The sky was the colour of urine from a patient with an excess of choler

This is not a medieval retelling of the spittoon joke. The guy at the right is passing around a flask or matula of urine so that the whole class can examine its colour, uromancy or uroscopy being the principal diagnostic tool of current medical practice. It's his own urine, and he half-filled the flask which is why he looks so smug, bladder size being one of these perennial signs of manhood that we XY-chromosome people put so much weight upon. His young assistant is taunting the rest of the class with a smaller flask: "Any of youse poofters want this little girlie pot to piddle in?"

Notice the 15th-century version of the cuckoo-clock / weather guide in the background; the drunken peasant is hoicking out one window, a sign of low humidity and good weather (when it's wetter, he goes in and his wife leans out the other window to hurl chunks). In 10 minutes time it will be 4 p.m. and the bust of the Emperor Elagobalus will pop out from the central roundel on a spring, squawking 'Peacock!' to mark the end of class.

The 'urine wheel' diagrams that illustrated these colour / disease connections were an early attempt to arrange the gamut of colour into some form of systematic order. A wealthy hypochondriac might buy his own 'urine tree' and plant it at the end of the garden, and if he remembered to water it regularly and heap up straw to shelter the roots during winter, every autumn he could expect a good harvest of multi-coloured matulae to relieve the diet of salted pineSHUT UP SMUT

Here's a tense scene from Uromancer where the Case character, attempting to break into the tightly-guarded vault where the Pseudo-Galen Codex is kept behind an astrological lock, tries not to trigger any of the interlacing laser detector beams. Have no fear, IT IS NOT A SPIDERWEB.