Mesentery

Overview
Mesentery is, in anatomy, the double layer of peritoneum that connects a part of the small intestine to the posterior wall of the abdomen. Its meaning, however, is frequently extended to include double layers of peritoneum connecting various components of the abdominal cavity.

Mesentery (proper)
The mesentery proper (i.e. the original definition) refers to the peritoneum responsible for connecting the jejunum and ileum, parts of the small intestine, to the back wall of the abdomen. Between the two sheets of peritoneum are blood vessels, lymph vessels, and nerves. This allows these parts of the small intestine to move relatively freely within the abdominal cavity. The brain, however, cannot map sensation accurately, so sensation is usually referred to the midline, an example of referred pain.

Development
The mesentery is derived from what is known in the embryo as the dorsal mesentery. The dorsal mesentery is larger than the ventral mesentery, which gradually becomes other parts of the peritoneum. Most parts of the ventral mesentery are associated with the liver.

Mesentery (general)
Mesenteries are composed of two layers of peritoneum. The peritoneum that lies on the walls of the abdominopelvic cavity (parietal peritoneum) invaginates at certain parts, with an organ inside this invagination. This invaginated peritoneum (visceral peritoneum) will often surround all but a part of the organ ("bare area"), through which the organ transmits blood vessels and nerves. If this organ is invaginated far enough into the peritoneum, the visceral peritoneum will come in contact with itself, forming the organ's mesentery.

Mesenteries in the body: The fetal pig is a good reference for this subject
 * Mesentery (proper) - surrounds parts of the small intestine (the jejunum and the ileum)
 * Mesocolon - surrounds parts of the colon
 * Meso-appendix - peritoneum of the vermiform appendix
 * Transverse mesocolon - peritoneum of the transverse colon
 * Sigmoid mesocolon - peritoneum of the sigmoid colon
 * Broad ligament of the uterus - peritoneum of the uterus, uterine tubes, and ovaries

Invertebrate anatomy
In invertebrate animals, the term mesentery is used for any tissue that divides the body cavity (coelom) into partitions.