodged in a shallow depression, the colic impression; here it bends abruptly forward and to the left, forming the right colic flexure (hepatic) where it becomes the transverse colon.

3. Transverse colon.  the longest and most movable part of the colon, passes with a downward convexity from the right hypochondrium region across the abdomen, opposite the confines of the epigastric and umbilical zones, into the left hypochondrium region, where it curves sharply on itself beneath the lower end of the spleen, forming the splenic or left colic flexure. Theright colic flexure is adjacent to the liver.

4. Descending colon. The descending colon of humans passes downward through the left hypochondrium and lumbar regions, along the lateral border of the left kidney.

5. Sigmoid colon. The sigmoid colon (pelvic 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 thepelvis, but on account of its freedom of movement it is liable to be displaced into the abdominal cavity.

6. Rectum.  is the final straight portion of the large intestine in some mammals, and the gut in others. The human rectum is about 12 centimetres (4.7 in) long,  and begins at the rectosigmoid junction (the end of the sigmoid colon), at the level of the third sacral vertebra or the sacral promontory depending upon what definition is used.  Its caliber 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 (the level of the puborectalis sling) or the dentate line, again depending upon which definition is used. In humans, the rectum is followed by the anal canal, before the gastrointestinal tract terminates at the anal verge.

7. Anal canal. is the external opening of the rectum. Like other animals, its closure is controlled by sphincter muscles. Feces are expelled from the body through the anus during the act of defecation, the primary function of the anus.

 

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