These are four in number; they are articulated together, and form a bony chain which runs from the membrana tympani to the fenestra ovalis, following a broken line. They have been named the hammer (malleus), the anvil (incus), the lenticular bone, and the stirrup (stapes), from their form or their functions. Special muscles act upon the malleus and the stapes, which are at the two extremities of the chain; the incus and the lenticular bone serving as media for the propagation of the vibrations. The motion impressed upon one of these extremities is communicated to the other by a sort of see-saw movement of the little bones, the mechanism of which is pretty nearly represented by that of a bell. One extremity of the hammer, the handle, is fitted into the membrane of the tympanum, and when the muscle of the hammer contracts, the membrane tightens, a phenomenon which will be discussed further on. The muscle of the stirrup attaches the flat part of this bone to the fenestra ovalis, and according to M. Longet prevents it from being forced in a contrary direction under the influence of the muscle of the hammer, of which it is the antagonist.