The lumbar plexus is formed by the anterior rami of spinal nerves L1 to L3 and part of L4 (see Chapter 4, pp. 398–401). The rest of the anterior ramus of L4 and the anterior ramus of L5 combine to form the lumbosacral trunk, which enters the pelvic cavity and joins with the anterior rami of S1 to S3 and part of S4 to form the sacral plexus (see Chapter 5, pp. 480–486).