HOW WE HEAR.--Whenever one body strikes another in the air, waves are produced, just as when we throw a stone into the water a series of concentric circles surrounds the spot where it sinks. These waves of air strike upon the membrane. This vibrates, and sends the motion along the chain of bones in the middle ear to the fluids of the labyrinth. Here bristles, sand, and stones pound away, and the wondrous harp of the cochlea, catching up the pulsations, [Footnote: The original motion is constantly modified by the medium through which it passes. The bristles, otoliths, and Cortian fibers of the ear, and the rods and cones of the eye (p. 239) serve to convert the vibrations into pulsations which act as stimuli of the appropriate nerve. The molecular change thus produced in the nerve fibers is propagated to the brain.--See _Popluar Physics_, p. 182.] carries them to the fibers of the auditory nerve, which conveys them to the brain, and gives to the mind the idea of sound. CARE OF THE EAR.--The delicacy of the ear is such that it needs the greatest care. Cold water should not be allowed to enter the auditory canal. If the wax accumulate, never remove it with a hard instrument, lest the delicate membrane be injured, but with a little warm water, after which turn the head to let the water run out, and wipe the ear dry. The hair around the ears should never be left wet, as it may chill this sensitive organ. If an insect get in the external ear, pour in a little oil to kill it, and then remove with tepid water. The object of the Eustachian tube is to admit air into the ear, and thus equalize the pressure on the membrane. If it become closed by a cold, or if, from any cause, the pressure be made unequal, so as to produce an unpleasant feeling in the ear, relief may often be obtained by grasping the nose and forcibly swallowing. (See p. 350.) 5. SIGHT. FIG. 61. [Illustration: _The Eye._] DESCRIPTION.--The eye is lodged in a bony cavity, protected by the overhanging brow. It is a globe, about an inch in diameter. The ball is covered by three coats--(l) the _sclerotic_, _d_, a tough, horny casing, which gives shape to the eye, the convex, transparent part in front forming a window, the _cornea_, _d_; (2) the _choroid_, _e_, a black lining, to absorb the superfluous light [Footnote: Neither white rabbits nor albinos have this black lining, and hence their sight is confused.] and (3) the _retina_, _b_, a membrane in which expand fibers of the _optic nerve_, _o_. The _crystalline lens_, _a_, brings the rays of light to a focus on the retina. The lens is kept in place by the ciliary processes, _g_, arranged like the rays in the disk of a passion flower. Between the cornea and the crystalline lens is a limpid fluid termed the _aqueous humor_; while the _vitreous humor_--a transparent, jelly-like liquid fills the space (_h_) back of the crystalline lens. The pupil, _k_, is a hole in the colored, muscular curtain, _i_, the _iris_ (rainbow). (See p. 352.)