What is the nerve root that leaves the cord toward the rear of the body?
It depends on how specific you need the answer to be. These would collectively be referred to as sacral nerves. They form in the area known as the sacral plexus. The bundle is known as the cauda equina, and refers to the segments s1-s5. The band of nerves continues to form the sciatic nerve. S5 is the furthest toward the rear. S3,S4, and S5 are the nerves that supply the perineal or genital area. These nerves also supply the legs, bladder, and genital areas. A good description is provided in the second link of references below.
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