The “devil’s backbone” refers to the medical condition of spina bifida, which occurs when a foetus’ spine and spinal cord do not join in the womb. In small Spanish towns, until slightly after the 1930s, it was considered a divine sign that a) the children were not meant to live, and/or b) their parents had sinned deeply. It was only much later that this congenital defect was understood by the medical world.

Halfway through the film, when asked by Carlos if he believes in ghosts, Casares shows him a series of specimen jars containing deformed foetuses, one of which has spina bifida. Casares relates how the rum-based embalming fluid of this particular foetus is sold as a cure for, among other things, impotence. He then offers Carlos a sip of the fluid to “cure” him of his superstitions. After the chastened Carlos declines and lives, Casares drinks the fluid himself – indicating that the futility of his situation, and his own unrequited love for Carmen, has somehow led him to believe in irrationality himself.