Part of
Large Intestine |
Short Description |
1. |
Ileum |
At about 4 metres (13 feet) in length, the ileum is the final section of the small intestine. It follows the duodenum and jejunum and is separated from the cecum by the ileocecal valve. The functions of the ileum include absorbing vitamin B12, bile salts and any products of digestion that were not absorbed by the jejunum. Undigested food, waste and water pass onwards from the ileum to the colon part of the large intestine by the action of peristalsis. |
2. |
Ileocecal sphincter (valve) |
The ileocecal valve (also known as the ileocaecal valve) is located at the junction of the ileum of the small intestine and the colon of the large intestine. The ileocecal valve restricts the reflux of the contents of the colon back into the ileum. |
3. |
Caecum (= cecum) |
The caecum, which also called the cecum, is a pouch that connects the ileum with the ascending colon of the large intestine. |
4. |
Appendix |
The appendix, which is also known as the vermiform appendix, is a blind-ended tube connected to the caecum.
The human appendix often described as a vestigial structure, meaning that it has lost most or all of its original function through the process of evolution*, so that it is now only a shrunken evolutionary remainder of a structure that had an important function in an ancestor of humans.
* Although some people do not accept the theory of evolution this medical view is generally expected in exam answers for life science courses that include the human digestive system.
Some people have speculated that it might have contained bacteria that helped with the digestion of cellulose molecules in plants. Modern humans can't digest more than a very small amount of cellulose per day. |
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5. |
Mesentery |
The mesentery labelled on the diagram above is not really part of the large intestine but is attached to the ileum (included in the diagram because it is the route by which material enters the large intestine from the small intestine).
The mesentery connects the jejunum and ileum (parts of the small intestine) to the back wall of the abdomen. It consists of two layers of peritoneum between which are blood vessels, lymph vessels, and nerves. |
6. |
Colon |
The colon is the last major part of the human digestive system.
It consists of four sections:
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7. |
Ascending Colon |
The ascending colon is the first of the four main sections of the colon. It ascends (i.e. "goes upwards") on the right side of the abdomen as far as the hepatic flexure which is located at the underside of the liver where the colon makes a left turn into the transverse (section of) the colon. |
8. |
Transverse Colon |
The transverse colon is the second of the four main sections of the colon. It traverses (i.e. "goes across") from the hepatic flexure on the right-side of the abdomen to the splenic flexure on the left-side of the abdomen where the colon turns downwards into the descending (section of) the colon. |
9. |
Descending Colon |
The descending colon is the third of the four main sections of the colon. It descends (i.e. "goes downwards") on the left side of the abdomen from the splenic flexure located beneath the lower border of the spleen, as far as the sigmoid (section of) the colon. |
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10. |
Sigmoid Colon |
The sigmoid colon is the fourth of the four main sections of the colon. It begins near the iliac crest of the left hip / pelvic bone and forms a loop that terminates in the rectum. |
11. |
Rectum |
The human rectum is often defined as the last 6 to 8 inches (approx 15 cm) of the adult large intestine.
However, that definition does not include (as part of the large intestine) the anal canal to which the rectum is attached. The function of the rectum is to store solid indigestible material and gases until its contents leaves the body via the anal canal and then the anus. |
12. |
Anal Canal |
The anal canal is the terminal part of the large intestine. It is located between the rectum and the anus, some texts say "extending from the anorectal junction to the anus".
The anal canal has three parts, the zona columnaris (upper half of the anal canal, above the pectinate line) and the zona hemorrhagica and zona cutanea below the pectinate line and separated by Hilton's white line. It is directed downwards and backwards. |
13. |
Anus |
The anus is the external opening of the rectum. Its opening / closure is controlled by sphincter muscles.
The function of the anus is the expulsion from the body of faeces, the process of which is called defecation. |
14. |
Right colic flexure (hepatic flexure) |
The right colic flexure is the sharp bend between the ascending colon and the transverse colon.
It is also known as the hepatic flexure because it is located next to the liver. |
15. |
Left colic flexure (splenic flexure) |
The left colic flexure is a sharp bend between the transverse colon and the descending colon.
It is also known as the splenic flexure because it is located near to the spleen. |
16. |
Taeniae coli |
The taenia coli (also known as the teniae coli, terms derived from the Latin for "flat band") is a band of longitudinal smooth muscle that passes along the outside of each of the four sections of the colon - i.e. the ascending, transverse, descending and sigmoid colons.
It consists of three separate longitudinal ribbons of smooth muscle called:
- the mesocolic coli, or mesocolic taenia
- the free coli or free taenia,
and
- the omental coli or omental taenia
These three ribbons or "bands" of smooth muscle are visible on the outer-surface of the large intestine and shown in the diagram above, together with some appendices epiploicae.
The teniae coli contracts lengthwise to produce haustra, which are the bulges in the sections of colon that form the large intestine. |
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17. |
Epiploic appendages
(= epiploicae appendices) |
When looking at the outer surface of the large intestine some appendages are seen attached to the taenia coli. These are called epiploicae appendages or epiploicae appendices (in the plural, the singular terms* being epiploic appendage or epiploic appendix). These are sacs of omentum distended with fat.
*Different textbooks use slightly different words so it is useful to know and recognize both versions.
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18. |
Haustra |
The 4 main parts of the colon (ascending colon, transverse colon, descending colon and sigmoid colon) are drawn as sections that appear to consist of a series of bulges - see diagram above. These rounded "bulges" are called haustra (plural) and are formed due to contractions of the taeniae coli gathering the main structure of the 4 sections of the colon into a series of pouches. |
19. |
Haustrum |
Descriptions of the anatomy of the large intestine generally mention the haustra (as above, a plural term). It is useful to also know the singular word, which is haustrum. |
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