The region behind the knee. The muscles attached to the bones immediately above and below the knee bound a diamond-shaped space through which pass the main artery and vein of the limb (known in this part of their course as the popliteal artery and vein); the tibial and common peroneal nerves (which are the continuation of the sciatic nerve from the thigh down to the leg); the external saphenous vein; and several small nerves and lymphatic vessels. The muscles – which bound the upper angle of the space and which are attached to the leg bones by strong prominent tendons – are known as the hamstrings. The lower angle of the space lies between the two heads of the gastrocnemius muscle, which makes up the main bulk of the calf of the leg.