[Location] Localized in the prone position. In the region of the sacrum, medial and inferior to BL33 (Zhongliao), in the fourth posterior sacral foramen. (Fig.62)
 
[Regional Anatomy] Skin→subcutaneous tissue→the gluteus maximus→the erector spinae. Superficial layer, the distribution of the intermediate gluteal cutaneous nerve; deep layer, the branches or the subordiantial ramus of the superior and inferior gluteal arteries and veins, inferior gluteal nerve, the fourth sacral nerve, and the posterior branches of the lateral sacral artery and vein.