The large intestine is the terminal part of the alimentary canal.
The large intestine runs from the appendix to the anus. It frames the small intestine on three sides.
Despite its being about one-half as long as the small intestine, it is called large because it is more than twice the diameter of the small intestine, about 3 inches.

Parts of Large Intestine

1. Caecum:

It is the first part of the large intestine.
It is a sac-like structure that is suspended inferior to the ileocecal valve.
It receives the contents of the ileum and continues the absorption of water and salts.
The appendix (or vermiform appendix) is a winding tube that attaches to the cecum.
Although the 7.6-cm (3-in) long appendix contains lymphatic tissue, suggesting an immunologic function, this organ is generally considered vestigial.

2. Colon:

It is the longest segment of the large intestine.
It is 1.25 meter long in humans and has an inverted U-shaped structure.
The colon has longitudinal bands called taeniae coli and small pouches called haustra
It can be further divided into four subparts:
- Ascending colon: It ascends in the right side of the abdomen from the appendix to the lower margin of the liver.
- Transverse colon: The ascending colon bends and traverses from the right side of the abdomen and lower margin of the liver to the spleen.
- Descending colon: It descends down in the left side of the abdomen from the spleen to the left groin (superior level of pelvic girdle).
- Sigmoid colon: It is S-shaped, enters the pelvis and joins the rectum (in the midline of the body).

3. Rectum: Food residue leaving the sigmoid colon enters the rectum in the pelvis.

4. Anal Canal:

Finally, food residue reaches the last part of the large intestine, the anal canal, which is located in the perineum, completely outside of the abdominopelvic cavity.
The mucosa of the anal canal is folded into several vertical folds called anal columns.
The anal columns are also called columns of Morgagni.

Cardinal Features of Large Intestine

The colon has three longitudinal bands made up of longitudinal muscles. These are called taeniae coli.
Tonic contractions of the taniea coli bunch up the colon into a succession of pouches called haustra (singular = haustrum), which are responsible for the wrinkled appearance of the colon.
Attached to the taeniae coli are small, fat-filled sacs of visceral peritoneum called epiploic appendages. The purpose of this is unknown.

Histology of Large Intestine

The wall of the large intestine has a few enzyme-secreting cells.
There are no circular folds or villi.
Other than in the anal canal, the mucosa of the colon is simple columnar epithelium made mostly of enterocytes (absorptive cells) and goblet cells.
The wall of the large intestine has far more intestinal glands.
These goblet cells secrete mucus that eases the movement of faeces and protects the intestine from the effects of the acids and gases produced by enteric bacteria.
The enterocytes absorb water and salts as well as vitamins produced by intestinal bacteria.