[Location] Localized in the prone position. In the region of the sacrum, medial and inferior to the posterior superior iliac spine, in the second posterior sacral foramen. (Fig.62)
 
[Regional Anatomy] Skin→subcutaneous tissue→the erector spinae→the second posterior sacral foramen. Superficial layer, the distribution of the intermediate gluteal cutaneous nerve; deep layer, the second sacral nerve, and the posterior branches of the lateral sacral artery and vein.