DESCRIPTION.--The ear is divided into the _external_, _middle_, and _internal_ ear. 1. _The External Ear_ is a sheet of cartilage curiously folded for catching sound. The auditory canal, _B_, or tube of this ear trumpet, is about an inch long. Across the lower end is stretched _the membrane of the tympanum_ or drum, which is kept soft by a fluid wax. FIG. 60. [Illustration: _The Ear._] 2. _The Middle Ear_ is a cavity, at the bottom of which is the Eustachian tube, _G_, leading to the mouth. Across this chamber hangs a chain of three singular little bones, _C_, named from their shape the _hammer_, the _anvil_, and the _stirrup_. All together these tiny bones weigh only a few grains, yet they are covered by a periosteum, are supplied with blood vessels, and they articulate with perfect joints (one a ball-and-socket, the other a hinge), having synovial membranes, cartilages, ligaments, and muscles. 3. _The Internal Ear_, or labyrinth, as it is sometimes called from its complex character, is hollowed out of the solid bone. In front, is the vestibule or antechamber, _A_, about as large as a grain of wheat; from it open three _semicircular canals_, _D_, and the winding stair of the _cochlea_, or snail shell, _E_. Here expand the delicate fibrils of the auditory nerve. Floating in the liquid which fills the labyrinth is a little bag containing hair-like bristles, fine sand, and two ear stones (_otoliths_). All these knocking against the ends of the nerves, serve to increase any impulse given to the liquid in which they lie. Finally, to complete this delicate apparatus, in the cochlea are minute tendrils, named the fibers of Corti, from their discoverer. These are regularly arranged,--the longest at the bottom, and the shortest at the top. Could this spiral plate, which coils two and a half times around, be unrolled and made to stand upright, it would form a beautiful microscopic harp of three thousand strings. If it were possible to strike these cords as one can the keyboard of a piano, he could produce in the mind of the person experimented upon every variety of tone which the ear can distinguish.