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To identify the mesentery of the jejunum and ileum, pull the loops of small intestine to the left. The mesentery is composed of two layers of the peritoneum with various amounts of fat, blood and lymphatic vessels, and nerves between them. The intestinal border of the mesentery, which is about 18 to 21 feet in length, attaches to the jejunum and ileum. The root of the mesentery is attached to the posterior abdominal wall and is about 8-12 inches in length. It runs obliquely from the left side of the vertebral column from the level of L2 vertebra towards the cecum located in the right iliac fossa.

Links and References:
Grant's: 2.71, 2.76
Netter: 253, 254
Rohen/Yokochi: 287, 288, 290