*Value of Sound Waves to the Body.*—From a physiological standpoint, the value of sound waves is not easily overestimated. In addition to the use made of them in the communication of ideas, they serve the purpose of protecting the body, and in the sphere of music provide one of the most elevating forms of entertainment. Sounds from different animals, as well as from inanimate objects, may also be the means of supplying needed information. The existence of two kinds of sound instruments in the body—the one for the production, the other for the detection, of sound—is certainly suggestive of the ability of the body to adjust itself to, and to make use of, its physical environment. Both the larynx and the ear are constructed with special reference to the nature and properties of sound waves. THE LARYNX *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 reënforce 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 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 _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.