A common motif of northern European painting in the late Middle Ages and Renaissance was the "memento mori" (remember that you die), a skull placed in the picture as a reminder of the transitoriness of earthly life. Paintings of St. Jerome in his study are especially likely to have one. They are often accompanied by a little trompe-l'oeil label saying something like "respice finem" (keep in mind the end [of your life]). A popular late medieval literary genre was the "Ars moriendi," "How to Die."

Life imitates art, in this case: this particular memento mori I keep on my desk in my office. I haven't yet had a student brave enough to ask me about it.