600. Sound is varied by the velocity of the expelled current of air, and the tension of the vocal ligaments. The size of the larynx, the volume and health of the lungs, the condition of the fauces and nasal passages, the elevation and depression of the chin, the development and freedom of action of the muscles which are attached to the larynx, the opening of the mouth, the state of the mind, and general health of the system, influence the modulations of sound. -=-=-=-=-=-=-=-=-=-=-=-= What effect has the combined action of these muscles? 599. How is sound produced? What have experiments shown? What effect has disease of the vocal ligaments upon the voice? 600. How is sound varied? Mention other conditions that contribute to the modulation of sound. -=-=-=-=-=-=-=-=-=-=-=-= CHAPTER XXX. HYGIENE OF THE VOCAL ORGANS. 601. _The voice can be changed and modified by habit._ Sailors, smiths, and others, who are engaged in noisy occupations, exert their vocal organs more strongly than those of more quiet pursuits. This not only affects the structure of the vocal organs, but varies the intonation of the voice. 602. _The voice is strong in proportion to the development of the larynx, and the capacity of the chest._ Singing and reading aloud improve and strengthen the vocal organs, and give a healthy expansion to the chest. The enunciation of the elementary sounds of the English language, aids in developing the vocal organs, as well as preventing disease of the throat and lungs. This exercise also conduces to the acquisition of musical sounds. 603. _The attitude affects the modulation of the voice._ When an individual stands erect, the movements of the whole respiratory apparatus are most free and effective. The larynx is brought forward by the erect position of the head and the elevation of the chin. The muscles of the arytenoid cartilages are then brought to a proper relation for action, by which a tension of the vocal cords is produced, that favors clear and harmonious enunciation. _Experiment._ Read with the head bowed forward and the chin depressed; then read with the head erect and the chin elevated, and the difference in the movement of the vocal organs, together with the difference in the voice, will be manifest. -=-=-=-=-=-=-=-=-=-=-=-= 601-616. _Give the hygiene of the vocal organs._ 602. How may the voice be strengthened? 603. What effect has the erect attitude upon the modulations of the voice? Give the experiment. -=-=-=-=-=-=-=-=-=-=-=-= [Illustration: Fig. 107. An improper position; but one not unfrequently seen in some of our common schools, and in some of our public speakers.] [Illustration: Fig. 108. The proper position for reading, speaking, and singing.]