Physiology and Hygiene for Secondary Schools Part 41

*The Sound-producing Mechanism of the Body* consists of the following parts:

1. Delicately arranged bodies that are easily set in vibration.

2. An arrangement for supplying the necessary force for making these bodies vibrate.

3. Contrivances for modifying the vibrating parts so as to produce changes in pitch and intensity.

4. Parts that reenforce the vibrations.

5. Organs by means of which the sounds are converted into the forms of speech.

The central organ in this complex mechanism is

*The Larynx.*-The larynx forms a part of the air pa.s.sages, being a short tube at the upper end of the trachea. Mucous membrane lines the inside of it and muscles cover most of the outer surface. The framework is made of cartilage. At the top it is partly encircled by a small bone (the hyoid), and its opening into the pharynx is guarded by a flexible lid, called the _epiglottis_. The cartilage in its walls is in eight separate pieces, but the greater portion of the structure is formed of two pieces only. These are known as the _thyroid cartilage_ and the _cricoid cartilage_ (Fig.

149). Both can be felt in the throat-the thyroid as the projection known as "Adam's apple," and the cricoid as a broad ring just below.

[Fig. 149]

Fig. 149-The larynx.-_A._ Outside view. _B._ Vertical section through larynx, showing inside. 1. Thyroid cartilage. 2. Cricoid cartilage. 3.

Trachea. 4. Hyoid bone. 5. Epiglottis. 6. Vocal cord. 7. False vocal cord.

8. Lining of mucous membrane.

The _thyroid cartilage_ consists of two V-shaped pieces, one on either side of the larynx, meeting at their points in front, and each terminating at the back in an upward and a downward projection. Between the back portions of the thyroid is a s.p.a.ce equal to about one third of the circ.u.mference of the larynx. This is occupied by the greater portion of the _cricoid cartilage_. This cartilage has the general shape of a signet ring and is so placed that the part corresponding to the signet fits into the thyroid s.p.a.ce, while the ring portion encircles the larynx just below the thyroid. Muscles and connective tissue pa.s.s from the thyroid to the cricoid cartilage at all places, save one on each side, where the downward projections of the thyroid form hinge joints with the cricoid. These joints permit of motion of either cartilage upon the other.

At the summit of the cricoid cartilage, on each side, is a small piece of triangular shape, called the _arytenoid cartilage_. Each arytenoid is movable on the cricoid and is connected with one end of a vocal cord.

[Fig. 150]

Fig. 150-*Vocal cords* as seen from above. _A._ In producing sound, _B._ During quiet breathing.

*The Vocal Cords* are formed by two narrow strips of tissue which, connecting with the thyroid cartilage in front and the arytenoid cartilages behind, lie in folds of the mucous membrane. They have the general appearance of ridge-like projections from the sides of the larynx, but at their edges they are sharp and smooth. The open s.p.a.ce between the cords is called the _glottis_. When sound is not being produced, the glottis is open and has a triangular form, due to the spreading apart of the arytenoid cartilages and the attached cords. But when sound is being produced, the glottis is almost completely closed by the cords. Above the vocal cords, and resembling them in appearance, are two other folds of membrane, called the _false vocal cords_ (B, Fig. 149). The false cords do not produce sound, but they aid in the closing of the glottis.

*How the Voice is Produced.*-The voice is produced through the vibrations of the vocal cords. A special set of muscles draws the arytenoid cartilages toward each other, thereby bringing their edges very near and parallel to each other in the pa.s.sage. At the same time other muscles act on the thyroid and cricoid cartilages to separate them at the top and give the cords the necessary tension. With the glottis now almost closed, blasts of air from the lungs strike the sharp edges of the cords and set them in vibration (Fig. 150). The vocal cords do not vibrate as strings, like the strings of a violin, but somewhat as reeds, similar to the reeds of a French harp or reed organ.

