Large Intestine and Its Histology - Practice Questions & MCQ

• 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.

• 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.
   

• 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.