The current issue of Cabinet Magazine, devoted to infrastructure, contains a fascinating article on an architectural detail of Catholic churches called the piscina. The author, Yara Flores, describes it like this:
Often set apart under an ornate lid and sealed under lock and key, the piscina has the air of a tabernacle but the appearance of a hooded washbasin. Indeed, its cover opened, the modern piscina tends to look like a very ordinary little sink. The deep difference lies not in its exposed hardware (an enamel basin, common chrome or brass fittings, etc.) but rather in its hidden plumbing, since the drainpipe of a piscina, rather than running into the sewer system, instead passes in pristine segregation down through the floor and foundation of the church and there vents into a small, sepulchral cave sunk in consecrated ground. The piscina is a sink that eads to a grave. What we have here is less a drainpipe than a burial chute.
As Flores goes on to explain, the purpose of the piscina can be traced back to the Christian dogma of transubstantiation. After a centuries-long debate, this rather mindbending theological doctrine was formulated at the Fourth Lateran Council in 1215 to mean that during the Eucharist the consecrated bread and wine actually change into the body and blood of Jesus. (At which point, it is bread and body, wine and blood simultaneously. To understand this fully, be prepared for some very subtle Aristotelian philosophy on substance.)
This mystically paradoxical dogma - or contradiction, depending on where you stand theologically - provided the central ritual in Christian mass, and also embodied (excuse the pun) that other mystical paradox of Christianity, the socalled hypostatic union of Jesus being both fully God and fully man.
However, all this theological subtlety aside (a couple of centuries later the protestants would start the debate afresh), the dogma of transubstantiation created an interesting practical problem, which led to the construction of the piscina (the basin) and the sacrarium (the drain) in churches.
If the priest was handling the body of Christ - which was, of course, the "body" of God Himself - extraordinary precautions were called for. He needed to wash his hands, certainly, both before and after. Before, fine, he could just give them a ritual rinse. But after? What about the crumbs? He was presumably here washing away tiny bits of the Almighty, and it clearly would not do to have such fragments, however minute, sloughed to the strawstrewn flagstones, or carried out by charwomen in the bilge. One was here disposing of sacred remains, indeed the most sacred remains possible to conceive.
The full article is online, titled 'Drain Pipes, Dream Pipes, Pipe Dreams' (PDF).