284 THE EAR The translucency of the drumhead varies considerably in health and, while the above description refers to one of average translucency, additional structures may be seen when the membrane is more transparent. The long process of the incus is frequently observed as a white line behind and parallel to the handle of the malleus (Fig. 157), but extending only about half its Fig. 156. Using the electric auriscope as a pneumatic speculum. Fig. 157. Diagram of right tympanic membrane. 1, Anterior superior quadrant; 2, Anterior inferior quadrant; 3, Posterior inferior quadrant; 4, Posterior superior quadrant. length. It is sometimes seen to end in a round spot which is the head of the stapes. At right angles to this, and extending to the postertor margin of the membrane, the tendon of the stapedius muscle presents a similar appearance. In the lower and posterior quadrant a round shadow may be seen at the circumference of the membrane which corresponds to the niche leading to the round window. The centre of the membrane may be of a pale yellow colour, due to the promontory shining through, because the membrane is concave while the promontory is convex, and thus the medial wall of the middle ear cavity is much closer to the drumhead at this part. It is occasionally possibk to see the tympanic opening of the Eustachian tube through the most anterior part of the membrane.