[Location] Localized in the prone position. In the region of the sacrum, between the posterior superior iliae spine and the midline of the back, in the first posterior sacral foramen. ( Fig.62)
 
[Regional Anatomy] Skin→subcutaneous tissue→the superficial layer of the thoracolumbar fascia→the erector spinae→the first posterior sacral foramen. Superficial layer, the distribution of the intermediate gluteal cutaneous nerve; deep layer, the first sacral nerve, and the posterior branches of the lateral sacral artery and vein.