From right to left, the figure shows the concha and lobule of the ear in profile; the external meatus (abbreviated); the drum, divided vertically, its posterior half visible; the hammer-bone, with the tip of its long arm attached to the drum, an arrow indicating the point of attachment and line of action of the tensor tympani muscle; the anvil attached by a ligament to the bony wall of the middle ear; the stirrup, with its foot-plate almost filling the oval window; the labyrinth, with the three semicircular canals above, and the scala vestibuli below. The curled black line shows the situation of the scala media, or ductus cochleæ (which contains the organ of Corti). Pulsations of sound which move the membrana tympani are transmitted by the three bones to the oval window. They shake the perilymph, producing waves which travel along the scala vestibuli to the apex of the cochlea, whence they return by the scala tympani to the round window (if they do not take a shorter course through the ductus cochleæ). The Eustachian tube opens out of the lower part of the middle ear.