The Larynx and the Ear : Part 2

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 22 of 27)

The Larynx. - The larynx forms a part of the air passages, 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 cricoids cartilage. Both can be felt in the throat - the thyroid as the projection known as "Adam's apple," and the cricoids as a broad ring just below.

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 space equal to about one third of the circumference of the larynx. This is occupied by the greater portion of the cricoids 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 space, while the ring portion encircles the larynx just below the thyroid. Muscles and connective tissue pass from the thyroid to the cricoids 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.

The Vocal Cords are formed by two narrow strips of tissue which, connecting with the thyroid cartilage in front and the arytenoids 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 space 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 arytenoids 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. 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 arytenoids cartilages toward each other, thereby bringing their edges very near and parallel to each other in the passage. At the same time other muscles act on the thyroid and cricoids 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. 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 passages 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 passages (mouth, nostrils, and pharynx) supply resonance chambers for reinforcing 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 cricoids cartilages upon each other. 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 reinforced by the peculiar structure of the upper air passages, 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 classed 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 associated 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 assume an attitude of mind such that the signals may be interpreted. In learning a language, both the sounds of the words and their associated 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 associated 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.

D.C. Heath and Co. - Publishers
Original copyright 1909