PATHOLOGY The eustachian tube, the narrow passage through which air reaches the tympanic cavity and mastoid cells, has three distinct parts, namely, the pharyngeal, the isthmus and the tympanic. The longest, the pharyngeal, with its widely open orifice in the nasopharynx, has a cartilagenous and membranous base with muscular tissue underlying its ciliated lining mucous membrane, and is constantly dilating and closing. This part of the tube is a diverticulum from the nasopharynx, containing within its mucous membrane lymphoid tissue, and is the site of the same kinds of inflammation, infection and pathologic invasion as occurs in the nasopharynx, in which there is such an abundance of lymphoid tissue. The outer and tympanic end of the eustachian tube is devoid of muscular tissue and action, its lining membrane is the mucoperiosteum of its underlying bony wall or base. It is a recess and a diverticulum from the tympanic cavity,