one of the ventricular organs, a highly vascular mound of tissue along the margin of the caudal part of the fourth ventricle; consists of many large capillaries, many glial, and some small nerve cells; receives fibers directly from the vagal and glossopharyngeal nerves; its efferent fibers go directly to the nucleus solitarius and the parabrachial nuclei and indirectly to other areas; it lies outside of the blood brain barrier and appears to function as a chemoreceptor trigger zone for emesis.