The sciatic nerve is the largest nerve in the body, and consists of the medially placed tibial nerve and the laterally placed common peroneal nerve.  It is formed from the ventral rami of L4, L5, S1, S2 and S3, and is a continuation of the upper band of the sacral plexus.  

It leaves the pelvis through the greater sciatic foramen, below the piriformis muscle, and descends between the greater trochanter of the femur and the ischial tuberosity.  Initially deep to piriformis, it runs inferiorly and laterally posterior to the ischium, crossing over the nerve to quadratus femoris (Inferior to piriformis, it lies deep to the gluteus maximus muscle).  It passes inferiorly crossing the obturator internus, the gemelli and quadratus femoris muscles.  The posterior cutaneous nerve of thigh and the inferior gluteal artery lie on its medial side.  Descending vertically, it enters the thigh at the lower border of gluteus maximus, where it lies on the posterior surface of adductor magnus.  It gives off nerves to the hamstring muscles.  The nerve is crossed obliquely on its superficial aspect by the long head of biceps femoris.  The nerve ends at the upper aspect of the popliteal fossa by dividing into the tibial (ventral divisions of the ventral rami of L4, L5, S1, S2, S3) and the common peroneal nerves (dorsal divisions of the ventral rami of L4, L5, S1, S2).  

The nerve can be represented on the back of the thigh by a line drawn from just medial to the midpoint of the line from the ischial tuberosity to the apex of the greater trochanter down to the apex of the popliteal fossa.  

Supply
It supplies articular branches to the hip joint, with muscular branches to biceps femoris, semitendinosus and semimembranosus and the ischial head of adductor magnus.  The nerve to the short head of biceps is from the common peroneal division, with the other muscular branches emerging from the tibial division.