*The Cochlea* is the part of the internal ear directly concerned in hearing. It consists of a coiled tube which makes two and one half turns around a central axis and bears a close resemblance to a snail sh.e.l.l (Figs. 151 and 152). It differs in plan from a snail sh.e.l.l, however, in that its interior s.p.a.ce is divided into three distinct channels, or ca.n.a.ls. These lie side by side and are named, from their relations to other parts, the _scala vestibula_, the _scala tympani_, and the _scala media_. Any vertical section of the cochlea shows all three of these channels (Fig. 153).

*The Scala Vestibula and the Scala Tympani* appear in cross section as the larger of the ca.n.a.ls. The former, so named from its connection with the vestibule, occupies the upper position in all parts of the coil. The latter lies below at all places, and is separated from the channels above partly by a margin of bone and partly by a membrane. It receives its name from its termination at the tympanum, or middle ear, from which it is separated only by a thin membrane.(122) Both the scala vestibula and the scala tympani belong to the outer portion of the internal ear and are, for this reason, filled with the perilymph. At their upper ends they communicate with each other by a small opening, making by this means one continuous ca.n.a.l through the cochlea. This ca.n.a.l pa.s.ses from the vestibule to the tympanum and, in so doing, goes entirely around

*The Scala Media.*-This division of the cochlea lies parallel to and between the other two divisions. It is above the scala tympani and below the scala vestibula, and is separated from each by a membrane. The scala media belongs to the membranous portion of the internal ear and is, therefore, filled with the endolymph. It receives the terminations of fibers from the auditory nerve and may be regarded as the true sense organ of hearing. The nerve fibers terminate upon the membrane known as the _basilar membrane_, which separates it from the scala tympani. This membrane extends the length of the cochlear ca.n.a.ls, and is stretched between a projecting shelf of bone on one side and the outer wall of the cochlea on the other. It is covered with a layer of epithelial cells, some of which have small, hair-like projections and are known as the _hair cells_. Above the membrane, and resting partly upon it, are two rows of rod-like bodies, called the _rods of Corti_. These, by leaning toward each other, form a kind of tunnel beneath. They are exceedingly numerous, numbering more than 6000, and form a continuous series along the margin of the membrane.

[Fig. 154]

Fig. 154-*Diagram* ill.u.s.trating pa.s.sage of sound waves through the ear.

*How We Hear.*-The sound waves which originate in vibrating bodies are transmitted by the air to the external ear. Pa.s.sing through the auditory ca.n.a.l, the waves strike against the membrana tympani, setting it into vibration. By the bridge of bones and the air within the middle ear the vibrations are carried to and concentrated upon the liquid in the internal ear (Fig. 154). From here the vibrations pa.s.s through the channels of the cochlea and set into vibration the contents of the scala media and different portions of the basilar membrane. This serves as a stimulus to the fibers of the auditory nerve, causing them to transmit impulses which, on pa.s.sing to the brain, produce the sensation of hearing.

Much of the peculiar structure of the cochlea is not understood. Its minute size and its location in the temporal bone make its study extremely difficult. The connection of the scala vestibula with the scala tympani, and this with the middle ear, is necessary for the pa.s.sage of vibrations through the internal ear. Its liquids, being practically incompressible and surrounded on all sides by bones, could not otherwise yield to the movements of the stapes. (See Practical Work.) The rods of Corti are thought to act as dampers on the basilar membrane, to prevent the continuance of vibrations when once they are started.

*Detection of Pitch.*-The method of detecting tones of different pitch is not understood. Several theories have been advanced with reference to its explanation, one of the most interesting being that proposed by Helmholtz.

This theory is based on our knowledge of sympathetic vibrations. The basilar membrane, while continuous throughout, may be regarded as made up of many separate cords of different lengths stretched side by side. A tone of a given pitch will set into vibration only certain of these cords, while tones of different pitch will set others into vibration.

Another theory is that the basilar membrane responds to all kinds of vibrations and the a.n.a.lysis of sound takes place in the brain.

