An embalming tent at Camp Letterman that was set up after the Battle of Gettysburg, 1863.
The doctors in this photo are demonstrating embalming to two Union officers, with what appears to be some of their work displayed on either side. There is some question whether those are really soldiers posing as embalmed bodies for the photo though! The setup pictured was probably typical of that found near a battlefield or field hospital.
This is the stuff of nightmares. The American Civil War was the first “modern war” in the sense of weapons killing power, but the medical and tactical part of it was antiquated. (Which accounts for the high casualty counts.)