It´s a "memento mori," a graphic way to remind all who pass that death is always just a breath away. Common imagery in Christian art, esp. the counter-reformation when everyone in Spain was VERY emotional about their Catholicism!
These kinds of gruesome things were common also in the Victorian era in the USA and England and western Europe. You see inscriptions on tombstones and graveyard gates with sunny sayings like "As you are, so once was I, and as I am you soon shall be: Prepare for Death and follow me."
I´m sure it´s being recycled as we speak by some Horror Rap star...
Reb