The location of the vocal cords in the air pa.s.sages enables the lungs and the muscles of respiration to aid in the production of the voice. It is their function to supply the necessary force for setting the cords in vibration. The upper air pa.s.sages (mouth, nostrils, and pharynx) supply resonance chambers for reenforcing the vibrations from the vocal cords, thereby greatly increasing their intensity. In ordinary breathing the vocal cords are in a relaxed condition against the sides of the larynx and are not acted upon by the air as it enters or leaves the lungs.

*Pitch and Intensity of the Voice.*-Changes in the pitch of the voice are caused mainly by variations in the tension of the cords, due to the movements of the thyroid and cricoid cartilages upon each other.(118) In the production of tones of very high pitch, the vibrating portions of the cords are thought to be actually shortened by their margins being drawn into contact at the back. This raises the pitch in the same manner as does the shortening of the vibrating portion of a violin string.

The _intensity_, or loudness, of the voice is governed by the force with which the air is expelled from the lungs. The vibrations of the cords, however, are greatly reenforced by the peculiar structure of the upper air pa.s.sages, as stated above.

*Production of Speech.*-The sounds that form our speech or language are produced by modifying the vibrations from the vocal cords. This is accomplished by "mouthing" the sounds from the larynx. The distinct sounds, or words, are usually complex in nature, being made up of two or more elementary sounds. These are cla.s.sed either as _vowels_ or _consonants_ and are represented by the different letters of the alphabet.

The vowel sounds are made with the mouth open and are more nearly the pure vibrations of the vocal cords. The consonants are modifications of the vocal cord vibrations produced by the tongue, teeth, lips, and throat.

*Words and their Significance.*-In the development of language certain ideas have become a.s.sociated with certain sounds so that the hearing of these sounds suggests the ideas. Our words, therefore, consist of so many sound signals, each capable of arousing a definite idea in the mind. To talk is to express ideas through these signals, and to listen is to a.s.sume an att.i.tude of mind such that the signals may be interpreted. In learning a language, both the sounds of the words and their a.s.sociated ideas are mastered, this being necessary to their practical use in exchanging ideas.

From spoken language man has advanced to written language, so that the sight of the written or printed word also arouses in the mind the a.s.sociated idea.

THE EAR

*The Ear* is the sense organ which enables sound waves to so act upon afferent neurons as to excite impulses in them. The effect upon the mind which these impulses produce is known as the _sensation of hearing_. In the performance of its function the ear receives and transmits sound waves and also concentrates them upon a suitable exposure of nerve cells. It includes three parts-the _external ear_, the _middle ear_, and the _internal ear_.

*External Ear.*-The external ear consists of the part on the outside of the head called the _pinna_, or auricle, and the tube leading into the middle ear, called the _auditory ca.n.a.l_ (Fig. 151). The pinna by its peculiar shape aids to some extent the entrance of sound waves into the auditory ca.n.a.l.(119) It consists chiefly of cartilage. The auditory ca.n.a.l is a little more than an inch in length and one fourth of an inch in diameter, and is closed at its inner end by a thin, but important membrane, called

*The Membrana Tympani.*-This membrane consists of three thin layers. The outer layer is continuous with the lining of the auditory ca.n.a.l; the inner is a part of the lining of the middle ear; and the middle is a fine layer of connective tissue. Being thin and delicately poised, the membrana tympani is easily made to vibrate by the sound waves that enter the auditory ca.n.a.l. In this way it serves as a receiver of sound waves from the air. It also protects

[Fig. 151]

Fig. 151-*Diagram of section through the ear*, showing relations of its various parts. (See text.)

*The Middle Ear.*-The middle ear, or tympanum,(120) consists of an irregular cavity in the temporal bone which is lined with mucous membrane and filled with air. It is connected with the pharynx by a slender ca.n.a.l called the _Eustachian tube_. Extending across the middle ear and connecting with the membrana tympani on one side, and with a membrane closing a small pa.s.sage to the internal ear on the other, is a tiny bridge formed of three small bones. These bones, named in their order from the membrana tympani, are the _malleus_, the _incus_, and the _stapes_ (Fig.

