Large intestine

The large intestine is formed of the caecum, appendix, colon and rectum. It coils around the small intestine and is characterised by:

• three bands of longitudinal muscle

• deep, longitudinal folds of mucosa which increase in the rectum

• numerous tubular glands which secrete mucus from their goblet cells.

Transverse colon

Transverse colon

Ascending colon Appendix

Rectum

Fig 11.6 The large intestine

Parts of the large intestine:

Descending colon

Sigmoid colon

Caecum

Small pouch to which the appendix is attached and into which the ileum opens through the ileo-caecal valve

Appendix

Sac attached to caecum of large intestine No known function in humans

Colon

Main part of the large intestine and is divided into four sections - ascending, transverse, descending and sigmoid colons

Ascending colon - this is the part that passes upwards on the right side of the abdomen from the caecum (a pouch at the junction of the small and large intestines) to the lower edge of the liver

Transverse colon - this is the longest and most mobile part and extends across the abdomen from right to left below the stomach

Descending colon - this is the part that passes downwards along the left side of the abdominal cavity to the brim of the pelvis

Sigmoid colon - this is the S-shaped part of the large intestine between the descending colon and the rectum

Rectum

Last part of the large intestine extending from the sigmoid colon to the anal canal. It is firmly attached to the sacrum and ends about 5 cm below the tip of the coccyx where it becomes the anal canal. Faeces are stored in the rectum before defecation

Anus

An opening at the lower end of the alimentary canal. It is the anal canal through which faeces are discharged. The anus is guarded by two sphincter muscles:

• internal sphincter - this is composed of smooth muscle under involuntary control

• external sphincter - this is composed of skeletal muscle under voluntary control The anus remains closed except during defecation

The functions of the large intestine are:

• absorption of most of the water from the faeces in order to conserve moisture in the body

• formation and storage of faeces which consists of undigested food, dead cells and bacteria

• production of mucus to lubricate the passage of faeces

• the expulsion of faeces out of the body through the anus.

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