THE EAR.

CHAPTER VII.

HOW WE HEAR. SOUND AND SOUND WAVES. THE
THE STRUCTURE OF SPEECH.

VOCAL CHORDS.

On either side of the head, lodged in a cavity which they do not completely fill, and situated in the midst of a dense and solid mass of bone, entering into the base of the skull and forming the temporal bone, are two membraneous bags called the membraneous labyrinth and the scala media of the cochlea. Each bag is filled with a liquid, and is also surrounded and supported by a fluid which fills the cavity in which they are lodged. Certain small, hard bodies, free to move around, lie in the fluid of the bag. The ends of

the auditory nerve of hearing are distributed around the wall of the sac, so that they are subjected to the blows of the little particles of calcareous sand, or otoconia, as they are called, whenever the fluid in the bags is disturbed.

The membraneous lining on which the ultimate ends of the nerves are spread is virtually a sensitive beach, and the little otoconia, showers of pebbles and sand, which are raised and let fall by each succeeding wavelet of sound. This wonderful mechanism constitutes the inner ear.

The ear, as a whole, consists of three parts: the outer ear, which is a trumpet-shaped passageway called the pinna serving to collect the sound waves and pass them on through

CHAMMER

ANVIL

STIRRUP.

FIG. 121. The ossicles.

the auditory canal to a small membrane called the eardrum; the ossicles, a series of three little bones, the hammer, the anvil, and the stirrup, they are called; and the inner ear just described.

The foot of the stirrup is connected with an oval membrane, which closes a hole in the inner ear. Sounds passing through the auditory canal cause the drum to vibrate and send tremors through the bones to the liquid in the little sacs. The tumbling of the "pebbles" against the filaments of the auditory nerve sends the intelligence to the brain.

The impression which the mind receives through the organ of hearing is called sound. All bodies which produce sounds are in a state of vibration, and they communi

cate their vibrations to the surrounding air and thus set it into waves, just as a stick waved back and forth in a pool of water creates ripples.

Sound implies vibration, and whenever a sound is heard

FIG. 122.-Bon jour ("good day" in French) as represented by a wave picture. The picture was made by a mirror arranged to move under the influence of the voice and to cast a beam of light upon a strip of sensitized paper.

some substance, a solid, a liquid, or a gas is in vibration and the surrounding air is in unison with it.

Sound has been likened to a picture painted not in the space and color of substance but in time and motion. What really passes out from the source is merely a rhythmical motion of the air particles, manifesting themselves as changes in pressure, spreading out in ever-widening spheres through the atmosphere. The order of these compressions is different for every sound. The musical sounds of an orchestra embody a different set of vibrations for

each note of each particular instrument. If the fluctuations in pressure of a sound wave are irregular and non-periodic, the sound is called a noise; if they are cyclic, and follow a regular and sufficiently rapid periodic lag, the sound is musical.

We may easily satisfy ourselves that in every instance in which the sensation of sound is produced the body from

FIG. 123.-Experiment showing sounding bodies are in vibration.

whence the sound comes must have been thrown into a state of rapid tremor, implying the existence of a motion to and fro of the particles of which it consists.

If the face of a tuning fork prong be touched with a small ball of cork suspended from a fine silk fiber, after the fork has been struck and caused to emit its note, the cork will be violently repelled from the latter. Why? Because the prong of the fork is in vibration.

If a small wire or bristle is fastened to the prong of the fork and a piece of smoked glass drawn across it while the fork is giving forth a sound, the trace of the point will appear as a wavy line, showing that while the glass was drawn along the prong went to and fro many times.

The vibrations or disturbances set up in the air by a sound emitting body are known as sound waves. These waves consist of a series of condensations and rarefactions

succeeding each other at regular intervals, each air particle swinging to and fro in a very short path.

Air waves cannot be seen by the naked eye, but their nature may be easily represented or illustrated. Fig. 126

FIG. 125.-Wavy line made by a bristle attached to a tuning fork prong in vibration when passed over smoked glass.

gives a pictorial representation of the crowding together of the air particles during the passage of a wave. The loudness of the sound depends upon the amount and suddenness of the change in pressure, and the note or pitch on the number of complete to and fro motions of the par ticles per second.

The timbre of a sound or the quality that distinguishes the note of a violin from that of a piano depends upon the