The large intestine or colon is continuous with the small intestine and ends at the anus. It is about 1.5 meters (5 ft. long) and consists of the following parts:
2. The ascending colon
3. The transverse colon
4. The descending colon
5. The pelvic or sigmoid colon
6. The Rectum
7. The anal canal
The caecum is the first part of the colon and is a dilated portion which has a blind lower end and is continuous above with the ascending colon. Just below the junction of the two, the ileocaecal valve opens. This valve is a sphincter and prevents the caecal contents passing back into the ileum.
The Vermiform appendix is a fine tube closed at one end, which opens out of the caecum, about 2 cm below the ileo-caecal valve. It is usually about 13 cm (5 inches) long and has the same structure as the walls of the colon but contains more lymphoid tissues.
The ascending colon passes upwards from the caecum to the level of the liver where it bends acutely to the left of at the right colic flexure to become the transverse colon.
The transverse colon is about 50 cm in length and passes across the abdomen to the under surface of the spleen. Where it forms the left colic flexure, by bending acutely downwards to become the descending colon.
The descending colon is about 25 cm in length and passes down the left side of the abdomen to the inlet of the lesser pelvis, where it becomes the sigmoid colon.
The pelvic or sigmoid colon has an S-shaped curve in the pelvis and it continues downwards to become the rectum.
The Rectum is about 12 cm long and is a slightly dilated part of the colon. It leads from the pelvic colon and terminates in the anal canal.
The anal canal is a short canal about 3.8 cm (11/2 inches) long in adults and leads from the rectum to the exterior. There are two sphincter muscles which controls the anus- The internal sphincter surrounds the upper the three quarters of the canal and consists of smooth muscle fibers. The external spinster and consists of striated muscle. It is the tone of these sphincters which keep the anal canal and the anus Closed.
In structure, the large intestine consists of the same four layers of the alimentary canal as described above with a few modifications.
1. The arrangement of the longitudinal muscle fiber is modified in the colon. They do not form a smooth continuous layer of tissues, but are collected into three bands called taenia coli situated at regular intervals round the colon. These bands are shorter than the other layers of the large intestine and so produce a typical puckered or sacculated appearance.
2. In the sub mucous layer, there are more lymphoid tissues than in any other part of the alimentary canal.
3. The mucus lining of the colon and the upper part of the rectum contains large number of goblet cells, which secrets mucus.
Functions of the large intestine:
In the colon, water, mineral, salts and some drugs are absorbed into the blood capillaries.
Colon has only one secretion, mucin which lubricates the faeces and facilitates their passage through the rectum and anus.
Many bacteria are present here which act on various food residues which have not been digested or absorbed in the small intestine.
Excess of calcium, iron and drugs of heavy metals, such as bismuth, are excreted from the walls of the large intestine and mix with the faeces.
Defecation is the process of emptying the rectum or the passage of faeces out of the body. This is achieved by the gastro-colic reflex, which occurs by reflex action with the infant whereas in adults, is under the control of the will and is carried out in response to the desire to empty the bowel produced by distension of the rectum with faeces.