151). Where the malleus joins the membrane is a small muscle whose contraction has the effect of tightening the membrane. The Eustachian tube admits air freely to the middle ear, providing in this way for an equality of atmospheric pressure on the two sides of the drum membrane. The bridge of bones and the air in the middle ear receive vibrations from the membrana tympani and communicate them to the membrane of the internal ear.

*Purposes of the Middle Ear. *-The middle ear serves two important purposes. In the first place, it makes it possible for sound waves to set the membrana tympani in vibration. This membrane could not be made to vibrate by the more delicate of the sound waves if it were stretched over a bone, or over some of the softer tissues, or over a liquid. Its vibration is made possible by the presence of air on _both_ sides, and this condition is supplied, on the inner side, by the middle ear. The Eustachian tube, by providing for an _equality_ of pressure on the two sides of the membrane, also aids in this purpose.

In the second place, the middle ear provides a means for _concentrating the force of the sound waves_ as they pa.s.s from the membrana tympani to the internal ear. This concentration is effected in the following manner:

1. The bridge of bones, being pivoted at one point to the walls of the middle ear, forms a lever in which the malleus is the long arm, and the incus and stapes the short arm, their ratio being about that of three to two. This causes the incus to move through a shorter distance, but with greater force than the end of the malleus.

2. The area of the membrana tympani is about twenty times as great as the membrane of the internal ear which is acted upon by the stapes. The force from the larger surface is, therefore, concentrated by the bridge of bones upon the smaller surface. By the combination of these two devices, the waves striking upon the membrane of the internal ear are rendered some thirty times more effective than are the same waves entering the auditory ca.n.a.l.

*The Internal Ear*, or labyrinth, occupies a series of irregular channels in the petrous process of the temporal bone.(121) It is very complicated in structure, and at the same time is very small. Its greatest length is not more than three fourths of an inch and its greatest diameter not more than one half of an inch. It is filled with a liquid which at one place is called the _perilymph_, and at another place the _endolymph_. It is a double organ, being made up of an outer portion which lies next to the bone, and which surrounds an inner portion of the same general form. The outer portion is surrounded by a membrane which serves as periosteum to the bone and, at the same time, holds the liquid belonging to this part, called the perilymph. The inner portion, called the _membranous labyrinth_, consists essentially of a closed membranous sac, which is filled with the endolymph. The auditory nerve terminates in this portion of the internal ear. Three distinct divisions of the labyrinth have been made out, known as the _vestibule_, the _semicircular ca.n.a.ls_, and the _cochlea_ (Fig. 152).

[Fig. 152]

Fig. 152-*General form, of internal ear.* The ill.u.s.tration represents the structures of the internal ear surrounded by a thin layer of bone. 1.

Vestibule. 2. Cochlea. 3. Semicircular ca.n.a.ls. 4. Fenestra ovalis. 5.

Fenestra rotunda.

*The Vestibule* forms the central portion of the internal ear and is somewhat oval in shape. It is in communication with the middle ear through a small opening in the bone, called the _fenestra ovalis_, at which place it is separated from the middle ear only by a thin membrane. Sound waves enter the liquids of the internal ear at this point, the foot of the stapes being attached to the membrane. Six other openings lead off from the vestibule at different places. One of these enters the cochlea. The other five open into

*The Semicircular Ca.n.a.ls.*-These ca.n.a.ls, three in number, pa.s.s through the bone in three different planes. One extends in a horizontal direction and the other two vertically, but each plane is at right angles to the other two. Both ends of each ca.n.a.l connect with the vestibule, though two of them join by a common opening. The inner membranous labyrinth is continuous through each ca.n.a.l, and is held in position by small strips of connective tissue.

The purpose of the semicircular ca.n.a.ls is not understood. It is known, however, that they are not used in hearing. On the other hand, there is evidence to the effect that they act as equilibrium sense organs, exciting sensations necessary for balancing the body. Their removal or injury, while having no effect upon the hearing, does interfere with the ability to keep the body in an upright position.

[Fig. 153]

Fig. 153-Diagram showing the divisions of cochlear ca.n.a.l.