Every time the stirrup bone is pushed in and drawn out of the oval window, the watery fluid (the perilymph) in the vestibule and inner ear is set in motion more or less violently, according to the intensity of the sound. The membranous labyrinth occupies the central portion of the vestibule and the passages leading from it. When, therefore, the perilymph is shaken it communicates the impulse to the fluid (endolymph) contained in the inner membranous bag. The endolymph and the tiny grains of ear-sand now perform their part in this marvelous and complex mechanism. They are driven against the sides of the membranous bag, and so strike the ends of the nerves of hearing, which transmit the auditory impulses to the seat of sensation in the brain. It is in the seat of sensation in the brain called the _sensorium_ that the various auditory impulses received from different parts of the inner ear are fused into one, and interpreted as sounds. It is the extent of the vibrations that determines the loudness of the sound; the number of them that determines the pitch. Experiment 158. Hold a ticking watch between the teeth, or touch the upper incisors with a vibrating tuning-fork; close both ears, and observe that the ticking or vibration is heard louder. Unstop one ear, and observe that the ticking or vibration is heard loudest in the stopped ear. Experiment 159. Hold a vibrating tuning-fork on the incisor teeth until you cannot hear it sounding. Close one or both ears, and you will hear it. Experiment 160. Listen to a ticking watch or a tuning-fork kept vibrating electrically. Close the mouth and nostrils, and take either a deep inspiration or deep expiration, so as to alter the tension of the air in the tympanum; in both cases the sound is diminished. Experiment 161. With a blindfolded person test his sense of the direction of sound, _e.g._, by clicking two coins together. It is very imperfect. Let a person press both auricles against the side of the head, and hold both hands vertically in front of each meatus. On a person making a sound in front, the observed person will refer it to a position behind him. 347. Practical Hints on the Care of the Ear. This very delicate and complicated organ is often neglected when skilled treatment is urgently needed, and it is often ignorantly and carelessly tampered with when it should be let alone. Never insert into the ear canal the corners of towels, ear spoons, the ends of toothpicks, hairpins, or any other pointed instruments. It is a needless and dangerous practice, usually causing, in time, some form of inflammation. The abrasion of the skin in the canal thus produced affords a favorable soil for the growth of vegetable parasites. Illustration: Fig. 143.—Diagram of the Middle and Internal Ear.