I would add that CSF has many
diagnostic uses, and is sometimes extracted for analysis. For example, most of what we know about the role of
serotonin shortage in
depression comes from looking not at brains (which is hard and destructive), but looking at CSF, which we can assay for
5-HIAA, the main metabolite of
5-HT, or serotonin.
There are persistent rumors that
LSD (acid) and
MDMA (
ecstasy) leave crystals or other residues in the CSF, or otherwise harm or deplete it. These are demonstrably false.