(Anat.) A cavity, or one of the cavities, of an organ, as of the larynx or the brain; specifically, the posterior chamber, or one of the two posterior chambers, of the heart, which receives the blood from the auricle and forces it out from the heart. See Heart. Note: The principal ventricles of the brain are the fourth in the medulla, the third in the midbrain, the first and second, or lateral, ventricles in the cerebral hemispheres, all of which are connected with each other, and the fifth, or pseudocoele, situated between the hemispheres, in front of, or above, the fornix, and entirely disconnected with the other cavities. See Brain, and Coelia.