A 40-year-old man is admitted to the hospital for severe constant abdominal pain with nausea and vomiting for 1 day with the pain radiating to the back. Past medical history is insignificant. He drinks one or two 6-packs of beer each weekend. He denies having diarrhea or fever. The serum amylase and lipase levels are markedly elevated. What is the anatomical location of the structure involved?