Medical Latin phrase that means "through the anus". Notice that it is per anum and not per anus (per requires an accusative). Not to be confused with per annum.

Examples:
"introduce the index finger of the left hand per anum" (Hippocrates, On Fistulae)
"Turning to the merits, appellant cites respectable authorities in support of his argument that at common law the "abominable and detestable crime against nature" could only be committed per anum and not per os." (court decision, Robert William DELANEY, v. STATE of Florida, 9/28/66)