A third view is that the filaments from the hair cells, rather than the basilar membrane, respond to the vibrations and in turn stimulate the terminations of the nerve fibers.

[Fig. 155]

Fig. 155-*Diagram* showing how wax may plug the auditory ca.n.a.l and cause deafness.

*Hygiene of the Ear.*-The ear, being a delicate organ, is frequently injured by careless or rough treatment. The removal of the ear wax by the insertion of pointed instruments has been found to interfere with the natural method of discharge and to irritate the membrane. It should never be practiced. It is unnecessary in the healthy ear thus to cleanse the auditory ca.n.a.l, as the wax is pa.s.sed by a natural process to where it is easily removed by a damp cloth. If the natural process is obstructed, clean warm water and a soft linen cloth may be employed in cleansing the ca.n.a.l, without likelihood of injury. Clean warm water may also be introduced into the auditory ca.n.a.l as a harmless remedy in relieving inflammation of the auditory ca.n.a.l and of the middle ear. Children's ears are easily injured, and it goes without saying that they should never be pulled nor boxed.

It frequently happens that a ma.s.s of wax collects in the auditory ca.n.a.l and closes the pa.s.sage so completely as to cause deafness (Fig. 155). This may come about without pain and so gradually that one does not think of seeking medical aid. Such ma.s.ses are easily removed by the physician, the hearing being then restored. Both for painful disturbances of the ear and for the gradual loss of hearing, the physician should be consulted.

*The Hearing of School Children.*-School children not infrequently have defective hearing and for this reason are slow to learn. The hearing is easily tested with a watch, the normal ear being able to hear the watch tick at a distance of at least two feet. Pupils with defective hearing should, of course, have medical attention, and in the cla.s.sroom should be seated where they can hear to the best advantage.

*Summary.*-Sound waves const.i.tute the external stimuli for the sensation of hearing. They consist of progressive vibratory movements of the air that originate in vibrating bodies. Through the larynx and the ear, sound waves are utilized by the body in different ways, but chiefly as a means of communication. The larynx produces sound waves which are reenforced and modified by the air pa.s.sages. The ear supplies suitable conditions for the action of sound waves upon nerve cells. Both the ear and the larynx are constructed with special reference to the nature and properties of sound waves, and they ill.u.s.trate the body's ability to adjust itself to, and to make use of, its physical environment.

*Exercises.*-1. For what different purposes are sound waves employed in the body?

2. How do sound waves originate? How are they transmitted? How do they differ from the waves on water?

3. How are sound waves able to act as nerve stimuli?

4. Describe two methods of reenforcing sound waves. Which method is employed in the body?

5. Name all the parts of the body that are directly or indirectly concerned in the production of sound.

6. Describe the larynx.

7. Describe the condition of the vocal cords in speaking and in ordinary breathing.

8. How are sounds differing in pitch and intensity produced by the larynx?

9. How is the sound produced by the vocal cords changed into speech?

10. What parts of the ear are concerned in transmitting sound waves?

11. Give the purposes of the middle ear.

12. Trace a sound wave from a bell to the basilar membrane, and trace the impulse that it causes from there to the brain.

13. Give the purpose of the Eustachian tubes; of the rods of Corti; of the semicircular ca.n.a.ls.

14. Give directions for the proper care of the ear.

PRACTICAL WORK

*To ill.u.s.trate the Origin of Sound.*-1. Strike a bell an easy blow and hold some light substance, as a pith ball attached to a thread, against the side, noting the result. 2. Sound a tuning fork by striking it against the table. Test it for vibrations as above, or by letting the vibrating p.r.o.ngs touch the surface of water. 3. Pluck a string of a guitar or violin, and find proof that it is vibrating while giving out sound.

*To show the Transmission of Sound.*-1. Vibrate a tuning fork and press the stem against a table or desk. The vibrations which are reenforced in this way will be heard in all parts of the room. Now press one end of a wooden rod, as a broom handle, against the table, and bring the stem of the vibrating fork against the other end. The vibrations now move down the stick to the table, from whence they are communicated to the air. Observe that the sound waves, to reach the ear, must pa.s.s through the rod, the table, and the air. 2. Fasten the tuning fork to a flat piece of cork by pressing the stem into a small hole in the center. Vibrate the fork and let the cork rest on the surface of water in a half-filled tumbler on the table. The sound will, as before, pa.s.s to the table and then to the air.

