third ventricle: a narrow ventricle in the midplane below the corpus callosum; communicates with the fourth ventricle via the Sylvian aqueduct
ventricle: one of four connected cavities in the brain; is continuous with the central canal of the spinal cord and contains cerebrospinal fluid
polaroid: (trade mark) a plastic film that can polarize a beam of light; often used in sunglasses to eliminate glare