The terminal junction of the ileum joins the large intestine at the ileocecal valve. It is approximately 4 cm long in an adult and consists of a ring of thickened smooth muscle. Relaxation and contraction of this sphincter controls the rate of entry of chyme into the colon. The ileocecal valve may also have an important role in preventing the movement of bacteria from the large bowel into the ileum. This sphincter is normally closed, but when peristalsis takes place in the terminal ileum, distension of the region causes a reflex relaxation of the sphincter muscle. This allows a small amount of chyme to enter the large intestine. The rate of entry into the colon is appropriately slow to allow salt and water absorption from the chyme before the next portion of chyme enters. The wall of the small intestine is composed of the following layers:
• Serosa: an outer layer composed of peritoneum and connective tissue. • Muscularis: containing outer longitudinal and inner circular layers of muscle, separated by the myenteric plexus nerve network. • Submucosa: connective tissue, which contains blood vessels, lymphatic tissue and a submucosal nerve plexus. • Mucosa: inner mucosa layer.