Observe that in this case the vibrations are transmitted by a liquid, a solid, and by the air. Compare this action with the transmission of sound waves by different portions of the ear.

*To show Effects of Sound Waves.*-1. Place two large tuning forks of the same pitch, and mounted on thin boxes for reenforcing their vibrations, near each other on a table. Vibrate one of the forks for a moment and then stop it by means of the hand. Observe that the other fork has been set in vibration. (This experiment does not work with forks of different pitch.) 2. While holding a thin piece of paper against a comb with the open lips, produce musical tones with the vocal cords. These will set the paper in vibration, producing the so-called "comb music." 3. Examine the disk in a telephone which is set in vibration by the voice. Observe that it is a thin disk and, like the membrane of the ear, has air on both sides of it.

*To show the Reenforcement of Sound.*-1. Vibrate a tuning fork in the air, noting the feebleness of the tone produced. Then hold the stem against a door or the top of a table, noting the difference. 2. Hold a vibrating tuning fork over a tall jar, or bottle, and gradually add water. If the vessel is sufficiently tall, a depth will be reached where the air in the vessel reenforces the sound from the fork. 3. Hold a vibrating fork over the mouth of a small fruit jar, partly covered with a piece of cardboard.

By varying the size of the opening, a position will be found where the sound is reenforced. If not successful at first, try bottles and jars of different sizes.

*To ill.u.s.trate the Manner of Vibration of the Liquid in the Internal Ear.*-Tie a piece of dental rubber over the end of a gla.s.s or wooden tube about half an inch in diameter and six inches in length. Fill the tube entirely full of water and, without spilling, tie a piece of thin rubber tightly over the other end. Holding the tube horizontally, press the rubber in at one end and note that it is pushed out at the other end. Make an imitation of a vibration with the finger against the rubber at one end of the tube and note the effect at the other end. To what do the tube and the rubber on the ends of the tube correspond in the internal ear?

[Fig. 156]

Fig. 156-*Simple apparatus* for demonstrating the larynx.

*To show the Plan of the Larynx.*-Cut from stiff paper four pieces of different shapes as indicated in Fig. 156. (The piece to the left should have a length of about six inches, the others proportionally large.) The largest represents the thyroid cartilage, the next in size the cricoid, and the two smallest the arytenoid cartilages. By means of pins, or threads, connect these with each other according to the description of the larynx on page 253. With this simple model the movements of the different cartilages and their effect upon the vocal cords may be ill.u.s.trated.

*To show the Relation of the Movements of the Vocal Organs to the Production of Different Sounds.*-1. Lightly grasp the larynx with the fingers while talking. Observe the changes, both in the position and shape of the larynx, in the production of sounds of different pitch. 2. Observe the difference in the action of the muscles of respiration in the production of loud and faint sounds. 3. p.r.o.nounce slowly the vowels, A, E, I, O, U, and the consonants C, F, K, M, R, S, T, and V, noting the shape of the mouth, the position of the tongue, and the action of the lips in each case.

*To demonstrate the Ear.*-Examine a dissectible model of the ear, locating and naming the different parts. Trace as far as possible the path of the sound waves and find the termination of the auditory nerve. Note also the relative size of the parts, and calculate the number of times the model is larger than the natural ear. _Suggestion_: The greatest diameter of the internal ear is about three fourths of an inch.

In an extended course it is a profitable exercise to dissect the ear of a sheep or calf, observing the auditory ca.n.a.l, middle ear, bridge of bones, and the tympanic membrane with attached malleus and tensor tympanic muscle. Pa.s.s a probe from the nasal pharynx through the Eustachian tube into the middle ear. With bone forceps or a fine saw, split open the petrous portion of the temporal bone and observe the cochlea and the semicircular ca.n.a.ls. By a careful dissection other parts of interest may also be shown.