Eating animal brains and spinal cords is associated with mad cow disease.
A human version of mad cow disease called variant Creutzfeldt-Jakob disease (vCJD) is believed to be caused by eating beef products contaminated with central nervous system tissue, such as brain and spinal cord, from cattle infected with mad cow disease. For this reason, the USDA requires that all brain and spinal cord materials be removed from high-risk cattle -- older cattle, animals that are unable to walk, and any animal that shows any signs of a neurological problem. These cow products do not enter the U.S. food supply. The USDA believes this practice effectively safeguards U.S. public health from vCJD.
http://www.webmd.com/brain/mad-cow-disease-basics