In the olden days of tanning, each individual hide was tanned with the brain of the animal killed.
Theory has it that the brain within the animal was sufficient to tan the hide.
I tan I need a larger, continuous supply of brains. Ninety eight percent of the hides I tan, have been obtained long after the death of the animal and I do not have access to the animal's brains. Instead I use fresh or freshly thawed cow brains that I order in bulk through grocery stores or meat packing plants. When I purchase brains, but are not able to use them for a matter of days I freeze them until I am ready to use them. Brains begin to rot even faster than meat and should be used fresh for this reason.