Yosef Karo, Shulchan Aruch — 31: The law of perforation of the brain or its membrane
“ Seif 2 The brain itself, if a bit of it is rotted, or mashed, but the meninges is intact -- it's kosher. But if it runs like water or wax -- it's treif. And if fluid is found inside the meninges, and there's lacking from the brain the volume of the fluid, if the brain encloses the fluid so it's not exposed at all even to the meninges -- it's kosher. ”
